LEFT- AND RIGHT-HANDERS DISTRIBUTE THEIR ATITNTION ASYMMETRICALLY O N STIMULUS-RESPONSE COMPATIBILITY TASKS

A Thesis

Presented to The Faculty of Graduate Studies

of The University of Guelph

by JASON IVANOFF

In partial fulfilment of requirements for the degree of Master of Arts August, 1998.

0 Jason Ivanoff, 1998.

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LEFT-AND RIGHT-HANDERS DISTRIBUTE 'MEIR ATTENTION ASYMMETRICALLY ON STIMULUS-RESPONSE COMPATIBILFTY TASKS

Jason Ivanoff University of Guelph, 1998.

Advisor: Professor M. Peters

Subgroups of left- and right-handers completed three stimulus-response compatibility experiments to investigate the relation between hand preference and performance. Although each handedness group demonstrateci Simon effects in al1 three experiments, the performance patterns of four theoretically distinct handedness groups were dissimilar. The differences between the two subgroups of lefi-handers, however,

were much les subtle than they were between the two subgroups of right-handem. The results suggest that subjects distributed their attention asymmetricaily on these stimulusresponse compatibility tasks. Furthemore, the direction of the aîîentional asymmetry was not aiways in accordance with the writing hand. Attentional precues altered the

magnitude, and sornetimes the direction, of the attentional asymmetrïes.

Acknowledgements. 1 wish to thank Gwen Keamey fbr the eutry ofdata in Experiment 2; Bruce Mckay for buildmg the respoase boxes; Wray Hutten fOr his assistance in the programming ofthe

experimeats; and Dr.Harvey Mannurek and Dr.Ernest Dalrymple-AlfOfd fbr their heipfid discussions and comrnents on eariier cirafis of t h thesis. I wish to &end m e r appreciatioa to

Dr.Michael Peters fbr his gracious support and guidance throughout the development of this ady-

TABLE OF CONTENTS

The Role of Handedness and Controlled Spatial Attention in the Simon Effea

38

LIST OF TABLES

Table 1:

Table 2:

Table 3 :

Table 4:

Table 5:

LIST OF FIGURES

Figure 5 :

Mean reachn th^ ofthe inconsistent kfl-î~ndersof

--eat_eateat.-_----_---.didididididididididi-

Experiment 2.

Figure 8:

116

Approximately ninety percent of people prefer to use their nght hand for a majority of everyday activities. Traditionaily, handedness has been assessed via

preference questionnaires (Peters, 1998). if'the goal is to leam which hand people would use in a given situation, one rnay sirnply ask (as in a questionnaire). A common cornplaint regarding these questionnaires, however, is that they are too subjective. One

rnay prefer to use a given hanci, in a given situation, but this hand may or may not be the moa proficient. This problem has compelled researchers to validate handedness

questionnaires by correlating manual performance with preference profiles. Some manual performance tasks, however, do not correlate well with manual preference, nor do some manual tasks correlate we11 with others (Rigal, 1992). This problem led some researchers to reason that relations between performance and preference relate simply to those tasks that are over-learned or well-practised (Provins, 1997; Provins & Magliaro, 1993). According to this view, the direction of one's handedness is largely a function of past experience.

This viewpoint encounten difficulties, however, when reliable performance asymmetries emerge between handedness groups (deterniined by preference inventories) on simple (Le., 't~nskikilled"~) motor tasks that appear devoid of prior expenence. Finger

tapping is such a measure (Peters, 1996). When subjects are instnicted to tap rapidly, one hand at a time, the writing hand taps faster than the other hand. The better manual

performance is uniikely related to task experience (since finger tapping is probably not practised) and may be attribut4 to the underlying differences in neural architecture.

The subjectivity of preference questionnaires, and the dilemma of task-specific

unimanual experience on performance measures, prompted some researchers to employ altemate methodology in handedness research. Unconstrained reaching tasks (Bishop, Ross Daniels, & Bright, 19%; Gabbard, Iteyq & Rabb, 1997) and modifieci peg-board tasks (Bryden, Singh, Steenhuis, & Clarkson, 1994) have emerged as instruments of so-

cailed objective preference measures. Researchers who use the reaching preference tasks record which hand subjects prefer to use to make ipsilateral and contralateral reaches. The long peg-board task requires subjects to make unimanuai "eap-frog-like" movements with a pair of pegs, begiming with the same hand-hemispace pairing. The point when subjects switch hands is taken to be an indication of the strength of one's hand preference. Although ipsilateral hand-hemispace pairings are preferred, some

groups of subjects will demonstrate hand preference almon irrespective of hernispace. Subjects demonstrate this bias even in the face of awkwardness (i-e.,contralateral reaches require some degee of stretching, and some loss of motor degrees of fi-eedom, since the

a m must cross midline). This devotion to the preferred hand (in contraiateral space) has been correlateci with manual preference between (Gabbard et al., 1997), and within (Bishop et ai., 1996) handedness groups. It is, nonetheless, apparent that subjects do not prefer to make al1 reaches (in left- and nght hernispace) with ody one particular hand. Why is this so? At first blush, the answer to the preceding question seems unrnistakable.

Anatomically, it is simply awkward to rnake contralateral reaches. The muscular systems 2

of these handedness groups may work to constrain particular reaches in space. It is easier

to reach out to objects in right hemispace with the right hand just as it is easier to reach with the lefl hand in left hemispace. Hence, the ipsilaterai advantage may sirnply reflect

anatomical "cornfort." The story, however, is more complicated beuuise people tend to initiate ipsilateral reaches fister than contraiateral ones (Fisk & Goodale, 1985; Stins & Michaels 1997). This effect (ipsilateral reaches initiateci faster than contralateral ones) is similar to s~atialcom~atibilitvand Simon effects found using choice reaction tirne

experiments. These effects will be discussed in detail. The fin section of this thesis will consider current theoretical positions of the

Simon effect. The issues of handedness, briefly discussed in this introduction, will be reconsidered with reference to spatial processing, spatial compatibility, and the Simon effect. Three experiments that compare the performance profiles of four handedness groups follow.

THE THEORETICAL PERSPECTIVES OF THE SIMON EFFECT

Generally, the S-R compatibility effect refers to the accuracy or reaction time (RT) advantage for stimulus ensembles that share physical or perce*

similarity with

response ensembles comparai to stimulus-response (S-R)ensembles that are dissimilar (Fitts & Seeger, 1953). The physical arrangement of early novaops provides a good example of this effect. The control panel was often aligneci horizontally (in a row) in front of the four bumem. The four burners, on the other han& were aligned such that 3

each bumer was positioned in ûich corner of the square stovetop (this is ail1 the cornmon arrangement ofthe burners). Confiision occurs when one tries to determine the control dial that operates a p h c u i a r bumer. The physical anangement of the controls is said to be incompatible with that of the burners. As the name implies, the locus of S-R

compatibility effects is thought to reside at the response-selection processing stage (i-e.,it occurs d e r the qualities of the stimulus are processed, but before response execution).

S-R cornpatibility effects are, presumably, different in kind fkom stimulus-stimulus (S-S) wngruency effects. S-Scongruence is presumed to occur when two dimensions of the stimulus match. S-S incongruent pairs are those where the two dimensions of the stimulus conflict. Research on S-R compatibility has a long hiaory. It is beyond the scope of this thesis to consider al1 types of compatibility effects. Rather, two classes of

S-R cornpatibility eflects will be discussed here: spatial compatibility and the Simon effect. Both types of compatibility effects refer to the RT and accuracy rate advantage when the spatial position of the stimulus and respond correspond than when they do notl

Consider a task where a stimulus is presented to the lefl or to the nght of a fixation point, and the correct response is contingent upon stimulus location. Two blocks of tnak mua be envisageci. In one block, the left hand responds to lefi side stimuli and the right hand responds to right side stimuli (S-Rcorrespondence). In the other, the lefl hand responds to right side stimuli and the right hand responds to Ieft side stimuli (S-R non-correspondence). Spatial compatibility refers to the situation where responses in the

S-R corresponding block of trials are faster and more accurate than the responses in the S-R non-comesponding block. In the late 1 9 6 0 ~Richard ~ Simon and his associates (see Simon, 1990, for a 4

review) ernployed an experimental situation where spatial wding was irrelevant to the selection of the correct response. In these studies correct responses depended upon nonspatial characteristics of the stimulus (such as colour o r shape). For example, in one of the earliest dernonstrations of this effect, Simon and Small(I969) used an auditory version ofa Simon-type task where high and low tones were presented independently to the left or nght ear (thus restricting auditory stimulation to lefi and right hemispace). Subjects were to respond with the left or right key according to the tone. It did not matter whether the stimulus was presented to the Iefl or right ear. Nevertheless, corresponding

S-R pairings (e-g.,lefi key-press to a tone that signalled a left reqonse presented to the lefi ear, and a right key-press to the other tone that ngnaled a right response presented to the right ear) were faster than non-corresponding ear-hand pairings (left key-press to the

"lefi"tone presented to the right ear, and right key-press to "'right" tone presented to the lefi ear). Simon and Rudeil (1967) attributed the faster conesponding responses to " a

strong natwal tendency t o associate right-ear stimulation with right-hand response and lefi-ear stimulation to lefi-hand response" (p.303). In other words, the Simon effect reflects an orienting bias where one nanirally responds in the direction of the stimulus. The cornparison between spatial compatibility and Simon effects raises the question of whether they are mily unique phenomena. It seerns reasonable to suspect that the spatial cornpatibility and Simon tasks

rnanhal similar cognitive processes. O'Leas, and Barber (1993) noted three experimental factors that distinguish the spatial cornpatibility effect fiom the Simon effect. First, the Simon effect uses a stimulus attribute (e-g., a colour, word, shape, etc.) that does not match the response attribute (e.g., a locarion)'. There is nothing inherent in 5

the colour '%luey7 or a square that wuld relate to a spatial location. Spatial compatibility, on the other hanci, depends on the spatial mapping between stimulus and response. For example, the EüGHT hand does relate to the RIGHT spatial position of the target. Second, the spatial location of the stimulus is exûaneous information for a Simon-type task (i.e., it is task irelevant), but it is relevant for a spatial compatibility task. The Simon task requires abjects to respond according to the stimulus properties (e-g., "press the button with the right hand when you see a blue circle, but press the button with the

left hand for the r d circle"). The spatial compatibility task requûes subjects to base their response solely on the spatial location of the stimulus (e-g.,"press the right button for the stimulus on the right and the lefi button for the hmulus on the lefl"). Finally, S-R corresponding and non-corresponding painngs are mixed randomly within a given block of trials for the Simon task. The spatial wmpatibility task, on the other han& separates corresponding and non-corresponding trials in a given b lock. Kornblum and his colleagues (1990, 1994) have proposeci a taxonomy and a mode1 of S-R compatibility. In their account, the relationship (Le., similarity, or dimensional overlap) between stimulus and response (defined as relevant or imelevant to task demands) constitutes the distinction between certain kinds of compatibility effects. In their words dimensional overlap is the "degree to which a stimulus set and a response set, or two or more aspects of a stimulus set or a response set, are perceptually,

stnicturally, or conceptually simila?' (Kornblum & Lee, 1995, p.875). Their account of compatibility niggests that processing occurs in two, functionally separate, modules. The stimulus identification module produces a stimulus vector (that consias of al1 the

percephial qualities of the stimulus) that is eventuaily passed dong to the response 6

module. The response module has an automatic response activation route and a response identification route. In Kornblum's (1994) taxonomy of S-R compatibility, the Simon effect is a Type 3 ensemble and spatial cornpatibility effect is a Type 2 ensemble. The spatial attribute of the stimulus is task irrelevant and relevant in Type 3 and 4 ensembles,

respectively. The mode1 presumes that Type 3 ensembles require the response identification route, whereas the spatial compatibility paradigm does not. Thus, according to Komblum's mode1 spatial compatibility and Simon effeas appear to be different f o m of S-Rcompatibility. The spatial Stroop task is very similar to the Simon task; the oniy difference is that the stimulus is a directional word (e.g., 'ZEFT"or 'WRIGHT') and it is informative rather than uninformative. Lu and Proctor ( 1999, however, suggested that this distinction is rather unimportant; they emphasised converging similarities from empirical findings in both sets of literature. However, considenng that the identity of the aimulus inherently holds relevant information one could argue that the spatial Stroop task possesses stimulus-stimulus (S-S) congruence coexisting with

S-R compatibility. in

Komblum's (1994) taxonomy, the spatial Stroop task is a Type 7 ensemble. Does S-S congruence influence S-R compatibility? O'Leary and Barber (1993) compareci the spatial Stroop and Simon effects with similar displays of "LEFT" and C?UGHT"(vocal responses were employed for the spatial Stmop task, and manual responses were used for the Simon task) at various distances from fixation. The Stroop task required subjects to respond to the position of the word, regardless of its meaning. The Simon ta& on the other han& required subjects to respond to the meaning of the printed worci, rather than its position. Since the 7

magnitudes of the Simon and spatial Stroop effects were comparable (24 and 23 rns, respectively), and neither effect was significantly influenceci by the distance of the stimulus from fixation, the authors wnciuded that Ymilar cognitive processes underlie these tasks. The spatial Stroop and Simon effects appear to share similar cognitive resources. The influence of the S-S congniity (i.e., the association between the form and the spatial position of the stimulus) does not appear to influence S-R compatibility (but see Hommel, 1997a).

As an experimental phenomenon, the Simon effect is quite robust under a variety of experimental manipulations. ïhe theoretical accounts of the effect, however, are diverse. There has been a recent flood of experimental studies on the Simon effect. In 1994, Psycholoaical Research dedicated a whole issue to this topic. It is believed that

this line of research holds much promise in terms of understanding the role of spatial processing in response selection. Aithough much work in experimental psychology has been wnducted on spatial attention and response generation, little work has focused on their interaction. The Simon paradigrn provides the oppominity to explore the interaction between spatial and response qualities. The following is a bnef review of some of the foremost explanations of the Simon effect (see Lu & Proctor, 1995, for a review of the Simon and spatial Stroop effects).

The Stimufus-Stimulus or Perce~tud-IdcntifmtionAccount

Hasbrouq and Guiard (1991) have attempted to explain the Simon effect in terms 8

of stimulus identification congruence (Le., S S congruence). Since a colour is assigned a spatial meaning by experimental instructions (e-g., red signais lefi, blue signals right),

and because it is placed in either a congruent (e-g., left) or incongruent (e-g., nght) spatial location, the stimulus meanings are either intrinsicaily congruent or incongruent. This account assumes that the meaning and location codes are compared during early stages of information processing, specificaily at stimulus presentation When both stimulus meaning and stimulus location correspond, the RTs are faner than when they provide conflicting information. The "typical" Simon task usually has a 1 : 1 correspondence berneen stimuius meaning and response selection so rhat one cannot dissociate a response selection account (which will be disnissed shortly) fiom a stimulus identification account. Hasbroucq and Guiard ( 199 1) tested their proposal in a task where the spatial meaning (Le., colour) of the stimuli changed across trials. The lateraiized stimulus (Le., a colour) and the response keys appeared in one of two colours (red or green, determineci randomly). Subjects, presumably, were unable to pre-program a response code before each trial since they did not know which key would possess which response code. Subjects were instructed to press the key below the stimulus light with the same colour. No Simon effect was observed. Consequently, Hasbroucq and Guiard ( 199 1) proposed that the Simon effect occurs before response selection, that is, during

stimulus processing. The results of Hasbroucq & Guiard (1991) have corne under empirical scrutiny. Homme1 (1995; Experiment 4) replicated the Hasbroucq and Guiard (1991) study, with a larger sarnple of subjects, and obtained similar results. Further, Homme1 asked his subjects to explain their task strategies. The majority of subjects contendeci that they 9

perceived an imaginary diagonal line between stimulus and response key during the spatially non-corresponding trials. This imaginary '%ne7'was straight in the spatially corresponding trials. In other words, subjects perceived the display as some sort of global figure, rather than one with Ieft and right distinct compownts. Homme1 ( 1995; Experiment 5) slightiy modified the Hasbrouq and Guiard ( 1991) task

nich that a black foil divided the display globally as two salient columns (left

and right). With this slight modification, a Simon efféct appeared! The black foil used in this experimem presumably prevented subjects âom easily employing the percephial

strategy mentioned earlier in the direct replication of Hasbroucq and Guiard (1991). This result provides mong evidence againa the stimulus identification model. Consequently,

many researchers have given credence to the traditional response-selection model of the

Simon effect (dirussed shortiy).

Bernis~hericModels and Interhernis~hericTransfer Time

As already mentioned, the earliest interpretation of the Simon effect presumed it to be an automatic orientation to the location of the stimulus (Simon & Rudel4 1967). Another interpretation suggests that some S-Rcompatibility effects (e-g., Simon and spatial compatibility effects) confound neuroanatornical co~ectivÏty:corresponding S-R pairs are restricted to a single cerebd hemisphere, while non-corresponding S-R pairs

require processing across the cerebrai hemispheres. Hence, an important digression is required in this discussion. Can the Simon effect be explained simply in terms of 10

interhernisphenc transfer tirne? Poffenberger ( 1912) recordeci simple-RT to visual stimuli presented to the lefi and right of fixation. The underlying prernise for his shidy was rooted in knowledge of nwoanatomy: a stimulus presented to one hemifiefd is singly projected to the contralateral hemisphere. Likewise, the motor output is sent Ekom a particular cerebral hemisphere to the connalateral hand. When one rnakes a unimanual response to stimuli presented to the left or right visual field, a very small RT hemifield difference (2-3ms) is observed. This RT difference (interhemispheric transfer time: EUT)was taken to be the time for neural impulses to cross hemispheric-space via the fibres of the corpus calIosum. Subsequent snidies (e-g., Bashore, 1981) have supporteci this early finding. The three millisecond iHïT has typically been reported as statistically nonsignificant (e-g.,Braun, Villeneuve, & Achim, 1996;Clarke & Zaidei, 1989). Furthemore, some studies have failed to find the 3 millisecond iHTT (e.g., Peters, 1983) and other studies using goho-go and choice-RT procedures have found MTTs as large as 30 milliseconds (see Bashore, 1990). As Bashore ( 1990) points out, the goho-go

procedure does not produce the expected difference because of additional cognitive resources that increase RT differences. The use of choice-RT procedures also employs additional, although presumably different, cognitive resources. According to Bashore ( 1WO), the measurement of iHïT is only possible with simple-RT procedures.

Peters (1983) compared tadile stimuli presented to the Iefi venus right hand. in

the first three experiments, subjects knew which hand was to receive the stimulus. No signi ficant differences emerged between ipsilateral and contralateral S-Rconditions.

ï h i s finding was atrributed to subjects allocating attention to the responding hand (an4 possibility, 'Tgnoring" the non-responding hand). When subjects did not know which

hand to respond with before the stirnuii were presented (EXpenment 4)- a spatial compatibility effect occurred where ipsilateral responses were significantly faster than contralaterai responses. Peters ( 1983) amibuted this result to differences in attention allocation and interhemispheric transfer, as one did not necessarily rule out the other. Apparent 1y, pre- programming may focus attention exclusive1y t O one response or another, but similar attentionai rnechanisms cannot be directed to both responses simultaneousl y. Rubichi, Nicoletti, h i , and Umilta (1997) pointed out that the Simon effect is

ofien greater in right hernispace than in lefi hemispace. These authors suggested that this asymmetry might relate to the findings in the hemispatial neglect literanire. Hemispatial neglect is a neuropsychological phenornenon where patients appear to ignore one side of

space. The right hemisphere directs atrentional processes t o both hemifields, but the left hemisphere directs attention only to the right hernifield (Verfaellie & Heilman, 1990). This view coincides with the observation that damage to either hemisphere may result in spatial neglect, but patients with damage to the right hemisphere manifest greater deficits (Heiiman, Watsoq & Valennein, 1993). Individuals with an intact corpus callosum, presumably, have attentional syaems that work in an integrated, synergistic fashion (although the

of labour" might be asymrneaic). This synergistic pattem is

likely responsible for similar simple-RTs to left and right hemifields with lefi and right hands. The allocation of attention is distributed in choice-RT paradigms since it is impossible to pre-program a response. According to this perspective, split-brain patients 12

cannot synthesize the attentional processes evident in the disconnected hemispheres. The hemisphenc-attentional mode4 however, is not without its weaknesses. tfthe hands are crossed (i-e., left hand in right space and nght hand in lefi space), one wght to expect a Simon (or spatial compatibility) effect relative to the position of the hands. The right hand (in left space) should be Eister when responding to right, as opposed to lefi,

stimuli locations. This, however, is not the case. Although RTs are longer for crossed (e.g., right hand in left space) versus uncrosseci (right-hand in nght space) conditions, the

Simon effect occurs relative to response location rather than response hand (Hommel, 1993%Wallace, 1971). Furthemore, the Simon effect is present (and slightly enhancesi) when the choice-RT is between fingers on the same hand (Buckolz, O'Do~ell,& McAuliffe, 1996). Such demonstrations suggea that response (or effector) Iocations influence the appearance of the Simon effect. Hence, it is possibie that attentional processes will ovemde any consideration of inherent neural architecture. Homme1 (1 993a) used a two-tone, auditoiy Simon task where subjects made manual responses contingent upon the tone. The response of the subject generated an ipsilateral or contralateral (relative to response hand) light stimulus. Several combinations of left-right light, tone, and hands (crosseci-uncrossed) were usd. The question was whether the Simon effect occurred relative to the effector (anatomical position of the response) or to the effect (e.g., the light) of the response. Simon effects were found when subjects had to respond appropriately to the respective tone. This was evident even when the light-key press relationship was inverted (e-g., lefl press tumed on the right light). The Simon eRea reversed with the incompatible response-light pairing

(i.e., a left response triggered the right light, and the right response triggered the 1eA 13

light), but only when subjects were instnicted to match the light to the tone. Furthemore, when the hands were placed in a crossed position, a similar pattern of results was obtained wch that the Simon effect occurred relative to the position of the light rather

than the response hand. ui other words, the Simon effect occurred relative to the response goal (or "action code," Hommel, 199%) rather than response location. Another strike against a "purely" hemispheric model of the Simon effect was the finding that the Simon effect can be found even when al! stimuli are presented to the left or right hemifield. Umiltà and Liotti ( 1987; Expriment 3) presented stimuli to the lefi and right hernifields (aithough subjects rnaintained fixation upon a centred cross, they were free to shif? covert attention to different spatial locations). In each hemifield, however, there were two alternative spatial positions (lefi and right, relative to each other). Manual responses were signalled by stimulus shape (rectangle versus square). The crucial finding was that the Simon effect occurred relative to spatial location, not

hemifield. Thus, the hemispheric-attention model of Verfaellie and Heilman ( 19%) is presently an insuficient account of the Simon and spatial compatibility effects. Moreover, it fiils to make any predictions where S-R compatibility effects appear to reverse (Hedge & Marsh, 1975; Wallace, 1971) with attentional manipulations. As Lu

and Proctor (1995) pointed out, 'Yhese [hemisphenc,] attentional orienting accounts are not given much credence currently because ad hoc assumptions are required to explain the fact that the Simon effect can be obtained when the relative leWright stimulus andlor response locations are in the sarne hemispace" (p. 180).

The interhemisphenc vansfer models cannot account for al1 of empincal findings on the Simon effe*. The cerebral hernispheres of normal individuals typically operate in

a synergistic pattern so as to produce goal-directed, unified behaviour. Current evidence has largely favoured response-selection hypotheses.

Many response-selection models presume that response cornpetition (or facilitation) occurs between responses that are generated fiom the identity (task relevant) and spatial (task irrelevant) codes of the stimulus. When the stimulus and response

correspond spatially, the response codes are compatible for the irrelevant (location) and relevant (identity) stimulus properties and any competition is absent, responses might even be facilitated (see Simon, 1990 for a review of facilitation versus interference hypotheses). Cornpetition occurs for spatially non-corresponding trials because the response codes for the irrelevant spatial position and relevant stimulus property conflict. This competition slows RTs and, in some of the more complex Simon tasks, error rates increase. It is on the details of the response selection account that most researchers disagree. A spatial coding account of the Simon effea (Umilta & Liotti, 1987; Wallace,

1971) suggens that two spatial codes are formed upon stimulus presentation. One spatial

code links the Urelevant stimulus location with a comesponding response code while the other spatial code links the relevant, non-spatial stimulus identity to the appropriate 15

response code. The onset spatial code is before the onset of the response code. The Umiltà and Liotti (1987) study, described earlier, supports the proposition that spatial coding can be made allocentncally (relative spatial codes) or egocentrically (location detedned by observer). In addition, Homme1 (1993a) poimed out that the spatial coding is made with respect to the intended action goal, not the response. Hence, it is both the stimulus and the response that are spatiaily codeci. A study by Hedge and Marsh (1975), however, posai difficulties for many

response-selection spatial coding theones (and, for that matter, the perceptual identification theory too) of the Simon effkct. The task is iilustrated in Figure 1. Hedge

and Manh (1975) used a Sirnon-type task where the response keys were pemanently coloured red and green (depicteci as shaded and white in Figure 1). In the 'hormal" Simon task, subjects pressed the key that correspondeci to the colour of the stimulus. The

RTs for the S-R non-corresponding trials were slower than the spatially corresponding trials (the Simon effect). In another task (the alternate colour task), subjects pressed the

key with the opposite coiour to that of the stimulus (e-g., red stimulus with green key). They found that the Simon effect reversed. That is, t d s with non-corresponding handlocation pairings were responded to faster than trials with corresponding pairs. The hypothesis that the Simon effect was the result of a natural tendency to respond in the direction of the stimulus (e-g., Simon & Rudell, 1967) could no longer hold tme, without fiirther modification.

Hedge and Manh (1 975) interpreted their findings in tenns of logical r d i n g . A

logical recoding rule was applied to the irrelevant spatial and the relevant non-spatial stimulus attribute (colour). When subjects responded in the reverse Simon task, they 16

needed to press the key with the opposite colour to the stimulus. This reversai rule, however, was inadvertentiy applied to the spatial location code, even though thîs stimulus

dimension was task irrelevant. Thus, a new response code for the newly produced spatial location code was crested (because of recoding) and responses were faster to noncorresponding, rather than corresponding spatial locations. Simon and his colleagues (Simon, Sly, and Vilapakkam, 1981;O'Leary, Barber, & Simon., 1994) have offered an alternative account of Hedge and Marsh's ( 1975)

findiriiys. They argued that the reverse Simon effect is not really Simon effect at d l . Specifically, they argued that the reversa1 occurred fiom display-control arrangement

correspondence. As seen in Figure 1 , the conditions with the faster responses are those where the stimuius and response panels match. Note that the shaded stimuli on the display and the response panels are aligned for the corresponding condition of the Simon effect and the non-corresponding condition of the reverse Simon effect. Simon and

colleagues have argued that this reverse Simon effect is not the result of spatial compatibility (hence, it is not a Simon effect). kend and Wandamacher (1987) employed a version of the Hedge and Marsh (1975) task with triangles and squares assigned to the lefi and right responses (assigrnent was counterbalanced between subjects). On sorne trials the figures contained a srnall dot. Subjects were instructed to press the key opposite to that indicated by the shape (e.g., a square instructed subjects to

press the right key, but a square with a dot in it insmicted subjects to press the left key). In this task, it seems, it is hard to argue for an intluence of displayantrol arrangement. Nonetheless, a Simon and reverse Simon effect were indeed obtained.

The referential-codina hv~othesisStates that the Simon effect occurs fiom some 17

reference point, like a fixation stimulus, relative to the imperative stimulus. Hmce, a target may be codeci as "IeW7or "right" relative to sorne reference point. Homme1 ( 1993c) conducteci a test of the referential-coding hypothesis such that dong with the

imperative stimulus, a non-idonnative stimulus was presented. The Simon effkct occurred, but it was relative to the non-informative stimulus. In another study, Proctor

and Lu (1994) used bilataal presentation of an imperative stimulus with a noninformative stimulus. The referential account supposes that the use of a non-informative distractor should not influence the magnitude of the Simon effect. The magnitude of the Simon effect, however, increased with a non-informative distractor, but not when the distractor and imperative stimulus were presented in dinerent colours. Hence, these results run counter to the predictions of the referential-coding account. Homme1 ( 1993c) did point out, however, that the predictions of the referential-coding account are not precise as one cannot predict the source of the reference (e.g., stimuli on a computer screen, the computer monitor, a room, and so forth). Such imprecision damages the referential-coding account. Another account, however, has been gaining in populafity.

The attention-shifting account of the Simon effect has becorne increasing popular (cf. Stoffer & Umiltà, 1997). Generally, these attentional mdels focus on understanding how the spatial code is created. It is thought that the spatial code fonns as a result of attention shifting to the location of the stimulus from fixation (Rubichi et al., 1997). Rubichi et al. (1997) have pimecl this mode1 to the "premotor theory of attention" (Riwlatti, Riggio, & Sheliga, 1994). The premotor theory holds that spatial aîîention is a produa of neural functioning within spatial, pragmatic maps (Le., attention is not a module in the brain, per se). The goaldirected action is intimately related to these spatial 18

maps. ï h e sudden onset of a lateralized stimulus causes a shifi of attention to its location. Shifting attention creates a saccadic motor program. The rnotor pmgram contains, arnongst other things, the direction of the stimulus (Le., how to get eyes on target). Responses are facilitated towards the direction of the attention-shift owing to the generation of the motor pmgram (Rubichi et al., 1997). In support of the attention-shifting account, Nicoletti and Umilta (1 994) conducted a task (Expenment 1) where subjects fixated on a certain point, but paid (covert) attention to a distinct marker (Le., away from fixation). The Simon effect

occurred relative to the marker, but not to fixation. M e n the task was modified such that subjects had to pay attention to a golna-go cue presented below fixation upon stimulus

onset, no Simon effect was present (Experiment 2). When a time interval was placed between the golno-go cue and the targec a Simon effect emerged (Expenment 3). Rubichi et al. (1997) used a similar procedure and drew comparable conclusions. When attention shified back to fixation (with the use of golno-go posicues) d e r response selection, a reverse Simon effect emerged (Rubichi et al., 1997). Thus, it appears that shifts of attention rnay influence the direction of the Simon effect. Verfaellie and Heilman (1990)found a Simon effect for both intentional (cue indicated response hand) and attentional (cue indicated target location) central precues. The terms attentional and intentional refer to processes that control stimulus detection and action preparation, respectively. Proctor, Lu, and Van Zandt (1992) replicated an

earlier study by Verfaellie, Bowen, and Heilman (1988; see also Verfaellie & Heilman, 1990). hecues predicted not only stimulus location (attentional precues) but also

respowe hand (intentionai precues). The attentional precue did not influence the Simon 19

effect; the intentional precues, on the other hand, eniarged the effect (by 17 ms to 34 ms). When the attentional precues were ernployed in isolation fiom the intentional precues, a Simon effect occurred but it was not influenced by precue validity. Invalid intentional, but not attentionai, precues reverseci the Simon effect. This finding hints at the crucial role played by intentionai aies for the preparatioa of the response. Thus, these results are in accord with the findings of Homme1 (1993a): intention plays an important role in the generation of the Simon effect. According to the attention-shifting hypothesis, however, directing aîtention to the side of the stimulus should nuIli@ the Simon effect. Nevertheless, Proctor et al. ( 1992) obtained the effect with attentional precues. Attention may be directed exoaenouslv wit h peripheral precues or endo~enously with central precues (e-g.,Jonides, 1981, 1983; Posner, 1980; Posner, Snyder, & Davidson, 1980; Rafal& Henik 1994). Endogenous shifts of attention are presumed to reflect slow, controlled, voluntary processes while exogenous shifts of attention are presumably fast and automatic. Klein ( 1994; Briand & Klein, 1987) cornpareci endogenous and exogenous precues using a choice-RT paradigm where subjects maintaineci specific expectancies about target fom. Whereas target form expectancies interacted with endogenous precues, t hey were additive in the exogenous precue condition. This result led Klein (1994) to suggest that exogenous precues influence earlier stages of perceptual processing (such as feature extraction and integration), while endogenous precues influence 1ater stages (such as those involving expectancies, decisions, or response selection). This distinction between automatic and controlled

shifls of attention utggests that Simon effeas based on exogenous and endogenous shifts of attention should differ.

Stoffer and Yakin ( 1994) tested the attention-shift hypothesis with a cenval (supposedly recniiting endogenous attention-shifts) and peripheral (supposedly recniiting exogenous attention-shifts) precue paradigrn. The Simon effecf with peripheral precues, was srnaller when a target appeared at an expected location relative t o a condition without

location precues (neutral precues). When precues were presented centrdly, the Simon

effect in a neutral condition was comparable to the Simon effect in the central precue condition at low (i.e.,50 ms) stimulus-onset asynchronies (SOAs: the interval between the precue and stimulus). At larger SOAs, the Simon effect increased for the neutral condition but decreased for the precue condition. Thus, the SOA is an important factor when one considers the influence of a precue. Thus, it appears that the Simon effect is influenced differentiaily when attention is directeci exogenously or endogenously toward the imperative stimulus. Stoffer and Yakin's (1994) study has been criticised, however, on the b a i s that precues were valid 100% of the time. Thus, subjects h e w , without uncertainty, where the stimulus would appear. This precue procedure rnay be contrasted with the typical procedure where precues are valid on most (but not d l ) trials (e.g., Posner, Snyder, & Davidson, 1980). Stoffer and Umilta ( 1997) have pointed out that using al1 valid precues is not a downfall; it actually provides a fair test of the attention-shift hypothesis. Subjects shifi their attention without concern o f "deceit." ûther researchers have disagreed with this interpretation, and they have failed to find a reduceà Simon effect with a precuing

paradigrn. Zimba and Bnto (1995) found that neither precue validity nor precue position (central or peripheral) influenced the magnitude of the Simon effect. Funhexmore, Zimba and Bnto ( 1995; Experirnent 2) found that the Simon effect was essentially absent when 21

SOAs were less than 50 ms (possibly because the stimulus spatial code may not have had time to fonn). Given these wnflicting results, it seems that fiutfier research is needed to elaborate on the role of attention in the formation of the Simon effect. Results in favour of the attention-shifting account of the Simon effect have been

used as corroborative evidence in favour of the premotor mode1 of attention. If aîtentionshifts necessarily lead to saccadic programming, however, then the Simon effect may be

an R-R (response-response) phenornenon (Weeks, Chua, & Bautista, 1997). In other words, the Simon effea may be the result of cornpatibility between preparatory (but not necessarily executed) oculomotor and (prepared and executed) manual responses. This question, as intriguing as it may be, definitely requires further consideration and is beyond the scope of this pape? So far, evidence in relation to the attention-shift hypothesis has been rnixed. The attention-shifting account, however, provides only half of the aory. The spatial location of a target only influences choice responses. Target detection, as occurs in simple-RT tasks, is not influenced by the spatial position. In the Rubichi et al. (1 997) study, a poacue (a gdno-go cue following response selection) influenced the direction of the Simon effect. If subjects have programmed their response (thus, the task is somewhat akin to a simple-RT task), why should a shifi of attention modifi the direction of the Simon effect? One may surmise that the original automatic response appears to be rather flexible, or, altemtively, it is reasonable that a second spatial code was produced, and it is oniy the second that matters (Rubichi et al., 1997). It is important to remember that the Simon effect only occurs under conditions of

response uncertainty (choice-RT, but not simple-RT îasks). When subjects have the oppomrnity to program a particular response before presentation of the target (hence, the

task becomes akin to one with simple-RT), it is reasonable to expect the absence of the Simon effect. This, however, is not the case (e-g, Proctor et al., 1992), as the magnitude

of the Simon e f k t is ofien increased with this manipulation. The explanation of this curious finding requires a model that considers cognitive motor factors. The final model reviewed is the action-concept (Hommel, 1997b), or goddirecteci (Pnw 1997), account of S-R compatibility. The action-concept model supplements, rather than replaces, current theories of S-Rcompatibility and the Simon effect (Proctor, 1997). Thus Far, the accounts of the Simon effect have concentrated on those cognitive processes responsible for the processing of the stimulus. Equally

i mponant, however, are the response codes (Wallace, 197 1). At least three experirnents have directly exarnined the role of response codes in the Simon effect. The fist was Hommel's (1993a) expenment, previously mentioned, where lateralized responses were joined to ipsilateral or contralateral flashes of light. The finding of importance was that the Simon effect would reverse when subjects were instructed to match the light to the imperative stimulus (a tone) with responses yoked to a contratateral visual stimulus. Ln other words, it was not the response that was important but the goal. In a similar vein, Guiard (1983) used an apparatus with a steering wheel

(with both hands on the bottom portion of the wheel). Responses were contingent upon auditory stimuli presented to the lefl and right ear. ï h e steering wheel caused a cursor on

a screen to move lefi or right with a counter-clockwise and a clockwise movernent of the wheel, respectively. Under this condition, the Simon effe* occurred relative to the 23

motion of the cursor (Le., the intended goal), rather than the direction of the manual movements of the nibjects' hands. With the cursor removed, however, the pattern of results was mixed: some subjects demonstrateci a regular Simon effect and others dernonstrated the reverse. It was Iikely that some subjects coded the response in terms of the hands and bottom portion of wheel rnovement (normal Simon effect) and othen coded it in ternis of the top portion of the wheel (reverse Simon effect). In the finai study (Riggio, Gawryszewski, & Umiltà, 1986)that supports an action-concept mode1 of the Simon effect, subjects made button responses with hand-held sticks. When the sticks were uncrossed, the typical compatibility effect was observed. With the sticks crossed, however, the compatibility effects emerged relative to the position of the response keys, rather than the hands. Hence, the response (or action) code is more than a code of movement direction; it is dependent on the intentions of the actor. ï h e action-code refers to the perceived, and expected, outcorne of some movement

(Hommel, 199%). The effect-code (Le., the amal overt behaviour) is often closely tied to the action-code, but they may be dissociated (as suggested by the three experiments outlined above). Another tenet of the action-concept mode1 assumes that stimulus and response sets occur within the same domain: they are not temporally separable. It was already mentioned that the Simon effect does not occur in simple-RT

experiments. When the responding hand is precued, however, so that subjects know in advance with which hand to make the response (i-e., there should be a pre-prograrnming

of response, hence it becornes a simple-RT task), one should expect a nuIl Simon effect. That, however, is not the case (Hommei, 1995; Hommel, 19%; Proctor et al., 1992; Proctor & Wang, 1997; Verfaellie & Heilman, 1990), as it is ofien observed that response 24

precues magnify the Simon effm (Proctor & Wang,1997). Hence, response certainty appears not to play a crucial role in the Simon effect. The action-concept mode1 is in accord with such a finding as "response selection is accomplished by activating the

correct response's action concept or, more precisely, those codes of the action concept that represent the relevant response feature(s)" (Hommel, 1997b, p.298). Why does the Simon effect not occur, then, with simpie-RT tasks? Homme1 (1997b)suggested two explmations to account for a lack of the Simon efF& with simple-RT ex@ rnents. Firn, the relative speed of simple-RT (compared to choice-RT) might wntnbute to a lack of wmpatibility differences. Resumably, it takes

some arnount of time for the irrelevant spatial code to corne into effect. The formation of the spatial code may occur after the features of the stimulus are processed (Treisman & Gelade, 1980). A relatively easy test of this hypothesis may be an experïrnent where subjects perform a simple-RT task but they hold off responding for a certain period (long enough for the spatial code to build). The second explanation holds that a response code will not be fomed when there is no alternative response to be coded. Homme1 (1996) conducted simple-RT experiments (response hand was blocked

as a within-subjects factor) with spatially presented stimuli. The crucial experiments showed significant hand by hemispace interactions. These interactions, although significant, were admittedy small (between 4-20 ms). in many simple RT tasks, thou& this interaction does not surface (note 4). What was interesting about these studies, however, was that the non-responding hand was placed on a response key. Although subjects knew -thatthe task involved oniy one hand, there were often hand erron (Le., the non-responding hand would often respond)! This suggests that subjects might have 25

spatidly coded the non-responding han& although it was not needed. In simple-RT tasks with the non-responding hand wmpletely out of the picture (Le., not placed on a response box), spatial compatibility does not play a significant role. Nevertheles, Homme1 (1996)

alluded to the possibility that the nonsignificant 3 4 rns advantage found in MTTs rnight result from small S-R compatibility effects rather than the tirne for information to cross

between the hemispheres, despite daims ( e g , Bashore, 1981, 1990) to the contrary.

Automatic and Cootroiled Processes

The distinction between controlled and automatic processes has an interesting history in psychology. As MacLeod ( 1991) pointed out in his review of the Stroop effect,

many accounts of automaticity proceeded from Cartell(1886). Cattell's notion was that some processes require less attention than others. In terms of a Stroop task, reading the printed word demands less attention than does narning the ink colour. Ln tems of the

Simon effect, one may presume that the position of a target in space autornaticaily activates a wrresponding response whereas the processing of the relevant response information (e-g., stimulus fom or colour) demands more aîtention or control. Recall that the dimensional overlap mode1 (Komblum, 1994; Komblum et al., 1990) assumes that responses are activated via an identification and an automatic route.

The automatic route ody plays a part when properties of the stimulus share perceptual, conceptual, or structural similarity with the properties of the response. This account presumes, however, that the task irrelevant and relevant features occur at discrete stages 26

of processing. This proposal may be contrasted with that of Hommel's ( 1993b) temporal overlap model that holds that the response and nimulus codes may overlap in varying

degrees. The magnitude of the Simon effect is directly proportional to the degree of overlap . Hommel (1997a) pitted the dimensional overlap model against the temporal overlap model with three experirnents that combined S-S congruence and S-R wmpatibility. In one experirnent, for example, responses were contingent upon a particular letter (H or S), while the spatial position of the stimulus was task irrelevant. The stimuli were flanked (to the lefl and nght) by congruent (same letter) or incongruent

(different letter) stimuli. The question was whether the congruency of the flanker interacts with the Simon effect. The dimensional overlap model predicts that the effects of S-S congruence and S-R compatibility should be additive*but not interactive (since these effects are thought to occur at distinct stages). The temporal overlap model, however, presumes that S-S and S-R effects should interact, since stimulus and response processing rnay overlap. Hommel (1997a) found that the Simon effect and the Stroop

effect combined additively, thus supponing the dimensional overlap hypothesis (Kornblum, 1994). Note that Hornmei (1997a) presumed that the Stroop effect is an S-S effect, while some authors (MacLeod, 1991) have argued that the Stroop effect is actually

an S-Reffect. Given this disagreement, it is hard to argue that piece of evidence supports one mode1 over the other. In his other experirnents, Homme1 (1997b;Experiment 2) used a flanker type task (where the imperative stimulus was surrounded by irrelevant stimuli that inherently signifi competing responses) as an S-S congnience meauire. Unlike the Stroop effect, this flanker effect significantly interacteci with the Simon effect.

Nonetheless, this locus of the flanker effect rnay reside closer to the response selection stage of information processing (Botella, 19%) since the (S-S) incongruent flankers also denote a non-corresponding response. Hence, it is still lefi to be determineci (conclusively)whether the stimulus and response codes overlap ternpodly.

The temporal overlap model, it seems, explains the magnitude of the Simon effect,but it does not explain why the automatic code forms regardless of task instructions. De Jong, Liang, and Lauber (1994) pmposed a dual-process model to explain the Simon effect and its reversal. Like the temporal overlap model, the dualprocess model presumes that the automatic and non-automatic (or less automatic) processes overlap in time. In addition, this account assumes (akin to Simon's, 1969, original hypothesis) that sudden stimulus onset immediately elicits a natural tendency to orient to its location. This tendency is automatic and unconditional (Le., not based on task instructions). It primes the spatially compatible response but dissipates rather quickly after stimulus onset. The second, conditionai (i-e.,it relies on task demands) component does not dissipate afler stimulus onset. It arises "'at the point in time when the transformation rule (identity or reversai) is applied to the relevant stimulus anribute and also, unintentionally, to the spatial stimulus code" (De Jong et al., 1994, p. 733). Thus. it is dependent upon task instructions. In agreement with Hedge and Manh (1975), De

Jong and colleagues argueci that a Iogical recoding rule seems to apply to the reversed Simon effect. They argued, however, that it "does not seem to represent a generalised-set

eRm but to arise at the point in time at which information related to the primary task is transformeci and the spatial code 'gets a piece of the action"' @e Jong et al., 1994, p. 745).

28

De Jong et al. ( 1994)tested their mode1 by separating the RTs of each subject into "bins." For example, the RTs (of each subject) were grouped into the fastest to the slowest in specified increments (in 1û??bins for Experiment 1 and 20% bins for Experiment 2). A Simon effect was aident for faster RTs, but it was absent for the slower RTs (e-g., at approximately 600 ms or slower), possibly owing to an inhibitory

mechanism. When the instructions gave incompatible rnappings (ive.,the key and stimulus were both coloured, subjects were told to press the opposite coloured key in these conditions), the reverseci Simon effect increased with sIower RTs. These resuhs fit well with De Jong et a l . 3 (1 994) prediction that the unconditional, automatic process decays with time (Hommei, 1994). The conditional rnechanism is not affected by time,

as it is time-locked to the key press. Accordingly, De Jong et al. reasoned that one should expect that the slope of the RT bins (i.e.,non-corresponding minus corresponding RT difference score versus average bin RT) would be linear for compatible and incompatible mappings. This is precisely what De Jong et al. found. Mthough the use of RT bins has not been accepted by al1 authors (Zhang & Komblum, 1997), a discussion of this rnatter is beyond the scope of this thesis. Toth, Levine, Stuss Oh, Winocur, and Meiran (1995) also tested the idea that dual processes underlie the Simon effect. Associative S a mapping (i.e.,"expectancy") is the first process that was assumed to reflect task characteristics. Thus fx,it has been assumed that the proportion of S-R non-corresponding trials to S-R corresponding trials

has been equal. The associative S-R mapping changes, however, when the proportion of non-corresponding to corresponding trials is manipulated. The idea of the nonassociative coding bias appears to be based on Simon and Rudell's (1967) idea that there 29

exists a natural tendency to respond to the source of stimulation. In addition, there are independent space- (irrelevant dimension) and form-based (relevant) processes. The degree to which these reflect the spatial-based (but not form-based) processes is different for different proporiions of corresponding versus non-corresponding trials. When there is a 1 : 1 correspondence between S-Rnon-correspondhg and corresponding trials within a block, there is little opportunity for the associative process and the non-associative process to bias subjects' responses. In the Toth et al. (1995) study, the proportion of S-R corresponding vernis non-corresponding trials was manipulateci. A reversed Simon

effect occurred when the proportion of correspondhg trials was low relative to the noncorresponding trials. The normal Simon effkt occurred when the proportion of S-R corresponding trials to non-corresponding trials was greater (or equal) to 0.5. It is possible t hat manipu1ating the ratio of corresponding to non-corresponding trials

generates an expectancy (Klein, 1994), such that nibjects corne to predict a certain response over another. Such expectancies appear to directly influence the direction of the

Simon effect. As evident fiom the nature of a Simon-task, the relevant and irrelevant dimensions of the stimulus appear to be processed almoa exclusively. Furthemore, Toth et al. (1995) separated fonn and spatially based processing (aigebraically) on the bais of

correct responses and probability equations based on a mode1 that assumes fiinctional independence between two processes. In other words, it was assumed a prion that the form-based processing and the spatial-based processing overlap marginally, but not wmpletely, so that some processing is redundant. Using this statisticai technique, the authors argued that two automatic (associative and non-associative) processes were 30

mutually exclusive of one another. Only when the proportion of spatially corresponding trials matched the proportion of spatidly nokcorresponding trials does the nonassociative naturaJ tendency to respond faster with the ipsilateral limb occur (for similar findings, see Hommel, 1994). Ahhough the theories of Toth et al. ( 1995) and De Jong et

al. ( 1994) propose interesting mechanisms that are consistent with current work (e-g., Vaile-Incl& 1W6a, 1996b; Wascher, Verleger, & Wauschkuhn, 1996) using ERP

(event-related potentiai) and LRP (lateralized readiness potential), these accounts do not explain the formation of the spatial code.

Surnmarv

The purpose of the foregoing review was to provide a hmework for understanding the general cognitive processes current1y bel ieved to operate during the Simon task. Generally, al1 accounts of the Simon effect presurne that (a) it arises at the response selection stage of information processing, (b) it is the result of simultaneously activated response codes (fiom the relevant and irrelevant dimension of the imperative stimulus) that either mutuaily correspond or mutuaily conflict. Thus far, however, only two accounts (Rubichi et al., 1997; Verfaellie & Heilman, 1990) have considerd the role of Simon effect asymmetries. Furthermore, right-handers have oflen been the sole subjects of investigation. Little attention, if any at ail, has been paid to lefi-handers (even though Verfaellie and Heilman suggested that the direction of this asymmetry may relate to handedness). Thus, a complete account of the Simon effect mua explain potential 31

individual di fferences.

ELAND PREFERENCE AND PERFORMANCE ASYMMETRIES

Given that the most likely locus of the Simon effect is at the response selection stage, its relationship to handedness is theoretically important. It has been argued that handedness differences result from an initial attentional bias (Peters, 1995). This

attentional bias presumably influences response-related processes. Recall that the attention-shifting account of the Simon efféct presumes that shifts of spatial attention generate the spatial codes of the stimulus. If left-handers shift their attention in a manner differently than right-handers, then subsequent spatial codes ought to differ. Hence, an initiai Iateral attentional bias may influence the properties of the Simon effect . On the other hand, right- and left-handers may also mentally represent spatial codes in a dissimilar fashion; hence Simon effect differences might occur owing to this difference in spatial processing. Evidence in support of the handedness differences in spatial

processing, nonetheless, has been equivocai.

S~atialAbilities and Handedness

A widespread conviction has been that lefl-handers, as a group, tend to be Iess

consistent1y lateralizd for language, compareci to nght-handers (Annett, 1985; Banich,

1997). This apparemly bilateral organisation of language, it has been argued, occurs at a

cost to the spatial capabilities of the right hemisphere. This is the cognitive crowding hypothesis (Lewis & Hams, 1990). Lewis and Harris (1990) reviewed both positive and negative reports of handedness differences on a variety of spatial tasks. A problem with the review, however, is that handedness groups may be classified in several different ways (Peters, 1990, 1995, 1998).

As Peten ( 1998) notes, there is no "gold standard" of

handedness preference and perfbrmance measures. Thus, handedness differences in spatial abilities might simply depend on how one defines the lefi-hander (or even the right-hander for that matter). Secondly, it appears that not al1 types of spatial abilities are lateralized the same way (Kossiyn et al., 1989). Kosslyn et al. (1989) have argueci that co-ordinate spatial processes (Le., Euclidean-based processes) are processed to a greater degree by the nght hemisphere and categoncai spatial relations (Le., those based on relative spatial positions) are largely processed by the left-hemisphere. Laeng and Peters ( 1995) tested the Kosslyn et al. ( 1989) hypothesis by presenting pairs of cartoon-like stimuli diainguished by categorical or co-ordinate relationships using a tachistoscope. Subjects were required to make speeded sarneldifferent judgements to the pair of stimuli. Overall, consistent lefi-handers (lefi-handers who use their left hand across a given set of preference items) had the m o a difficulty with this task compared to right-handers and inconsistent lefi-handers (lefi-handers who typically prefer their left hand for precision tasks and right hand for ballistic tasks). While righthanders demonstrateci the visual-field by spatial task interaction (lefl visual field advantage for coordinate relationships and a right visuai field advantage for categorical relationships) as predicted by Kosslp et al.'s (1 989) hypothesis, consistent left-handers 33

evidenced no interaction. Inconsistent left-handers dernonstrateci a co-ordinate advantage, over categoncal relations, for the right hemifieid only. These results imply that different Ends of spatial relations are processed differently across handedness groups.

The Role of Manuai Exmrience and Tareet Hemu~acein Handedness

There remains the possibility that left-hariders may code space differently than right-handers (as considered above). This differential coding rnay influence spatially directed responses. The second cognitive component, response selection, rnay also give nse to handedness differences on this task.

It can be safely presumed (for everyday, ecological situations) that the hand selected depends on the task to be performed. Manuai preference for highly skilled activities, such as witing, will be chosen on the basis of past experience. Under some circumstances, sume activities demand a particular limb to be used. Peters and Ivanoff (in press) compared the performance profiles of left- and right-hand computer mouse users on a cornputer mouse task. Subjects were required to use a mouse, with their preferred and non-preferred hands, to c i i r a a cursor to a stimulus presented on the

screen. Generally, the left-handers with right hand computer mouse exprieme performed similady to right-handers, while left-handers with left hand mouse experience did not. Since the task required that, for a given set of trials, only lefi or nght-hand responses needed to be prepared and directed towards a particular target, this experiment 34

was not a Simon-type task. ïhus, no response selection process was required; there was no influence of S-R compatibility effects. Right-handen, as a group, tend to be rather consistent with regards to their hand preferences (Peters & Murphy, 1993) for mon items used in handedness questionnaires (e-g.,throwing, drawing, hammering a nail, and so on). For some items (e-g.,"picking up a small abject"), however, responses tend to be rather inconsistem. These 'Cnconsistent"

items are typically unskilled, and they do not matter in terms of theü wnsquences (Peters, 1995). In addition, the within handedness group deviations may occur because

subjects rnay envision the object-to-be-rnanipulated(e.g., a coin) in either lefi or nght hemispace. Subjects may also envision their dominant hand engaged in another role (e.g., holding a pen). If one drops a coin,and it falls to one's lefi side, it rnakes more

sense to pick it up with the ipsilateral hand rather than change one's body orientation so that the "preferred" hand may perfonn the task. This raises an interesting question. At

whar point, dong a lefi-right visual continuum, would people switch hands to make ipsilateral or contralateral reaches?

Bryden et al. ( 1 994) compared the questionnaire answers of lefi- and nghthanders to their scores on the long pegboard. The long pegboard is an elongated piece of wood with 15 pairs of alternating small and large holes. Two pegs (smdl and large size)

are initially placed to the far lefi or right of the pegboard. Subjects were required to leapfrog the pegs, one over the other, dong the board using the hand ipsilateral to the initial position of the pegs. Subjects were instructed that they could switch hands when they felt it appropriate to do so. When subjects were classifieci according to "skilled"

activities (e.g., writing, throwing, and so on; Steenhuis & Bryden , 1989) a handedness 35

difference emerged oniy when the left hand initiateci the movement of the pegs from lefi to right (no significant difference when starting with the nght hand and moving to the left). Right-handers switched at the 13& (M=12.92) peg while lefi-handers switched at about the 15" &f=15.35) peg. Assuming that the pegboard was symmehically placed along subjects' midline, it is interesthg to note that lefi- and right-handed subjects did not differ according to the point that they switched hands when moving into left hemispace. Another study by Bishop et al. (1 996) compareci three groups of nght-handers (strong, w&

and predominant) on a reaching task. The aibjects were required to pick

up a card (the target) placed along a hemi-circle from left to right hemispace, and put it into a bin (placed at the midline). The predominant right-handers (nght-handers who responded with at least one lefi-hand answer on their handedness preference questionnaire) used their right hand significantly less across the hernispace than the weak or nrong right-handers did. Bishop et al. ( 1996) suggested that the between group differences reflected a greater "readiness to reach across the midline with the preferred hand" for the strong (or weak) nght-handers relative to the predominant right-handers (p. 284).

Gabbard et al. ( 1997)teaed lefi- and right-handers on an open reaching task. Subjects were blindfolded while a foam cube was placed within the left or right hemispace. Subjects then removed their blindfold and kept their eyes shut until an experirnenter gave the signal to "go" (Le., reach for the cube). Subjects opened their eyes, picked up the cube, and placed it in a box at theû midline. At the midline (i.e., the

cube was placed directly in fiont of subject), 95% of right-handers used their nght hand 36

while 75% of left-handers used their lefi hand. More interestingly, lefbhanded subjects were more likely to use their right hand in right space than right-handers using their left hand in Iefl space. According to Gabbard et al. (1997), the data support the proposition that left-handm are not as lateralized as right-handers are, but, in situations such as t hese,

the "less lateralized" left-handers experience an "advantage." When faced with a novel, yet unskilled, task, how does one decide which hand to

use? Provins (1997) has argued that the between hand performance differences on skilled tasks (e-g.,writing) that are well practised with one hand simply reflect the fact that one hand has more task experience than does the other. Since unskilled ta&

however, do

not have a learned component, between hand differences ought not to exist. Given that different handedness patterns have emerged on unskilled tasks, such as finger tapping (Peters, 1996; Peters & Servos, 1989), that are largely unpractised, it is questionable whether Provins' ( 1997) argument holds for every unpractised task. The reaching tasks are, by far, highly practised simple tasks. Asymrnetric patterns, as reflected in the tasks of Gabbard et al. (1997), Bishop et al. (1996),and Bryden et ai. ( 1994) rnight reflect an asymmetry within some system. They cannot, however, testif) as to the locus of the asymmetry. The greater tendency of the (strong) nght-handers to reach into left space with the right hand may owe to structural asymmetnes of the muscles that coopenite in left versus right reaches (Peters, personal communication). Alternative1y, the locus of this effect may occur as a result of a higher "decision-bias7' to make reiatively more fiequent right hand reaches in lefl space cornpared to lefi hand reaches in right space, a s Bishop et al. (1 996) believes. A Simontype task appears to be similar to these behavioural measures of handedness except that

37

subjects7responses are cunstrained by tasic instructions, and subjects do not make reaches across the midline. Furthemore, it is the influence of the spatial position of the stimulus that influences response selection in both paradigms that is theoretically interesting with reference to handedness in both paradigms, possibly for different reasons though.

TEE ROLE OF HANDEDNESS AND CONTROLLED SPATIAL ATfENTION IN THE SIMON EFFECT

The Simon task is peculiar in the sense that one may consider it both as bimanual and unirnanual. It is unirnanual in terms of rnovernent execution; however, it is bimanual before response seledon, as either hand must be "ready for action." In natural bimanual and unimanual situations, the hands have an a priori assigned role (Peters, 1995). When one is about to wrïte, it is the writing hand that is selected. When one is about to harnmer a naii, the lefl hand (of right-handers) initially holds the nail in place while the right hand uses the harnmer to tap the nail into the wood. Undoubtedly, these roles have been

assigned to their respective hands due to pnor task experience, an inherent lateral bias for skilled movements, and their interaction. It was mentioned early in the introduction to this paper that an ideal handedness

performance task is one that is not overly practised. Moreover, a task that is resistant to practice is even more desirable. Given that spatial compatibility effects are practice resistant ( M a & Proctor, 1992; lacoboni, Woods, & Mazziotta, 1996), a Simon-type

task becomes ideaily suited to examine handedness differences. These differences are 38

likely due to femres of relatively hardwired underlying neural architecture?r;nher than the result of an asymmetric experience differences between the hands (Peters, 1998;

Peters & Ivanoff, in press). The Simon task is "unfamiliar" because (a) people typically don? engage in simple, rapid key-presses to target stimuli (even contemporary video games are more sophisticated than this), and (b) there are few natural situations where people do not

know a priori which hand to use in a given situation (reaching to dropped items is one of few). Those situations where one has not selected an effector are those where hemispace, or some other variable, becorne the deciding factor. One may pick up a dropped pen with the hand closest to the pen (i.e.,the goal). Or, the hand used may be chosen by convenience. If one is holding groceries with the dominant hand, it is the %ee7" nondominant hand that would likely be used to pick-up fdlen coins. The studies by Bishop et al., ( 1996),Bryden et al. ( 1994), and Gabbard et al., ( 1997)examined the influence of hemispace and hand preference in response selection simply by noting which hand nibjects chose to use. One might take this investigation further by examining the performance profiles of different handedness groups on a Simon-type task. Specifically, different patterns of performance ought to exist for different handedness goups in a rnanner that corresponds to the aforementioned studies. Virtually ail studies of the Simon effect have been conducted on nght-handers. Some studies (e.g., h d o r et al., 1992) do not even specify the handedness of their abjects. Given that right -handers show different performance profiles from lefi-handers across different tasks, how do different handedness groups compare on the Simon task?

How does this nahual tendency to respond towards the location of the stimulus depend on 39

handedness? R e d 1 the study of Simon and Rudeil (1 %7), mentioned earlier, where wbjects (left- and right-handers) made key-press responses in response to mmmands issued t o the lefi and right ear. Right-handers responded fasts to the "'right" cornmand (with nght hand) and lefi-handers responded faster to the "left" one (with lefl hand). No other handedness difference emerged (i. e., the overall Simon e f f ' was quivalent for lefi- and right-handers). Handedness classificatio~however, was based on the subject's writing

hand. Given that subgroups of lefi-handers (and possibly right-handen, Bishop et al., 1996) demonstrate different preference and performance patterns on particular tasks (e.g.,

Peters, 1996; Peters & Servos, 1989), it seerns reasonable to suggest that subtle asymmetric patterns might have been hidden due to insufficient handedness classification. Ladavas (1987) compared eight right-handers and eight lefi-handers on a task where stimuli were presented above and below the line o f fixation. The hands were

placed in their respective hemispace. In both lefi-handen and right-handers, the respective dominant-hand responded to the top stimuli faster than the lower stimuli. The opposite patterns of results were s h o w for the non-dominant hand. ïhat is3the nondominant hands, of left-handers and right-handers, responded to the lower stimuli faster than the higher stimuli. Ladavas (1987) interpreted the resuits in ternis of the typical division of labour affiorded to the hands during bimanual tasks. The dominant hand, that p e r f o m the object manipulation, is typicaily higher than the nondominant hand that is perfoming a supporting role. Subjects may have adopted, however, a strategy where they mentaily rotated the stimulus and response arrays so that they correspond. Given 40

that right-handers appear to prefer rightward (or clockwise) mental rotations (Heil, Bajnc, Rosler, & Hennighausen, 1997), one might remnably speculate that lefl-handers prefer the opposite (hence, l e h a r d rotation). Although this explanation demands empirical validity, it c m o t necessarily be ruled out. At this point it is important to explain what c o n s t i ~ e as Simon effect. R e d , that the Simon effect has been descnbed as the RT (or accunicy) advantage of S-R pairs that spatially correspond compareci to those that do not. Consider, for example, a Simontype experiment where responses are contingent upon the orientation of a stimulus (e.g-, a

rightward arrowhead, ">", signals a key-press with the right han& while a leftward arrowhead, "