Hemispheric differences in stop task performance

Hemispheric differences in stop task performance

Menno Van der Schoot *, Robert Licht, Tako M. Horsley,Joseph A. Sergeant

Department of Clinical Neuropsychology, Vrije Universiteit, Van der Boechorststraat 1,

1081 BT Amsterdam, The Netherlands

Accepted 23 October 2002

Abstract

This study examined hemispheric specialization for stop task performance. It was found

that inhibitory performance was better for stop signals presented in the right visual field. This

result provided support for the hypothesis that, during stop task performance, subjects call

upon the left-lateralized neural system that is involved in active attention. It was suggested

that a stop task requires such a mode of attention because subjects maintain a tonic readiness

for inhibitory action while being engaged in the stop task�s go routine. Subjects are continu-

ously alert for possible stop signals while discriminating between go stimuli. The stop task may

be considered a typical activation task.

� 2002 Elsevier Science B.V. All rights reserved.

PsycINFO classification: 2330; 2346

Keywords: Stop task; Motor inhibition; Hemispheric differences; Attention

1. Introduction

The motor inhibition process has been studied using the stop signal paradigm

(Logan & Cowan, 1984), in which subjects perform a primary choice reaction time(RT) task and are occasionally presented with a stop signal that instructs them

to suppress the response. Logan and Cowan�s model accounts for response inhibi-

tion in terms of a �horse race� between the go process (triggered by the primary

task stimulus) and the stop process (triggered by the sudden presentation of the stop

Acta Psychologica 112 (2003) 279–295

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*Corresponding author. Tel.: +31-20-4448908.

E-mail address: [email protected] (M. Van der Schoot).

0001-6918/03/$ - see front matter � 2002 Elsevier Science B.V. All rights reserved.

PII: S0001-6918 (02 )00133-6

stimulus). The subject succeeds in suppressing his/her response if the stop process fin-

ishes before the go process.

The concept of inhibition has been the subject of intense interest in recent years.

Since the race model provides a powerful method for comparing inhibitory compe-

tence in different conditions, tasks or subject groups, the stop signal paradigm hasbeen used in a variety of studies. Especially within the fields of developmental and

clinical psychology, the inhibition function and stop signal reaction time (SSRT) –

the two main dependent variables in the stop signal paradigm – have proved to be

valid diagnostic tools for establishing inhibitory deficits. Research in the former do-

main tries to describe and explain differences in cognitive abilities between young and

old subjects in terms of age-related changes in inhibitory capacity (e.g., Bedard et al.,

2002; Kramer, Humphrey, Larish, Logan, & Strayer, 1994; Ridderinkhof, Band, &

Logan, 1999; Williams, Ponesse, Schachar, Logan, & Tannock, 1999). The latterdomain of investigation aims at uncovering the relation between a variety of psycho-

pathological (childhood) disorders – in particular that of attention deficit hyper-

activity disorder – and deficiencies in inhibitory control (e.g., Logan, Schachar,

& Tannock, 1997; Overtoom et al., 2002; Schachar & Logan, 1990).

In order to investigate the nature of the stop process, a number of studies have

also focused on within-subject factors affecting the SSRT (e.g., Kramer et al.,

1994; Ridderinkhof et al., 1999; Van den Wildenberg, Van der Molen, & Logan,

2002; Van der Schoot, Licht, Horsley, & Sergeant, submitted for publication).Van der Schoot et al. found that SSRT in an auditory stop signal condition was fas-

ter than SSRT in a visual stop signal condition. They attributed this modality effect

to differences in the neurophysiological processes underlying perception. Ridderink-

hof et al. found that stop processes were completed more slowly when the imperative

signal (a target arrow) was flanked by distractor stimuli that were associated with the

incorrect primary response (noncorresponding flanker arrows) and that response

suppression in the primary task was less efficient when stop processes were active si-

multaneously. According to Ridderinkhof et al., these results indicate that the oper-ation of response inhibition in the primary task processes and response inhibition in

the stop process affected one another negatively.

Other studies used psychophysiological variables to examine the control processes

that underlie response inhibition in the stop signal paradigm (e.g., De Jong, Coles,

Logan, & Gratton, 1990; Pliszka, Liotti, & Woldorff, 2000; Van der Schoot, Licht,

Horsley, & Sergeant, in press). In the study by De Jong et al., event related brain po-

tentials and electromyogram measures suggested that response inhibition involved

both the inhibition of central response activation and a more peripherally operatingmechanism. Recently, Band and Van Boxtel (1999) reviewed the literature on the

neural mechanisms underlying the stop process. Based on the available anatomical,

neurophysiological and psychophysiological data, they concluded that the prefrontal

cortex, basal ganglia and supplementary motor area are the most likely sources of

inhibitory activity.

From a neuropsychological point of view, it would be interesting to further exam-

ine the stop process by studying hemispheric differences in stop task performance.

Therefore, we decided to repeat the visual stopping task of the above-mentioned ex-

280 M. Van der Schoot et al. / Acta Psychologica 112 (2003) 279–295

periment under conditions of hemifield stimulation, flashing the stop stimuli in the

left or right visual fields. From a large number of behavioral experiments that em-

ployed this methodology, it has been established that left–right differences in speed

and accuracy can be interpreted as providing evidence for an asymmetry of cerebral

hemispheric function (e.g., Bradshaw & Nettleton, 1981). That is, if stop perfor-mance would be found faster and more efficient with the stop stimulus presented

in one or the other visual half field (VHF), one may conclude that the hemisphere

opposite to the side of better performance is in some way specialized for processing

a stop task. Below, it is hypothesized that there is a left-hemispheric superiority for

stop task performance.

1.1. Active versus receptive attention

It may be argued that subjects performing a stop task feel obliged to actively mon-

itor for stop signals while responding to the primary stimuli. That is, a stop task

causes subjects to give priority to their stopping record as a result of which they

try to maintain a tonic readiness for inhibitory action while being engaged in a pri-

mary task routine. A similar suggestion has been made by Douglas (1999), Kramer

et al. (1994), and Oosterlaan and Sergeant (1995). Kramer et al. suggested that dur-

ing go task performance, subjects are continuously on the alert for a stop signal.

Douglas argued that a stop task shares some characteristics with the vigilance orcontinuous performance task: �in both tasks, subjects must monitor stimuli carefully

while maintaining preparation to make the appropriate response (either inhibit or re-

spond) to rare and unpredictable stimuli�. Following a similar line of argumentation,

Oosterlaan and Sergeant (1995) suggested that a stop task assesses preparation, as

well as inhibition.

How is the above �attentional claim� that a stop task is presumed to make on a

subject related to hemispheric differences? Drawing from the Pribram and McGuin-

ness (1975) formulation of attentional control in the brain, Tucker and Williamson(1984) differentiated between an active attentional system controlling motor readi-

ness and a receptive attentional system controlling perceptual responsivity. Hemi-

spheric asymmetries have been observed in tasks that appeal to either of both

systems. The left hemisphere was found to be linked to an active, vigilant attentional

mode (Dimond & Beaumont, 1971, 1973; Kinsbourne, 1974). In the experiments by

Dimond and Beaumond, subjects showed a right visual field (RVF) superiority for a

task that required them to maintain attention to respond to an unwarned stimulus.

Attentional tasks that do not require the vigilance of the motor readiness system maybe performed better by the right hemisphere (Heilman, 1979; Heilman & Van Den

Abell, 1980; Mesulam, 1981; Semmes, 1968). Heilman and Van Den Abell observed

a significant asymmetry in the facilitation of reaction speed by laterally presented

warning stimuli, with a superior performance following left visual field (LVF)

warning stimuli. Assuming that a stop task includes both a stop component

(maintaining vigilance to respond to unwarned stop signals) and a go component

(producing orienting responses to warning stimuli and discriminating between sub-

sequent go signals), we hypothesize that the former component requires the active,

M. Van der Schoot et al. / Acta Psychologica 112 (2003) 279–295 281

left-lateralized, attentional mode and that the latter component calls for the recep-

tive, right-lateralized, attentional mode. The present study is focused on the first hy-

pothesis.

1.2. Activation and arousal

To gain insight into the possible operation of left-lateralized attentional processes

during stop task performance, we elaborate on the basic neural control systems that

are believed to be involved in active and receptive attention: activation and arousal

(see Pribram & McGuinness, 1975). Arousal is defined in terms of phasic physiolog-

ical responses to input. Activation is defined in terms of tonic physiological readiness

to respond. Pribram and McGuinness� formulation is founded on the distinct neural

circuits that underlie both attentional control systems. Arousal is regulated by thenorepinephrine and serotonine pathways that spring from brainstem nuclei and in-

nervate widespread brain areas to support alertness and the brain�s responsivity to

perceptual input (McGuinness & Pribram, 1980; Tucker & Williamson, 1984). Acti-

vation is hypothesized to increase neural activity to maintain a tonic readiness for

action. It is thought to be regulated by dopamine pathways from the substantia nigra

combined with cholinergic influences on the basal ganglia of the extrapyramidal

motor system (McGuinness & Pribram, 1980). Attention directed by the activation

system is more internally controlled and more integral to vigilance and motoroperations than attention directed by the arousal system. It is important to recognize

that the neurotransmitter pathways underlying activation have been found to be

asymmetric in their distribution and function. Glick, Ross, and Hough (1982)

showed that the levels of dopamine and choline acetyltransferase, an index of acet-

ylcholine activity, are higher in the left than in the right basal ganglia. PET scan in-

spection additionally demonstrated a similar asymmetry in the concentration of

dopamine receptors (Wagner et al., 1983). Kooistra and Heilman (1988) showed that

especially the globus pallidus appears to be a nucleus with significant left-laterality.Given the assumption that the stop component of a stop task demands that sub-

jects appeal to the left-lateralized activation system, we predict that the left hemi-

sphere will be favored in the present experiment. That is, subjects are not only

assumed to actively monitor for stop signals while being engaged in the go task rou-

tine, they are also expected to display a RVF-preference for them.

1.3. The visual half field procedure

In the present study, hemispheric differences in stop task performance were exam-

ined by using the VHF procedure: sensory input and motor response were restricted

to one hemisphere, and then speed and accuracy were measured in all possible pair-

ings. In the case of visual input, LVF information will project first to the right hemi-

sphere, and vice versa. Likewise, motor responses are contralaterally controlled.

Hence, latency differences between conditions in which stimulus perception and

response generation are controlled by the same hemisphere (uncrossed: LVF-LH

(left hand), RVF-RH (right hand)) and conditions in which stimulus perception and

282 M. Van der Schoot et al. / Acta Psychologica 112 (2003) 279–295

response generation are controlled by the opposite hemispheres (crossed: LVF-RH,

RVF-LH) reflect the time it takes the motor signal and, possibly, the sensory signal

to cross the corpus callosum. Whether there is a delay in response due to the cross-

callosal transfer of the laterally presented visual information depends on the model

one departs from. Two standpoints can be taken. First, the callosal relay model (Zai-del, 1983, 1985) presupposes that one hemisphere is unable to process the informa-

tion at all, leaving the other to be exclusively specialized for the task. Therefore, the

callosal transmission of the stimulus from the competent to the incompetent hemi-

sphere is included in the laterality effect. Second, the direct access model (Zaidel,

1983, 1985) assumes that both hemispheres are able to process the information

but one hemisphere may use less efficient strategies.

Since previous RT research (Davis & Schmit, 1971; Filbey & Gazzaniga, 1969;

Geffen, Bradshaw, & Wallace, 1971) revealed that the detection of laterally presentedvisual stimuli, exempting specific identification and analysis, is not sufficient to in-

duce RT differences between the hemispheres, we decided to adopt the direct access

interpretation of laterality effects as our working model. That is, when an inhibitory

response is required with either the right or left hand, as in the present experiment,

no crossing of information is assumed to emerge according to which hemisphere

originally receives the stop signal. As a consequence, laterality effects that cannot

be due to the cross-callosal transfer of the motor command from the input-receiving

hemisphere to the motor cortex of the hemisphere controlling the responding handnecessarily indicate a hemispheric difference in stop task competence. The main goal

of the present study is to examine the hypothesis that there is a left-hemispheric dom-

inance for stop task performance.

2. Method

2.1. Subjects

Eighteen students (10 males, 8 females, between 18 and 26 years of age, 8 left-

handed, 10 right-handed) were paid about €8 per hour for participation in the study.

All were healthy and had a normal or corrected-to-normal vision.

2.2. Apparatus

The stimuli were presented on a NEC Multisync 5FG monitor positioned 50 cmfrom the subject�s eyes. Subjects were lying on a couch in a dimly illuminated cubicle.

A response box was positioned on either side of the couch.

2.3. Task and stimuli

2.3.1. Primary task

Each trial began with the presentation of a square warning stimulus

ð1:40 cm� 1:40 cmÞ for 500 ms. It was followed by the primary task stimulus, which

M. Van der Schoot et al. / Acta Psychologica 112 (2003) 279–295 283

was displayed for 125 ms. After the imperative signal was extinguished, the screen

went blank for a 2375 ms intertrial interval. The stimuli for the primary task were

the uppercase letters X and O. Each letter was 1.80 cm wide and 2.90 cm high. Both

the warning stimuli and the stimulus letters were presented in black-on-white and

in the center of the screen. In the primary choice RT task a capital X required a re-sponse with the index finger of the right or left hand and a capital O required a re-

sponse with the middle finger of the same hand. We counterbalanced the hand used

so that half the subjects used the right hand (RH) first and half used the left hand

(LH) first. Across the right-hand-first-subjects and left-hand-first-subjects, mapping

of letters onto fingers was also counterbalanced.

Hemifield laterality experiments necessitate fixation. Therefore, we employed a

short primary task stimulus duration. This forced subjects to fix their eyes on the

center of the screen since a slight gaze deviation from the middle at the momentof presentation could lead to missing a signal. To further prevent subjects from scan-

ning – besides repeatedly instructing them to fixate their eyes on the center of the

screen – the 500-ms warning stimulus presently served as a signal for fixation along

with its warning function. In addition, a central fixation point (a little black �þ� sym-

bol) was displayed whenever the screen went blank.

2.3.2. Stopping task

A stop signal was presented on 25% of the trials, occurring equally often at eachof six stop signal delays, and equally often with an X and an O. The sequence of pri-

mary task stimuli, stop signals, and stop signal delays was pseudo-randomized. The

stop signal was a red circle (3.60-cm diameter) exposed for 200 ms. It was presented

in the center of the visual field (CVF), in the LVF, or in the RVF. The lateral stop

stimuli were placed 14.00 cm away from the center of the screen. The visual angle

between lateral and central stimuli was 15.6�.

2.4. Design and procedure

Subjects participated in two sessions either consistingof nine test blocksof 144 trials.

After the first session, the response handhad tobe changed.Apractice blockof 48 trials

preceded each session. The test blocks were arranged in groups of three, and a short

break was scheduled after each part. In between sessions, subjects took a longer rest.

In every block, 36 stop signals were presented: two CVFs (1 after an X and 1 after

an O), two LVFs (1 after an X and 1 after an O) and two RVFs (1 after an X and 1

after an O) at each stop signal delay. Consequently, in all six resulting conditions(CVF/RH, LVF/RH, RVF/RH, CVF/LH, LVF/LH, RVF/LH), a total of 18 stop

signals occurred at each delay.

Under the assumption that inhibition functions are linear, variation in mean pri-

mary task reaction time (MRT) will affect the intercept, i.e. the height, of the inhi-

bition function. To take account of differences between subjects in mean reaction

time (MRT) and strategy (e.g., a subject may have delayed a response in an attempt

to enhance the probability of inhibiting), stop signal delay was defined as the inter-

val between the onset of the stop signal and the expected MRT (i.e., MRT-delay). In

284 M. Van der Schoot et al. / Acta Psychologica 112 (2003) 279–295

order to set delays relative to MRT, alterations of MRT were tracked block-to-

block; that is, the MRT was calculated after each block of trials whereupon stop sig-

nal delays were adapted to it in the following block. More specifically, the MRT in

block n was used to set the stop signal delays in block nþ 1 equal to MRT-500,

MRT-400, MRT-300, MRT-200, MRT-100 and MRT-0 ms. The stop signal delaysin the first test block of each session were set relative to the MRT of the practice

block preceding it. Whereas it is almost impossible to inhibit a response to a stop

signal presented at the subject�s MRT (MRT-0), the more a stop signal delay approx-

imates zero (MRT-500), the greater the probability of inhibition. The intermediate

delays trace out the inhibition function between these extremes.

No negative delays were employed. In practice, this meant that the primary stim-

ulus and the stop signal were presented simultaneously in the MRT-500 condition (in

block n) in case the MRT was less than 500 ms (in block n� 1).Instructions for the primary choice reaction time task were given first. Subjects

were instructed to respond as fast and accuratly as possible. Then, the subjects were

instructed to try to withhold the response whenever a stop signal occurred. It was

clarified that stop signal delays were varied by the experimenter in such a way that

sometimes stop signals would be presented so late that it would be extremely difficult

to suppress the primary response. Finally, subjects were explicitly instructed not to

delay their responses to the go task in order to improve their odds of stopping.

2.5. Data analysis

2.5.1. Primary task measures

For each subject, the following primary task measures were derived from the no-

stop signal trials: mean reaction time (MRT), standard deviation (SD), percentage of

errors (pressing with the X-finger when O was presented or vice versa) and percent-

age of omissions (nonresponses).

2.5.2. Inhibition function

Inhibition functions were generated by computing the proportion of stop signal

trials, at each stop signal delay, on which subjects successfully inhibited their pri-

mary response (i.e. P ðInhibitÞ). Effects of all possible LVF/CVF/RVF-RH/LH pair-

ings on the probability of inhibition (over all delays) were examined in analyses of

variance (ANOVA) with repeated measures across delay. An interaction between

delay and experimental condition would then demonstrate differences in the shape

of inhibition functions.

2.5.3. SSRT

To explore more specific deficits in the stopping process, mean SSRTs were esti-

mated for each individual in each VHF/response hand condition by means of the fol-

lowing procedure. The point in time at which the stop process finishes was computed

from the data by setting it equal to the nth RT of the rank ordered go task RTs,

where n is the number of RTs that make up the distribution multiplied by the

observed P ðRespondÞ (¼ 1� PðInhibitÞ). Subtracting stop signal delay from this

M. Van der Schoot et al. / Acta Psychologica 112 (2003) 279–295 285

value yielded the SSRT. It is important to realize that this procedure was carried out

for each stop signal delay employed in the experiment. The mean SSRT was then ob-

tained by averaging over stop signal delays (see Logan & Cowan, 1984, for an exten-

sive, and more theoretical, discussion on the SSRT estimation procedure). We realize

that this method assumes that SSRT is a constant. However, Logan and Cowan(1984) showed that estimates obtained in this way approximate the estimates of

SSRT obtained by more advanced methods that treat SSRT as a random variable.

The effects of visual field and response hand on SSRT were examined in ANOVA

and subsequent paired-samples t-tests.

2.5.4. Estimating SSRT at the central stop signal delay

Band, Van der Molen, and Logan (2003) found the estimation of SSRT to be

most reliable around the central delay, where subjects have a 50% chance of success-ful inhibition. In a simulation study, Band et al. showed that the impact of using

noncentral delays on the estimation of SSRT is considerable. Especially the SSRTs

that are calculated at stop signal delays that yield a lower-than-50% inhibition rate

underestimate the true latency of the stop process (see also Logan & Burkell, 1986).

The reason for this is that there is variability in SSRT, and that at the late delays,

only the faster stop processes are able to win the race against the go processes. This

is unfortunate because we are interested in the mean SSRT. Since the classical

method mentioned above assumes that SSRT is a constant and since its estimationprocedure takes into account the P ðInhibitionÞ at each stop signal delay, we decided

to also estimate SSRT at the delay that would yield a 0.5 inhibition rate. The best

possible estimate of the central delay was obtained by fitting a regression line to

the most central observations of the inhibition function (MRT-500, MRT-400 and

MRT-300) and entering 0.5 in the computed regression equation (it should be recog-

nized that in some cases this procedure yielded a negative 50%-inhibition delay).

SSRT was then calculated by subtracting the central delay from the point in time

at which the stop process finished. The stop process finish time could be computedfrom the data by setting it equal to the median of the go task RTs.

3. Results

3.1. Primary task measures

For each subject, MRT, SD of MRT, percentage of errors and percentage ofomissions were obtained separately for the left (LH) and right hand (RH). Table 1

displays the means and standard deviations of each of these dependent measures

(along with SSRT).

The fractions of errors and omissions never exceeded 2%, indicating that subjects

fixated their eyes at the center of the screen throughout the experiment. Paired-sam-

ples t-tests showed no significant differences in accuracy between responding hands.

A t-test performed on the MRT revealed that response hand neither had an effect on

primary response speed ðtð17Þ ¼ 0:60Þ.

286 M. Van der Schoot et al. / Acta Psychologica 112 (2003) 279–295

Table 1

Performance on the stop signal paradigm as reflected by the means and standard deviations for the dependent measures for each response hand (go task) and

response hand/visual field (stop task) condition

Go task measures Left hand Right hand

M SD M SD

MRT (go task) 534.70 59.49 545.09 58.38

SD of MRT 116.11 28.22 116.00 22.87

% of errors 1.63 0.97 1.57 0.91

% of omissions 1.30 1.18 1.06 0.35

Stop task measure LVF CVF RVF LVF CVF RVF

M SD M SD M SD M SD M SD M SD

SSRT 270.14 44.24 275.21 43.70 279.31 55.23 289.95 48.00 272.84 47.69 264.03 46.21

Note. M: mean, SD: standard deviation, MRT: mean reaction time, SSRT: stop signal reaction time, LVF: left visual field, CVF: central visual field, RVF:

right visual field; all times are in ms.

M.VanderSchootetal./Acta

Psychologica

112(2003)279–295

287

3.2. Inhibition function

Table 2 presents the mean probability of inhibiting a response to the primary task

at each stop signal delay for each visual field/response hand condition.

Fig. 1 displays these probabilities of inhibition (P ðInhibitÞ) as a function of MRT-

delay graphically.

A three-way ANOVA with repeated measures across response hand (HAND)

(two levels: LH vs. RH), VHF (two levels: LVF vs. RVF) and delay (six levels)

Table 2

Probability of inhibition as a function of stop signal delay and response hand/visual field condition

Stop signal

delay (ms)Left hand Right hand

LVF CVF RVF LVF CVF RVF

MRT-500 54.32 52.16 50.00 51.24 50.93 57.10

MRT-400 50.93 48.77 47.53 45.99 52.47 53.70

MRT-300 43.52 37.65 38.27 33.95 41.36 48.15

MRT-200 32.72 31.79 31.17 29.63 34.57 33.95

MRT-100 28.70 29.01 28.09 25.93 27.78 33.02

MRT-0 21.91 22.53 23.46 25.31 26.85 28.40

Note. LVF: left visual field, CVF: central visual field, RVF: right visual field.

Fig. 1. The probability of inhibition as a function of MRT-delay for each response hand/visual field con-

dition. (MRT: mean reaction time, LH: left hand, RH: right hand, LVF: left visual field, CVF: central

visual field, RVF: right visual field).

288 M. Van der Schoot et al. / Acta Psychologica 112 (2003) 279–295

was conducted for the probability of inhibition. As predicted by the race model, the

probability of inhibition increased significantly as stop signal delay decreased ðF ð5;85Þ ¼ 62:10, p < 0:0001Þ. There was a significant HAND� VHF interaction ðF ð1;17Þ ¼ 14:90, p < 0:001Þ, even though these factors did not interact with delay, nei-

ther separately (F ð5; 85Þ ¼ 0:33 and F ð5; 85Þ ¼ 0:27, respectively), nor in combina-tion ðF ð5; 85Þ ¼ 1:44Þ.

3.3. SSRT

The SSRTs are graphically represented in Fig. 2. A two-way ANOVA confirmed

the interaction between VHF and HAND ðF ð1; 17Þ ¼ 8:77, p < 0:01Þ, denoting that

uncrossed–crossed differences resulted in interhemispheric transfer times (ITTs) of

9 ms (left hand: RVF–LVF) and 26 ms (right hand: LVF–RVF). According to thedirect access interpretation of laterality effects (Zaidel, 1983, 1985), these delays in

SSRTs can be attributed – at least in part – to the cross-callosal transmission of

the inhibitory motor command. Of greater significance here, however, is the hemi-

spheric asymmetry of 17 ms that was additionally uncovered by these ITTs.

A more meaningful way of specifying this discrepancy is to look at the LVF–RVF

difference within both the uncrossed and the crossed VHF/HAND-pair. From Fig. 2

it can be seen that the RVF stop signals speeded the executive process of inhibition

Fig. 2. The interaction effects between VHF and response hand on stop stimulus reaction time (SSRT).

(LVF: left visual field, RVF: right visual field).

M. Van der Schoot et al. / Acta Psychologica 112 (2003) 279–295 289

with 6 and 11 ms, respectively, suggesting that, for the benefit of a fast stopping pro-

cess, stop signals need to be presented to the left hemisphere. Although not signifi-

cant within uncrossed and crossed VHF/HAND-pairings, the mere LVF vs. RVF

effect on stopping speed indirectly manifested itself through the pattern of VHF ef-

fects on left and right hand performance. Planned comparisons indicated that theVHF effect on SSRT was significant for the right hand ðtð17Þ ¼ 2:83, p < 0:05Þ,but not for the left hand ðtð17Þ ¼ 0:94Þ.

It should be noted that the additional LVF–RVF difference of the right hand as

compared to the left hand (17 ms) reflects the sum of the �pure� RVF-facilitation of

SSRT in an uncrossed (RVF/RH) and crossed (RVF/LH) arrangement: the former

adds 6 ms to the VHF effect of the right hand, the latter subtracts 11 ms from the

VHF effect of the left hand. Together, they result in the differential outcomes of

the above post hoc test conducted separately for the right hand (significant VHFeffect) and left hand (nonsignificant VHF effect).

3.4. Central delay-SSRT

The SSRTs that were obtained by the central delay approach were 456.39

ðSD ¼ 300:63Þ, 514.11 (326.15), 536.94 (314.69) and 396.61 (215.86) ms, respectively,

in the LVF/LH, RVF/LH, LVF/RH and RVF/RH condition. These SSRTs are

slower than those obtained by the traditional SSRT estimation procedure, and theabsolute differences in SSRT between the VHF/HAND conditions are inflated. Still,

the VHF�HAND interaction was only marginally significant ðF ð1; 17Þ ¼ 4:33,p < 0:06Þ. Probably, the interaction failed to reach conventional levels of significance

because the between-subject variability in central delay-SSRT was large. The reason

for this is that in subjects with low and flat inhibition functions the operating regres-

sion procedure yielded fairly negative 50%-inhibition delays. Since these delays were

added to the median of the primary task RTs so as to obtain the central delay-

SSRTs, it may be argued that these SSRTs overestimate the true latency of the stopprocess in subjects with low inhibitory capacity. In part, these overestimates may

also account for the relatively slow mean SSRTs obtained by the central delay ap-

proach.

Nevertheless, post hoc tests performed on the central delay-SSRTs confirmed

that – although not significant within uncrossed ðtð17Þ ¼ 0:79Þ and crossed

ðtð17Þ ¼ 0:22Þ VHF/HAND-pairings – the mere LVF vs. RVF effect on stopping

speed indirectly manifested itself through the pattern of VHF effects on left and right

hand performance. Post hoc t-tests indicated that the VHF effect on SSRT was sig-nificant for the right hand ðtð17Þ ¼ 2:21, p < 0:05Þ, but not for the left hand

ðtð17Þ ¼ 0:73Þ.

3.5. Handedness

To minimize the chance that the effects resulted, in part, from handedness (i.e., the

characteristic preference that individuals show for one or the other hand for per-

forming unimanual tasks), left-handed ðn ¼ 8Þ and right-handed ðn ¼ 10Þ subjects

290 M. Van der Schoot et al. / Acta Psychologica 112 (2003) 279–295

were equally represented in the experiment. Furthermore, the ANOVAs that were

conducted for the probability of inhibition and SSRT were repeated with handedness

as the between-subject factor. These analyses showed that there was neither a main

effect of hand-preference on the probability of inhibition ðF ð1; 16Þ ¼ 1:50Þ and SSRT

ðF ð1; 16Þ ¼ 1:93Þ, nor did this factor interact with any of the within-subject vari-ables. These results are supportive of previous findings that there is no motor asym-

metry for response acts as simple as button-pressing (Alegria & Bertelson, 1970;

Beaumont, 1974).

4. Discussion

The most important implications of the present experiment apply to the (asymme-try of) hemispheric engagement in stop task processing. As previously stated, the

two uncrossed conditions and the two crossed conditions were anticipated to gener-

ate divergent SSRTs only in case of differential hemispheric functioning. Whereas

differences between uncrossed and crossed sensorimotor routes simply reflect the an-

atomic restraints of interhemispheric transfer, differences within each contingency

can differentiate between balanced and unbalanced hemispheric involvement during

stop task performance. Balanced hemispheric involvement predicts a fully symmet-

rical field by hand interaction that can be accounted for on a mere anatomical basis.Unbalanced hemispheric involvement predicts a deviation from this pattern as a

hemispheric difference in stop task competence will �bend� the interaction in favor

of one or the other hemifield. In the present study, the results showed a slight

improvement of stop task performance for the RVF.

The left-hemispheric dominance manifested itself indirectly. That is, the hemifield

effects on inhibitory speed (SSRT) did not reach significance within equivalent field/

hand pairings (i.e. uncrossed and crossed routes). However, when added up in the

subsequent post hoc tests, the within-route effects made the hemifield difference sig-nificant for the right hand, but not for the left hand. The RVF advantage within the

uncrossed field/hand pairing enlarged the RH-VHF effect and the RVF advantage

within the crossed field/hand pairing reduced the LH-VHF effect. The finding that

the VHF effect was significant for the right hand but not for the left hand reflects

the hypothesized left-hemispheric superiority for motor stopping in the stop signal

paradigm.

Accordingly, empirical support has been provided for the notion that the stop

task is actually composed of a go and stop component, the latter of which appealsto an active, left-lateralized, attentional system controlling motor readiness (see

Tucker & Williamson, 1984). It was reasoned that a stop task requires such a mode

of attention because subjects actively monitor for stop stimuli while being engaged in

its go routine (see also Douglas, 1999; Kramer et al., 1994; Oosterlaan & Sergeant,

1995).

In the introduction of this paper, it has been demonstrated that the hemispheric

specialization for active and receptive attention is equivalent to the asymmetry

observed in the neural substrates of the two major attentional control systems

M. Van der Schoot et al. / Acta Psychologica 112 (2003) 279–295 291

characterized by Pribram and McGuinness (1975). The arousal system produces a

phasic response to input and is found to be right-lateralized. The activation system

maintains a tonic readiness to respond, and the function and distribution of its un-

derlying neurotransmitter circuits are found to be left-lateralized (e.g., Glick et al.,

1982; Kooistra & Heilman, 1988; Wagner et al., 1983). The idea that subjects appealto this system during stop task performance is supported by the RVF advantage dis-

closed by the present findings.

We argue that because of the current experimental design, the Simon effect (Si-

mon, 1968, 1969, 1990) may have inflated the differences between uncrossed and

crossed arrangements of response hand and side of stop stimulus presentation.

The Simon effect represents the impact of compatibility between task-irrelevant stim-

ulus location and response location (see for a review Proctor & Reeve, 1990). When

left stimuli are responded to by pressing a left response key and right stimuli are re-sponded to by pressing a right response key, performance is better (in terms of RT

and error rates) than when left stimuli are paired with right responses and vice versa.

It should be emphasized that such phenomena occur even when the stimulus position

is completely irrelevant for the task. Usually, the Simon effect has been associated

with response selection. Simon (1969) speculated about an automatic tendency to re-

act toward stimulus location. Interestingly, Hommel (1995) demonstrated that the

Simon effect can occur under conditions comparable to those used in the present

study. He used arrows, pointing either to the left or to the right, that required a spa-tially corresponding reaction. Briefly, on stimulus presentation, subjects received a

second stimulus appearing randomly to the left or the right side. This was a green

Go or a red No-go signal, indicating whether the precued response should be per-

formed or suppressed. The location of the second color-stimulus was irrelevant (like

our hemifield stop signals) but still caused a Simon effect. Being focused on �go� RT

and proportion of errors, Hommel only related the Simon effect to the location of the

Go signal. It is conceivable, however, that the participants would have displayed an

ideomotor tendency to respond toward the source of stimulation in case of inhibitoryresponses to laterally presented No-go signals as well. It may be easier, in other

words, to suppress a right hand response after a right stop signal and a left hand re-

sponse after a left stop signal because of a spatial stimulus–response compatibility,

exerting a facilitatory influence on stopping speed and accuracy. Although the un-

crossed/crossed effects on inhibition function and SSRT should be interpreted pri-

marily in anatomical terms, a contribution of the Simon effect cannot be ruled out

in the present experiment.

Significantly, the above proposition is supported by the data. It was found thatinhibition functions tended to be higher when stop signals were displaced from the

center of the screen to the side of the active hand ðF ð1; 17Þ ¼ 4:36, p < 0:06Þ. Thatis, left stop signals facilitate left hand response suppression and right stop signals fa-

cilitate right hand response suppression, both in comparison with central stop sig-

nals. Since stimuli presented in the CVF equally innervate both hemispheres, these

effects cannot be explained in anatomical terms. Accordingly, another mechanism

has to be in operation. In view of the above, the Simon effect appears to be a likely

candidate. There may be an alternative explanation for the results, however. Since

292 M. Van der Schoot et al. / Acta Psychologica 112 (2003) 279–295

subjects used only the right hand or only the left hand in each block of trials, re-

sponse preparation may have shifted attention to the side of preparation. As a con-

sequence, this may have positively affected the stop process by facilitating the

detection of the stop signal.

Acknowledgement

We wish to thank Guido Band for his helpful comments on earlier versions of this

article.

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