The interaction of stimulus- and response-related processes measured by event-related lateralizations of the EEG

ELSEVIER

Electroencephalography and clinical Neurophysiology 99 (1996) 149-162

The interaction of stimulus- and response-related processes measured by event-related lateralizations of the EEG Edmund Wascher*, Bernd Wauschkuhn Medical University of Liibeck, Department of Neurology, Ratzeburger Alice 160, D-23538 Labeck, Germany Accepted for publication: 20 March 1996

Abstract

The present study focused on the relationship between movement- and stimulus-related asymmetries of the electroencephalogram (EEG). In seven tasks the saute bilateral stimuli containing asymmetric information were presented but response requirements differed. Three functionally distinct asymmetries were found: (1) an asymmetry over the motor cortex prior to unimanual movements, (2) an asymmetry over the posterior cortex beginning about 20 ms after the start of the movement, and (3) an early increase of negativity contralateral to a relevant stimulus (200-300 ms after stimulus onset) that was maximal at temporo-parietal sites but was also visible at central sites. Although related to stimulus side, this asymmetry was modulated by response requirements: it was largely abolished with simple responses, smaller with nogo than with Go stimuli and occurred twice when a sequence of simple and choice responses was required. Therefore, the early temporo-parietal asymmetry most probably reflects an interface between sensory and movement-related processes.

Keywords: Attention; Response preparation; Stimulus-response compatibility; Asymmetry of the electroencephalogram; Lateralized readiness potential

1. Introduction Prior to movements, negativity increases at electrodes over the motor cortex, contralateral to the limb involved in the movement. This phenomenon may be observed both before voluntary movements as lateralization of the Bereitschaftspotential (Vaughan et al., 1968) and before precued movements as movement-related lateralization of the contingent negative variation (CNV) (e.g. Kutas and Donchin, 1980). This unilateral increase of negativity can even be measured when no distinct motor-related event related potential (ERP) component is visible, overlapping the stimulus-evoked potentials (Kutas and Donchin, 1980). A convenient method of dissociating this lateralization from lateralizations evoked by other factors (e.g. by task demands) is computing the lateralized readiness potential (LRP), i.e. the difference potential between re-

* Corresponding author. Tel.: +49 451 5003544; fax: +49 451 5002489; e-mail: [email protected].

cordings contralateral and ipsilateral to the movement (de Jong et al., 1988; Gratton et al., 1988). The LRP can be used as a measure of response preparation (Coles et al., 1988). An increasing number of theoretical and experimental publications about the LRP has demonstrated the high informational value of this method. The LRP has been found to consist of two subcomponents. One of these subcomponents of the LRP reaches its maximum with the overt response and therefore reflects the preparation of the actual response. This part of the LRP is preceded by an initial lateralization, which was interpreted as early response tendency due to preliminary information extracted from the stimulus array (e.g. Coles et al., 1992; Eimer, 1995). If some 'partial information' is in conflict with the actually required response (e.g. in a noise compatibility task) a tendency towards a wrong response is visible in the initial part of the LRP (Coles et al., 1988). The magnitude of the early LRP may vary as a function of subject's degree of control of which responses are activated following stimulus presentation (Coles et al., 1992). Osman et al. (1992) reported such early lateralizations for laterally presented go and nogo stimuli. While

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E. Wascher, B. Wauschkuhn / Electroencephalography and clinical Neurophysiology 99 (1996) 149-162

the early lateralization returned to baseline for nogo stimuli, it increased for go stimuli. The onset latency of the difference between go and nogo lateralizations depended on the discriminability of the stimuli. However, asymmetries of the ERP are not only evoked by response preparation but also by stimulus-related processes. The posterior N1 is increased contralateral to a laterally presented visual stimulus. It was suggested that the N1 represents 'activity in the dorsal stream, projecting from the striate cortex to the parietal lobe and encoding spatial aspects of visual information' (Mangun, 1995, p. 16). Mangun (1995) reported evidence for two functionally distinct asymmetries of the N1, a first slight one (150-190 ms) at parietal sites and a second strong one (190-220 ms) at posterior occipital-temporal regions (see Mangun, 1995, Fig. 12). He proposed a hierarchical system of selection, with an initial segmentation of the visual world, altering the signal-to-noise ratio for inputs across the visual field, followed by selective attention. Luck and Hillyard (1994) reported an increase of the N2 contralateral to a relevant item of a stimulus array (pop-out) at parietal sites. Though the maximum of this asymmetry was markedly later than the N1 asymmetries mentioned above (at about 300 ms), it might also be interpreted as a shift of attention towards the spatial position of the relevant item. Yamaguchi et al. (1994) demonstrated that the latency of the asymmetry due to spatial attributes of a stimulus depends on the kind of stimulus. While peripheral stimuli evoked a strong asymmetry starting at 140 ms at anterior and posterior electrode sites, stimuli with symbolic spatial information (arrows) evoked lateralizations at posterior sites not earlier than 240 ms, which spread to anterior sites with time (Yamaguchi et al., 1994). These lateralizations were not associated with discrete negative components of the ERP (see also Luck and Hillyard, 1994, Fig. 9, lower panel). Therefore, in order to measure the latency of maximal asymmetry, Luck and Hillyard (1994) calculated the difference potential between recordings contralateral and recordings ipsilateral to the pop-out position (see Luck and Hillyard, 1994, Fig. 7). Since these stimulus-related asymmetries were found within the same time range as the first component of the LRP, the interaction of stimulus- and response-related asymmetries is of special interest. The interaction of these two systems might be the origin of psychological phenomena like the spatial S-R compatibility effect. This effect can be described as facilitation of responses ipsilateral to a presented stimulus compared to responses contralateral to a stimulus. This effect occurs even if the position of the stimulus is not task-relevant (for a review see Simon, 1990). The present study investigates the interaction of stimulus- and response-related asymmetries. To this end, first, 'event-related lateralizations' (ERLs) were calculated not only for response-related asymmetries ('LRP') at central electrodes over the motor cortex, but also for

stimulus-related asymmetries at centro-parietal, parietal, temporal and occipital sites the same way. Second, 5 different response requirements were introduced in separate blocks for the same stimuli. These were a choice response task, a simple response task, a go/nogo task, and two 'response pattern' tasks. The simple response task and the go/nogo task were introduced to investigate whether the spatial location of the stimulus has influence on response times and on ERP asymmetries even if no response selection is necessary. In the response pattern tasks, subjects had to respond to each stimulus with a sequence of a bimanual simple response and a unimanual choice response (either first simple then choice or first choice then simple). These tasks were introduced to investigate first, the influence of the stimulus location on more complex response requirements, and second, whether the advantage of a response ipsilateral to a presented stimulus exists even if this response is preceded by a simple response. Measuring ERLs at a large number of symmetrical electrode pairs was expected to provide information about the temporal overlap of stimulus- and response-related asymmetries at different locations. The different response requirements were expected to separate perceptual and motor asymmetries from each other and to illustrate their functional interactions. Although all tasks were conducted within one session, the data will be presented in two separate parts. First, various ways of measuring event-related asymmetries of the electroencephalogram (EEG) were compared in a choice response task. The purpose of this approach was to delineate stimulus- and response-related asymmetries and their relation to asymmetries of components of the ERP. Second, the different components of the ERLs that were found in Part I were investigated for several response requirements. The change of response demands for the same stimuli should evoke task-dependent variations in response-related but not in stimulus-related processes (except the task requirements imply altered stimulus evaluation, e.g. in the simple response task). In this way, the assignments of the ERLs to stimulus-related and to response related processes found in the choice response task were tested for validity. 2. General methods 2.1. Subjects

Eleven male students of the Medical University of Liibeck, aged 19-25 years, were recruited for this study. All subjects were right-handed, in good physical health, and had no history of psychiatric or neurological disorder. They had normal or corrected-to-normal vision. Subjects were paid 35 DM (approximately $24) on average for participation. The total pay-off was calculated from 30 DM base value plus a bonus of up to 10 DM for good performance.

E. Wascher, B. Wauschkuhn/ Electroencephalography and clinical Neurophysiology 99 (1996) 149-162 2.2. Stimuli and procedure Identical stimuli were presented in 7 separate blocks. In each block a different kind of response was required (for details see below). A white fixation cross in the center of the screen and two symmetrically positioned white frames (representing the possible stimulus positions) were displayed continuously. In each trial, a lette:r (A or B) and a filler (3 horizontal bars, similar to the letters in size and luminance) on the opposite location were presented for 200 ms. The distance of the two lateral frames from the fixation cross was 1.1 °. Stimuli were approximately 11 mm wide and 13 mm high (0.5 x 0.6 °) and of bright yellow color. Within each block of tria'~is the 4 types of stimuli were presented in randomized order. The different tasks were: performed by the subjects in a pseudo random order within one session, different for each subject. Each block consisted of 40 practice and 240 experimental trials. SOAs were random, between 1500 and 2500 ms. Response requirements were derived from the classification of Donders (1868-4i9) that contains simple, choice and go/nogo responses. This scheme was extended by response patterns. In these conditions a simple and a choice response or a choice and a simple response had to be performed in each trial one after the other. Subjects were requested to perform these response patterns in a fast continuous sequence. More details about the response requirements will be given in Part 2. Subjects were seated in a comfortable armchair in a sound-proof, electrically shielded chamber. Stimuli were presented on a 14 inch Multisync monitor with an observation distance of approximately 1.3 m. The presentation of the task was controlled by a PC.

151

force of 2 N. Deviations from this criterion will be described in detail in Part 2. Triggered by signals from the control computer, the data (EEG, EOG, and response force) were stored on another PC and digitized at 200 Hz, from 100 ms before to 1400 ms after the stimulus. Data preparation started with the elimination of rhythmic a-activity by means of an ideal 8-12 Hz digital filter 2 applied in the frequency domain. This procedure was introduced for two reasons: first, to be able to apply very strict artifact criteria (rhythmic a-activity is very often even larger than shift artifacts caused by body movements); and second, to reduce noise caused by phase differences between the two hemispheres that would affect the differential potentials twice as much as normal averages. This procedure affected neither the waveforms nor the amplitudes of the grand averages, but improved the signal to noise ratio in subjects' averages. These a filtered data were checked for artifacts. Trials with zero lines, out-of-scale values, slow drifts larger than 50/~V and fast shifts larger than 80/2V/500 ms were excluded from further analyses. Trials including saccades were rejected. The transmission of vEOG into the EEG was estimated by regression in areas of maximum vEOG variance. Transmission coefficients of hEOG for remaining small horizontal eye movements were taken from the literature (Anderer et al., 1992). EEGs were corrected by subtracting both EOG channels weighted by their transmission coefficients. 2.4. General data analysis 2.4.1. Response parameters Response time was defined as the moment when response force crossed the criterion of 2 N. Trials with incorrect responses were excluded from further analyses.

2.3. Recording and data processing EEG was recorded from Fz, C3', Cz, C4', CP5, CP1, CP2, CP6, P7, P3, Pz, P4, P8, PO5, PO1, PO2, PO6, O1, and O21 using Ag/AgC1 electrodes (Picker-Schwarzer) with electrodes affixed at the mastoids (linked by a 5 k ~ resistor) as reference. Electrooculogram (EOG) was recorded bipolarly both vei~tically from above and below the left eye (vEOG) and horizontally from the outer canthi of both eyes (hEOG). EEG and EOG were amplified and filtered by a Nihon-Kohden 4421 amplifier with 5 s time constant (0.03-70 Hz bandpass). Response force was recorded continuously from isometric weight elements which had to be pressed by the two index fingers. Criterion for a given response was a

I C3' and C4' were 1 cm in front of C3 and C4. P7 and P8 (Pivik et al., 1993) are equivalent to T5 and T6 in the nomenclature of Jasper (1958). PO5 and PO6 are equivalent to electrode sites called OL and OR (e.g. Mangun and Hillyard, 1991).

2.4.2. EEG parameters Detailed data analyses differ between the tasks and therefore will be described below. Difference potentials between hemispheres are reported. These potentials (ERLs) were calculated for all symmetrical electrode pairs (C3'/C4', CP5/CP6, CPI/CP2, P7/P8, P3/P4, PO5/ PO6, PO1/PO2, Ol/O2) in the same way as LRPs are computed (e.g. Gratton et al., 1988), by subtracting the EEG activity ipsilateral to an event from the activity recorded contralateral to the event: ERL(right event) --- ERP(left hemisphere) - ERP(right hemisphere); ERL(left event) = ERP(right hemisphere) - ERP(left hemisphere);

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ERL = [ERL(right event) + ERL(left event)I/2. For all unimanual responses the side of response was taken as side of event. For all bimanual responses and for nogo trials the side of event was defined as the spatial position of the letter. 3. Part 1: Methods 3.1. Task Subjects were instructed to respond to an 'A' with the left index finger and to a 'B' with the right index finger. Since the letters were positioned left or right from the fixation cross, the task was a 'Simon task' (Simon, 1990). Stimulus and response could be either spatially congruent (SC) or spatially incongruent (SI). Subjects were encouraged not to pay attention to the spatial position of the stimulus but to respond as fast as possible to the identity of the letters. 3.2. Data analysis 3.2.1. Response time Differences in response time between the two conditions were analyzed by a paired t test. 3.2.2. ERPs Data were averaged separately for all stimulus response combinations 3. N190, N2, and P3 were measured at lateral electrode sites. N190 was defined as the first distinct negative peak between 150 and 350 ms at P7, P8, PO5, and PO6. N2 was defined as the most negative peak between 250 and 400 ms at C3', C4' and as the second distinct negative peak within the same time window at P7, P8, PO5, and PO6. P3 was defined as the most positive peak between 300 and 600 ms at C3', C4', P7, P8, PO5, and PO6. These electrode sites were selected because lateralizations due to motor activity were expected at C3'1C4', at P7/P8 the largest lateralizations were observed in the present experiment (see below), and at PO5/PO6 lateralizations due to visuo-spatial processes had been reported (e.g. Mangun and Hillyard, 1991). Amplitudes of N190, N2, and P3 were tested separately for each electrode pair in ANOVAs including stimulus side (2), response side (2), and hemisphere (2) as factors. Since the present paper focuses on ERLs, only the interactions of stimulus side by hemisphere and of response side by hemisphere were of further interest. 3.2.3. ERLs ERLs for SC and SI were obtained from the 4 averages as described in Section 2. Measurements were taken in 3 3 (a) Left stimulus-left response (SC); Co) right stimulus-right response (SC); (c) left stimulus-rightresponse(SI); (d) right stimulus-left response(SI).

different ways. First, the maximal lateralization of the early ERL (between 150 and 300 ms) and of the subsequent late ERL (between 250 and 600 ms) was measured for the electrode pairs C3'/C4', P7/P8, and PO5/PO6. The overlap of the time windows for early and late ERLs was due to the interindividual variance of latencies. However, for each subject early and late ERLs were measured as two clearly distinct, consecutive peaks. The early ERL measured in the condition SI was inverted because this component was expected to be more negative contralateral to the stimulus, which would be spatially ipsilateral to the response. Differences between the two conditions were tested by paired t tests. C3'/C4' and P7/P8 were tested additionally by a 2 (anterior/posterior) by 2 (SC/SI) ANOVA. Second, the time course of asymmetries was estimated by point by point t tests for each sampling point. Third, mean amplitudes were calculated from 225 ms to 275 ms (±25 ms around the maximal deflection of the early ERL), and from 25 ms before to 25 ms after the mean response time of each subject (late ERL). Mean amplitudes were tested separately for each channel and each condition against baseline by paired t tests. Additionally two 2 (anterior/posterior) by 2 (early/late ERL) by 2 (SC/SI) ANOVAs were calculated, separately for the comparisons C3'1C4' versus P7/P8 and C3'/C4' versus PO5/PO6. A third area from 75 to 125 ms after each subject's mean response time was analyzed at P7/P8 and at PO5/PO6. Amplitudes of this area were tested against baseline by paired t tests. This quite complicated pattern of analyses was used to contrast the common methods of measuring movementrelated asymmetries (LRP) and attention-related asymmetries (lateralizations of ERP components). 4. Results 4.1. Response times As expected, response times were prolonged when the sides of stimulus and response did not correspond (t(10) = 2.50, P < 0.05). Response times were 437 ± 56 ms for SC and 453 ± 47 ms for SI, respectively. 4.2. Lateralizations of the ERP N190 was larger contralateral to the stimulus at temporo-parietal sites (stimulus side by hemisphere at P7/P8: F(1, 10) = 22.67, P < 0.001; at PO5/PO6: F(1, 10) = 25.45, P < 0.001; see Fig. 1, bold versus thin lines). This effect appears to be pronounced for right stimuli only. However, this result might be due to the overlap of taskrelated asymmetries. No interaction of response side by hemisphere was found (see Fig. 1, left-response versus right-response columns). N2 peaked at about 300 ms and was observed at parietal and temporo-parietal as well as at central electrode

E. Wascher, B. WauschkuhnI Electroencephalography and clinical Neurophysiology 99 (1996) 149-162

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Fig. 1. Grand averagesrecorded in the choice responsetask separatelyfor either stimulus side and responseside for 8 symmetricelectrode pairs. ERPs recorded from the left hemisphericare plotted as bold lines.

sites (see Fig. 1). The analysis of C3'/C4' together with P7/P8 showed a significant interaction of response side by hemisphere by anterior/posterior (F(1, 10)=5.06, P < 0.05). This interaction was caused by increased negativity contralateral to the response at central sites (F(1, 10)= 7.95, P < 0.05) and the lack of such an effect at temporoparietal sites (F(1, 10) = 0.14, P > 0.10). Interactions of stimulus side by hemisphere were not visible in any of the selected electrode pairs. P3 amplitude was reduced at central sites contralateral to the response (C3'/C4'; response side by hemisphere: F(1, 10)= 17.51, P < 0.01) which is equivalent to an increase of negativity within the P3 latency range contralateral to the response (see Fig. 1). At temporo-parietal sites no effect of response side by hemisphere was found. These results were reflected by an interaction of response side by hemisphere by anterior/posterior (C3"1C4" versus P7/P8: F(1,10) = 26.23, P < 0.001). Unexpectedly, P3 increased ipsilateral to the stimulus at PO5/PO6 (F(1, 10) = 10.43, P < 0.01). The following analyses were performed on the difference potentials (Fig. 2).

4.2.1. Maxima of lateralization The first peak of lateralization, measured between 150 and 350 ms at C3"1C4", P7/P8, and PO5/PO6 (mean latency between 250 and 267 ms; see Fig. 2), did not change its latency either as a function of scalp site or condition (SC versus SI). However, its amplitude was larger at temporo-parietal pairs (P7/P8; PO5/PO6) than over the motor cortex (C3'/C4' versus P7/P8: F(1, 10)= 21.27, P < 0 . 0 0 1 ; C3'/C4' versus PO5/PO6: F(1, 10)= 28.29, P < 0.01). The temporo-parietal amplitudes did not differ from each other (P7/P8 versus PO5/PO6: F(1, 10) = 0.39, n.s.). The second peak of lateralization measured between 300 and 600 ms at C3"1C4" had its maximum around the time of response. Consequently, shorter latencies were found for SC than for SI (t(10)= 2.38, P < 0.05). There was no difference in amplitude between the two conditions (t(10) = 0.45). 4.2.2. Time course of lateralization t tests against baseline for the difference potentials (see Fig. 2) showed significant deflections due to an increase of negativity contralateral to the stimulus at C3"1C4", be-

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Fig. 2. Topography of ERLs recorded in the choice response task for spatially congruent (bold lines) and spatially incongruent (thin lines) stimulus and response locations. For left- and for right-hand responses the EEG activity from the hemisphere ipsilateral to the responding hand was subtracted from the EEG activity contralateral to the responding hand. These difference potentials (asymmetries) for left-hand and for right-hand responses were collapsed. Negativity eontralateral to the responding hand ('correct') is plotted upwards. At central sites an asymmetry related to the stimulus position (early ERL between 150 and 300 ms) was followed by an asymmetry related to the response side (late ERL). The early ERL was largest at P7/P8. It was followed by an 'incorrect' asymmetry due to an increase of negativity ipsilateral to the responding hand at parietal and temporo-parietal sites.

ginning at 215 ms (SC) and 250 ms (SI), respectively. While for SI the difference potential returned to baseline and changed its polarity due to increasing negativity contralateral to the response (becoming significant at 325 ms), the laterality for SC was sustained until 515 ms after the stimulus. However, a local m i n i m u m was found at 310 ms, equivalent to the moment when lateralization reversed for SI. A t posterior sites, the lateralization differed earlier from baseline than at central sites (P7/P8: SC 180 ms, SI 150 ms; PO5/PO6: SC 170 ms, SI 150 ms). It disappeared after 275 ms in both conditions at P7/P8 and only a short time later (SC 285 ms, SI 290 ms) at PO5/ PO6 (see Fig. 2). 4.2.3. Mean amplitudes The first lateralization (250 _+ 25 ms; see Table 1, A1)

was larger at temporo-parietal sites, whereas the late lateralization (mean response time _+ 25 ms; see Table 1, A2) was maximal at central sites. Both analyses (C3'/C4' versus P7/P8 and C3"1C4' versus PO5/PO6) showed an interaction o f early/late lateralization by anterior/posterior topography (C3'1C4' versus P7/P8: F(1, 1 0 ) = 47.89, P < 0.001; C3'/C4' versus PO5/PO6: F(1, 1 0 ) = 7 0 . 6 4 , P < 0.001). The first lateralization was due to an increase o f negativity contralateral to the letter and was largest over temporo-parietal sites. The second lateralization was due to an increase of negativity contralateral to the response at central sites. At parietal and at temporo-parietal sites a reversed lateralization was visible, that was different from baseline at the moment of response for SI only. Fig. 2 shows that this 'ipsilateral' deflection developed later than the 'contralateral' distribution o f the LRP. Therefore,

E. Wasclu;r, B. Wauschkuhn/ Electroencephalography and clinical Neurophysiology 99 (1996) 149-162

155

Table 1 Mean amplitudes, t values (test against baseline) of ERLs in the choice response task C3'/C4'

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3.77 -2.96 -0.18 -2.73 -4.72 -4.28

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1.72 -1.48 0.28 -1.22 -0.97 -1.54

4.79 -3.50 0.80 -3.05 -3.75 -5.87

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Amp, mean amplitudes (positive values correspond to 'correct' activation, i.e. increased negativity contralateral to the required response); A1, mean amplitude around the maximum of the first ERL (±25 ms); A2, mean amplitude around the response (±25 ms); A2', mean amplitude around 50 ms after the response (i.e. 25-75 ms after the response). ***P < 0.001; **P < 0.01; *P < 0.05; #P < 0.10.

a third area (see Table 1, A2") was introduced. Between 75 ms and 125 ms after the response a strong lateralization was found for both conditions with more negativity ipsilateral to the response.

5. Discussion The peak amplitude of the N190 was enhanced contralateral to the relevant stiimulus. The analysis of ERLs showed that this lateralization reached its maximum at 250 ms, 60 ms later than the peak of the N190. Additionally, the ERLs added information about the functional meaning of the early deflection of the LRP in this paradigm using lateral stimuli. The first peak of the LRP was strongly influenced by posterior, stimulus-related asymmetries. The second asymmetry found at central sites appeared to be purely movement-related. The third lateralization of interest in the present data was inverse to the central motor-related lateralization at the parietal cortex, beginning after the initiation of the response. This posterior inversion can already be seen in the data of Kutas and Donchin (1980) but was not discussed in that paper. It might be due to reafferences from the finger that had been moved. The inverse polarity of this component might be due to surface-to-depth current flow in pyramidal tract neurons in the somatosensory cortex (Lang et al., 1994). While Luck and Hillyard (1994) reported an N2pc (where pc is 'posterior, contralateral'), in the present study an ' N 1 9 0 p c ' was found. The scalp distribution of these two components seems to be very similar. However, no stimulus-related later~dization of the succeeding N2 component was found in the present study. Most probably, this component was already overlapped by responserelated asymmetries. Nevertheless, the difference waves presented by Luck and Hiillyard (1994, Fig. 7) and by the present study are almost identical. In both studies, the asymmetry started at about 180 ms, reached its maximum at 250 ms, and decreased to a minimum at 300 ms. There-

fore, the same phenomenon was reflected in lateralizations of different negative ERP components. The most parsimonious conclusion is that the asymmetry is not a feature of these different ERP components but results from an endogenous component that overlaps them. Additionally, the asymmetry reached its maximum over the temporo-parietal cortex, whereas the N190pc and the N2pc were located rather occipitally. The temporal characteristics and the topography of the first asymmetry might be an argument in favor of a nonmotoric interpretation of the early LRP (e.g. that it is an effect of volume conduction from stimulus-related temporo-parietal asymmetries). On the other hand, the early LRP might reflect response tendencies towards the source of stimulation initiated by the spatial coding of the stimuli, which is reflected by posterior asymmetries within the same time range. To clarify the nature of the early LRP, ERLs were investigated for 6 additional response requirements, reported in Part 2.

6. Part 2: Methods 6.1. Tasks 6.1.1. Simple responses

Subjects were instructed to respond to every stimulus as fast as possible but to avoid premature responses. In one block the response was unimanual but the side of response changed in the middle of the block. In a second block subjects had to respond bimanuaUy, i.e. with a simultaneous press of both hands. Subjects were told that each stimulus would consist of a letter and a filler as in the other response requirements but they should not pay attention to the letters. Since the letters were positioned left or right from the fixation cross, letter and response could be SC or SI for unimanual responses. For bimanual responses, one hand was SC and the other hand SI to the letter.

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6.1.2. Go/nogo responses Subjects were instructed to respond to one letter and not to the other (half of the subjects responded to an A, the other half to a B) as fast as possible. As for simple responses in one block, the response was unimanual but the side of response changed after half of the trials. In a second block subjects had to respond bimanually. Subjects were told to ignore the position of the letter. 6.2. Response patterns

In two blocks subjects were instructed to respond with a response pattern that was composed of a bimanual simple response and a unimanual choice response. These two responses had to be performed in a fast sequence. Subjects were instructed to separate the two responses by relaxing the limb involved in the first response before starting the second response. In two separated blocks subjects had to perform first the simple then the choice response (simple and choice) or the other way around (choice and simple). 6.3. Data analysis 6.3.1. Response time Both for simple and for go/nogo responses response times were analyzed in a 2 (SUSC) by 2 (number of hands) ANOVA. For response patterns the criterion for the first responses was defined as in the previous tasks (force > 2 N). However, the criterion for a fully correct response pattern was a decrease of force after the first response by at least 1 N followed by a renewed increase of at least 1 N. Response times for the first response were compared to the response times of equivalent responses in other tasks, i.e. the first response in the choice & simple task was analyzed together with the response times of the choice response reported in Part 1 by a 2 (task) by 2 (SC/SI) ANOVA and the first response in the simple & choice task was analyzed together with the bimanual simple response. 6.3.2. EEG Amplitudes of ERLs were measured from 225 to 275 ms (+_25 ms around the maximal deflection of the early ERL reported in Part 1), and from 25 ms before to 25 ms after the mean response time of each subject (late ERL). Mean amplitudes were tested separately for each channel and each condition against baseline by paired t tests. Additionally, two 2 (anterior/posterior) by 2 (early/late ERL) by 2 (SC/SI) ANOVAs were calculated, separately for the comparisons C3'/C4' versus P7/P8 and C3"/C4" versus PO5/PO6, respectively. A third area from 75 to 125 ms after the mean response time of each subject was analyzed at P7/P8 and at PO5/PO6. Amplitudes of this area were tested against baseline by paired t tests.

7. Results 7.1. Response times

For simple responses, response times differed neither between uni- and bimanual conditions (number of hands: F(1, 10)=0.03, n.s.) nor due to spatial compatibility (F(1, 10)=0.91, n.s.). Response times for unimanual responses were 239 +- 34 ms for SC and 241 +- 35 ms for SI. Mean response times for bimanual responses were 241 + 36 ms both for SC and SI. Go/nogo responses were faster if stimulus and response were located at the same side than if they were not (compatibility: F(1, 10) = 5.08, P < 0.05). This effect tended to be larger for unimanual responses (SC 388 ms, SI 403 ms) than for bimanual responses (SC 397 ms, SI 398 ms; number of hands by compatibility: F(1, 10)= 4.29, P < 0.10). Responses were significantly delayed for the choice response in the choice and simple response pattern compared to the single choice responses that were reported in Part 1 (F(1, 10)= 6.07, P < 0.05). The effect of compatibility (F(1, 10)= 19.48, P < 0 . 0 1 ) did not differ between these two tasks (task by compatibility: F(1, 10) = 1.46, n.s.): responses at the same spatial location as the relevant stimulus were faster (480 ms) than responses at the opposite spatial location (509ms; t(10) = 3.54, P < 0.01) even if the choice response was part of a response pattern. Responses were also significantly delayed for the bimanual simple response in the simple & choice response pattern compared to single bimanual simple responses (F(1, 10)=14.44, P < 0 . 0 1 ) . Though followed by a choice response, the first response was not affected by the location of the stimulus (SC 314ms, SI 310ms; compatibility: F(1, 10)=0.49, n.s.; task by compatibility: F(1, 10) = 0.66, n.s.). 7. 2. ERLs 7.2.1. Simple responses Since mean response times and the latency of maximum early lateralization (250 ms) coincided for the simple response task, it is quite probable that also the different types of ERLs overlap. Especially for the unimanual task, stimulus-related processes, motor preparation and reafferences might influence ERLs within the same time range. This is demonstrated in Fig. 3 (left panel; see also Table 2). Over the motor cortex the ERL was dominated by the LRP. No incorrect lateralization was visible for SI but the LRP appeared to be markedly smaller than for SC (1.07 p V versus 1.97/~V for the stimulus-locked ERL and 1.07/~V versus 1.65/zV for the movement-locked ERL; see Table 2). It can be argued that these lateralizations are an addition of motor- and attention-related lateralizations. While the motor-related lateralization is of equal polarity

E. Wasctu.r, B. Wauschkuhn / Electroencephalography and clinical Neurophysiology 99 (1996) 149-162

157

Simple response

response attentionrelatedERLextracted unimanual

unimanual response

bimanual response

C3'/C4'

~

~

P7/P8

~

l.VE PO5/P06

-response I

/

I

I 0

L

/~--~

~

J

I

~

L.

I

t

J

J

L

._~

_.1 lOOms

Fig. 3. ERLs recorded in the simple response task at central and two selected posterior sites. The left panel shows the ERLs for the unimanual responses for SC (bold lines) and SI (thin lines) stimulus and response locations. These ERLs are charaeterized by the overlap of response- and stimulusrelated asymmetries. For bimanual responses only stimulus-related asymmetries were found. The extraction of the stimulus-related ERLs from unimanual responses (right panel) demonstrates the additivity of stimulus and response related ERLs (right panel - middle panel = left panel). in both conditions, the ;reversed polarity o f attentionrelated E R L s for S I r e d u c e d the a m o u n t o f the lateralization. O v e r the t e m p o r o - p a r i e t a l c o r t e x a t e n d e n c y o f correct/incorrect lateralizations for S C / S I is visible at about 200 ms but starting at 2510 m s an a s y m m e t r y inverse to the L R P is visible for both c o n d i t i o n s at posterior sites. O v e r the temporo-pariet~d cortex the bipolar attentionrelated lateralization was s u p e r p o s e d by this unipolar a s y m m e t r y . This c o m p l e x system o f overlaps is expressed in an interaction o f anterior/posterior by c o m p a t i b i l i t y

( C 3 ' / C 4 ' versus P7/P8: F ( 1 , 1 0 ) = 24.09, P < 0 . 0 0 1 ; C 3 7 C 4 ' versus P O 5 / P O 6 : F(1, 10) = 29.53, P < 0.001). T h e E R L s in the b i m a n u a l task (see Fig. 3, m i d d l e panel) should lack both the m o t o r - r e l a t e d lateralization and the lateralization due to reafferent signals. T h e r e f o r e , the E R L should represent the attention-related lateralizations only. Indeed, the lateralization was larger at temporo-parietal sites than at central sites. T h e lateralization at central sites did not reach significance, but the amplitude was about half the d i f f e r e n c e b e t w e e n the E R L s for

Table 2 Mean amplitudes, t values (test against baseline) of ERLs in the simple response tasks C3'/C4'

AI

A2

lh SC lh SI 2h l h SC lh SI 2h

P7/P8

PO5/PO6

Amp

t

P

Amp

t

P

Amp

1.97 1.0"t 0.21[ 1.65 1.07 0.2,1

4.58 2.48 1.77 4.39 3.27 2.00

** * ** ** #

0.13 -1.49 0.84 -0.06 -1.22 0.88

0.30 --4.81 3.98 -0,14 -4.48 3.78

** ** ** **

0,04 -1.05 0,56 -0.11 -0.79 0.61

Amp, AI, A2, see legend to Table 1; lh, unimanual response; 2h, bimanual response. ***P < 0.001; **P < 0.01; *P <: 0.05; #P < 0.10.

P 0.13 -3.64 2.90 -0.39 -2.98 3.06

** * * *

158

E. Wasche r, B. Wauschkuhn / Electroencephalography and clinical Neurophysiology 99 (1996) 149-162

go-nogo unimanual response

unimanual response

bimanualresponse

movement-related asymmetry

correct T incorrect ~¢ _

_P7/P8p4~

1 laV[A

PO5/P06

,

,-resp°nre

j/~,,,~"

I I I

o lOOms

Fig. 4. ERLs recorded in the go/nogo task at central and two selected posterior sites. The left panel shows the ERLs for the unimanual responses for SC (bold lines) and SI (thin lines) stimulus and response locations and for nogo stimuli (dotted lines). Nogo stimuli presented left or fight are collapsed, using the same procedure as for left and fight responses. The side of event was defined as the spatial position of the letter. After elimination of the stimulus-related ERL evoked by nogo stimuli (middle panel) pure motor-related asymmetries were expected. However, there might be remaining stimulus related asymmetry at 200-300 ms. The asymmetry in this time range might, on the other hand, be due to a inhibition of stimulus-related asymmetries with nogo stimuli as shown in the fight panel for bimanual responses. SC and SI in the unimanual condition. Additivity was more evident for the temporo-parietal sites. When the amplitudes o f the lateralization were measured for the first area, the average o f the lateralizations of unimanual SC and SI was almost exactly the lateralization of the bimanual task (see Table 2; P7/P8, 0.84 versus 0.82/zV; PO5/PO6, 0.56 versus 0.55/~V). This additivity is graphically demonstrated in the right panel of Fig. 3 where the lateralizations for the unimanual task were recalculated by subtracting the attention-related lateralizations found in the bimanual task 4. Thus, all three kinds of lateralizations reported in Part 1 were found for simple responses as well. Since the letters were o f no informational value the amplitude of the attention-related temporo-parietal peak was reduced but not diminished. The remaining early asymmetry might be due to the distinction between the letter and the filler even 4 The ERLs of the fight panel of Fig. 3 were calculated as follows: ERL(SC)' = ERL(SC) - ERL(bimanual); ERL(SI)' = ERL(SI) + ERL (bimanual).

if this distinction was not task-relevant, since the letter was task-relevant in all other tasks. Both the motorrelated central lateralization and the temporo-parietal lateralization due to reafferent signals were overlapped by the attention-related lateralization. 7.3. Go/nogo responses

The left panel of Fig. 4 (see also Table 3) shows that the lateralization at temporo-parietal sites was almost the same size for go and no-go stimuli in the unimanual task. The central lateralization was sustained for SC stimuli (in contrast to Part 1 where a minimum was visible at 310 ms). This effect was due to the slightly shorter response times in the go/nogo task compared to the choice response task. For SI the LRP started later and seemed to be of shorter duration. However, the overlap of attentionand motor-related lateralizations may confound these results as it was demonstrated for simple responses. Therefore, the middle panel of Fig. 4 shows the ERLs for the unimanual task after subtraction of the ERLs

E. Wasctu,r, B. Wauschkuhn / Electroencephalography and clinical Neurophysiology 99 (1996) 149-162

159

Table 3 Mean amplitudes, t values (test against baseline) of ERLs in the go/nogo tasks C3'/C4'

P7/P8

PO5/PO6

Amp

t

P

Amp

t

P

Amp

t

P

A1

lh lh lh 2h 2h

SC SI nogo go nogo

i .27 -0.11 0.47 0.8(I 0.2[~

4.76 -0.34 4.19 2.95 1.43

*** ** * -

1.74 -2.50 1.45 1.93 1.02

6.03 -6.91 5.25 3.31 2.38

*** *** *** ** *

1.74 -2.37 1.27 1.99 0.80

4.97 -5.57 3.81 3.83 2.24

*** *** ** ** *

A2

lh lh 1h 2h 2h

SC SI nogo go nogo

1.74 1.2(1 . 0.40 .

4.26 3.65

** **

-0.36 -1.37 . 0.18 .

-0.98 --4.04

**

-0.49 -1.02 . 0.11 .

-1.92 -3.02

# *

0.30

-

.

. 2.06

.

. #

.

.

.

. 0.38

.

. -

.

.

Amp, A1, A2, see legend to Table 1; lh, 2h, see legend to Table 2. ***P < 0.001; **P < 0.01; *P < 0.05; #P < 0.10.

evoked by nogo stimuli. Since the nogo-ERLs should be evoked by attention-related processes and by automatic response tendencies, the result of this subtraction should represent motor activity c,nly. In fact the resulting lateralization at central sites is dominated by the LRP. This LRP seems to start still earlier for SC. However, the temporal difference between tlhe lateralizations evoked for SC and SI was much smaller than before and corresponded with the difference in response times. At about 200300 ms a residual lateralization due to stimulus position is visible at temporo-parietal sites. Although not symmetrical for SC and SI, this re:~idual lateralization might have been due to larger early lateralization for go stimuli than for nogo stimuli. This effect is also seen for bimanual responses (see Fig. 4, right panel).

7.4. Choice & simple response patterns The ERLs evoked in this task look very similar to the ERLs evoked in the choice response task (Part 1; see Fig. 2 and Fig. 5, left panel (note that the time scale is different from previous figures)). However, one difference has to be mentioned: the early ERL (see Table 1, area A1 and Table 4, area A1 for cho si) seems to be more symmetrical for the response pattern than for the task of Part 1. No systematic lateralizations were found due to movement preparation of the second (bimanual) response (see Fig. 5 and Table 4, A3).

7.5. Simple & choice response patterns If the subjects had to perform a simple response first, the early component of the ERL seemed to be reduced at posterior channels (see Fig. 5, right panel, and Table 4, A1, si_cho). However, it was followed by a second stimulus-related asymmetry preceding the choice response. At central sites an increase of negativity contra-

lateral to the hand that had to be selected for the choice response is visible for the first time after the second stimulus-related lateralization. However, the fact that the second stimulus-related ERL at central sites was shifted above the baseline indicates that this component was already overlapped by a LRP, reflecting the tendency to prepare the correct hand. 8. Discussion

8.1. Response times Not only for choice responses but also for unimanual go/nogo responses and for choice responses at the beginning of a response pattern, an advantage of the hand ipsilateral to the relevant stimulus was found. Unimanual simple responses were not affected by the side of the letter. Equally, bimanual responses were not affected by the side of the letter, neither as single simple or go/nogo responses, nor at the beginning of a response pattern. To summarize, only unimanual responses that required response selection (go versus nogo; left versus right hand) were affected by the spatial location of the stimulus. The fact that bimanual responses were never affected by stimulus location deserves some comment. Both index fingers were moved simultaneously even if the bimanual movement was followed by a choice response and therefore the letter had to be attended. Thus, bimanual responses were planned as a compound movement. Also, for response patterns concerted planning might have been the reason for the delay of the first response compared to the equivalent single response. However, the initial simple responses in the simple & choice task were still faster than choice responses. Thus, the results suggest that even though response patterns were planned as compound movements, responses were initiated before the planning of the pattern was complete.

E. Wascher, B. Wauschkuhn /Electroencephalography and clinical Neurophysiology 99 (I 996) 149-162

160

responsepattern choice=>simple correct

simple=>choice

I

-C3'/C4:

A,~ ~ A . , d a ~ ,. ,s.~l,...t~'~ w'"'1,~1,_ t ~ "~ , , l ~

~

incorrect

t

i

I

I

I

I

1

i

I

I

0 lOOms

Fig. 5. ERLs recorded for response patterns at central and two selected posterior sites for spatially congruent (bold lines) and spatially incongruent (thin lines) stimulus and response locations. The left panel shows the ERLs from the choice & simple condition which is dominated by the choice response, and therefore very similar to Fig. 2. The right panel shows the ERL for the simple & choice condition. The main difference from all previously shown ERLs is a dual stimulus-related asymmetry.

8.2. ERLs

Three ERL components were found independently of response requirements in all tasks: (1) an increase of negativity contralateral to the stimulus, maximal at posterior sites; (2) an increase of negativity contralateral to the response, maximal over the motor cortex; and (3) an increase of negativity ipsilateral to the motor asymmetry at posterior sites. The second and the third asymmetry were time-locked to the response. The maximum of the second asymmetry coincided with the response time, and therefore represented response preparation, whereas the third asymmetry started about 20--30 ms after the beginning of the movement. This asymmetry might be due to reafferent information from the finger that had to be moved. Thus, whatever the precise functional role of the second and the third asymmetry might be, it is quite clear that these components are tightly coupled to response processing. In contrast, the meaning of the first asymmetry remains hard to understand. One reason for this difficulty might be that central and parietal asymmetries within this time range hitherto have been reported in

separate publications with different focuses of interest. While studies which reported parietal asymmetries (e.g. Mangun and Hillyard, 1988, 1991) focused on attentional effects, studies on early central asymmetries discussed these asymmetries in terms of response conflicts only (Coles et al., 1988, 1992; Smid et al., 1991, 1992; Eimer, 1995). The present results suggest that this distinction fails to explain the whole phenomenon. The analysis of the topography of the first ERL and the use of several response requirements provided valuable new information. Although the first posterior ERL occurred contralateral to the letter in all tasks, its magnitude was not determined exogenously by the appearance of lateral information. Rather, the size of the first ERL changed with task requirements: it was (1) reduced in simple response tasks, (2) smaller for nogo than for go trials, and (3) appeared twice in the simple & choice task, the first time following the presentation of the stimulus (as in all other tasks) and the second time between the execution of the initial simple response and the subsequent choice response. These results suggest that the first ERL reflects a shift of attention towards the relevant in-

E. Wascher, B. Wauschkuhn / Electroencephalography and clinical Neurophysiology 99 (1996) 149-162

161

Table 4 Mean amplitudes, t values (test a[;alnst baseline) of ERLs in the response pattern tasks C3'/C4' Amp A1

A2

A3

cho_si $C cho_si SI si_cho SC si_cho SI cho_si SC cho_si SI si_cho SC si_cho SI cho_si SC cho_si SI si_cho SC si_cho SI

0.75 --0.58 0.70 -0.31 1.25 1.10 0.69 -0.41 -0.04 0.11 1.60 1.51

P7/P8 t 2.45 -4.89 3.17 -1.41 ,- 3.85 3.21 3.68 -2.30 -0.16 0.24 3.48 8.55

PO5/PO6

P

Amp

t

P

Amp

t

P

* *** * ** ** ** * ** ***

1.70 -1.94 1.40 -0.98 -0.23 -1.30 1.45 -0.49 0.21 -0.98 -0.14 0.06

4.12 -4.46 2.79 -2.60 -0.06 -2.57 2.49 -1.26 0.61 -3.63 -0.23 0.19

** ** * * * * ** -

1.49 -2.16 1.11 -1.05 0.00 -1.52 0.99 -0.63 -0.13 -1.39 - 1.15 -0.73

4.31 --4.27 2.95 4.15 -0.01 -2.64 2.22 -1.53 -0.47 -2.54 -2.37 -2.06

** ** * ** * # * * #

Amp, A1, A2, see legend to Table 1; A3, mean amplitude around the maximum of the second posterior ERL (±25 ms); cho_si, choice & simple response; si_cho, simple & choice response. ***P < 0.001 ; **P < 0.01; *P < I).05; #P < 0.10.

formation. This conclusio~L is strongly in accordance with the interpretation proposed by Luck and Hillyard (1994) for the N2pc, measured to laterally presented pop-out stimuli. Rizzolatti et al. (1987) argued in their 'premotor theory of attention' that the attraction of attention by a lateral stimulus includes the programming of a saccade. The saccade may either be inhiLbited (covert attention) or executed (overt attention; cf. Posner, 1980) but the process of attention displacement and eye movement programming remains the same. Umilt~ and Nicoletti (1992) used the premotor theory of attention to account for spatial stimulus-response compatibility. They argued that due to the unconditional shift of attention, an irrelevant spatial code is formed that might inte,rfere with the relevant spatial response code delivered by the identity of the stimulus if the position of the stimulus and the side of response (that is coded in the stimulus) are not the same. The early ERL might reflect a shift of covert attention. An argument for this interpretation is the onset latency of the early ERL (about 200 ms) which is very similar to the latency of stimulus induced saccades (overt shift of attention) towards a lateral object (e.g. Bekkering et al., 1994). The effect of spatial stimulus-response compatibility was shown in the present data for all unimanual responses which required identification of the laterally presented letter in the bilateral stimulus array. Up to this point the present results are in accordance with the theories quoted above, but the fact that for the go/nogo task an effect of spatial stimulus-response compatibility was found argues against them. In this task no lateral response code was necessary because the decision was not 'left versus right' but 'go versus nogo'. This decision should not interfere with the spatial code of the stimulus. Therefore, the present data suggest that the ,;hift of attention towards a spatial location includes not only a tendency to move the

eyes towards this location but a more general tendency to move towards this location. The reciprocal influence of attention- and movementrelated processes, which was hypothesized to be reflected in the early asymmetry, might either be mediated by subcortical systems (Mangun et al., 1994) or be due to functional interactions inside the cortex. Evidence for corticocortical interaction was presented in a PET study by Corbetta et al. (1993). They report that both motor cortex and SMA were more activated when the stimuli were presented in the visual field ipsilateral to the responding hand. Additionally, stronger SMA and motor cortex activity was obtained when distractors were presented ipsilateral rather than contralateral to the movement (Corbetta et al., 1993). Single cell recordings in animals showed that there might be motor cells and interfacing cells next to sensory cells both in motor and in sensory areas of the cortex (Requin et al., 1988; Graziano et al., 1994). Therefore, a plain distinction between sensory and motor areas of the cortex, as well as between stimulus- and movement-related processes, has to be reconsidered. In summary, the combination of different response requirements with the measurement of ERLs of the EEG in high topographic resolution provided valuable information about the interaction of stimulus- and responserelated processes.

Acknowledgements The authors thank Rolf Verleger for many helpful comments on earlier drafts of the manuscript and support during all stages of the work, Meike Reinhard for conducting the experiments, and Piotr Jaskowski for introducing force measurement to our laboratory. This research was supported by a grant from the 'Deutsche Forschungsgemeinschaft' (Ve110/2-3) to Rolf Verleger.

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References Anderer, P., Semlitsch, H.V., Saletu, B. and Barbanoj, M.J. Artifact processing in topographic mapping of electroencephalographic activity in neuropsychopharmacology. Psychiat. Res. Neuroimaging, 1992, 45: 79-93. Bekkering, H., Adam, J.J., Kingma, H., Huson, A. and Whiting, H.T.A. Reaction time latencies of eye and hand movements in single- and dual task conditions. Exp. Brain Res., 1994, 97: 471--476. Coles, M.G.H., Gratton, G. and Donchin, E. Detecting early communication: using measures of movement-related potentials to illuminate human information processing. Biol. Psychol., 1988, 26: 69-89. Coles, M.G.H., Gehring, W.J., Gratton, G. and Donchin, E. Response activation and verification: a psychophysiological analysis. In: G.E. Stelmach and J. Requin (Eds.), Tutorials in Motor Behavior II. Elsevier, Amsterdam, 1992, pp. 779-792. Corbetta, M., Miezin, F.M., Shulman, G.L. and Petersen, S.E. A PET study of visuospatial attention. J. Neurosci., 1993, 13: 1202-1226. De Jong, R., Wierda, M., Mulder, G. and Mulder, Ld.M. Use of partial information in responding. J. Exp. Psychol. Hum. Percept. Perform., 1988, 14: 682-692. Donders, F.C. On the speed of mental processes. Onderzoekingen gedaan in het Physiologisch Laboratorium derUtrechtsche Hoogeschool, Tweede reeks, 1868-69, II, pp. 92-120. Translation by W.G. Koster in W.G. Koster (Ed.), Attention and Performance II. North-Holland, Amsterdam, 1969, pp. 412-431. Eimer, M. Stimulus-response compatibility and automatic response activation: evidence from psychophysiological studies. J. Exp. Psychol. Hum. Percept. Perform., 1995, 21 : 837-854. Gratton, G., Coles, M.G.H., Sirevaag, E.J., Eriksen, C.W. and Donchin, E. Pre- and post-stimulus activation of response channels: a psychophysiological analysis. J. Exp. Psychol. Hum. Percept. Perform., 1988, 14: 331-344. Graziano, M.S.A., Yap, G.S. and Gross, C.G. Coding of visual space by premotor neurons. Science, 1994, 266: 1054-1057. Jasper, H.H. The 10-20 electrode system of the International Federation. Electroenceph. clin. Neurophysiol., 1958, 10: 371-375. Kutas, M. and Donchin, E. Preparation to respond as manifested by movement-related potentials. Brain Res., 1980, 202:95-115. Lang, W., H611inger, P., Eghker, A. and Lindinger, G. Functional localization of motor processes in the primary and supplementary motor areas. J. Clin. Neurophysiol., 1994, 11: 397-419. Luck, S.J. and Hillyard, S.A. Electrophysiologicai correlates of feature analysis during visual search. Psychophysiology, 1994, 31: 291308. Mangun, G.R. Neural mechanisms of visual selective attention. Psychophysiology, 1995, 32, 4-18. Mangun, G.R. and Hillyard, S.A. Spatial gradients of visual attention:

behavioral and electrophysiological evidence. Electroenceph. clin. Neurophysiol., 1988, 70: 417--428. Mangun, G.R. and Hillyard, S.A. Modulations of sensory-evoked brain potentials indicate changes in perceptual processing during visualspatial priming. J. Exp. Psychol. Hum. Percept. Perform., 1991, 17: 1057-1074. Mangun, G.R., Hillyard, S.A., Luck, S.J., Handy, T., Plager, R., Clark, V.P., Loftus, W. and Gazzaniga, M.S. Monitoring the visual world: hemispheric asymmetries and subcortical processes in attention. J. Cogn. Neurosci., 1994, 6: 267-275. Osman, A., Bashore, T.R., Coles, M.G.H., Donchin, E. and Meyer, D.E. On the transmission of partial information: inferences from movement-related brain potentials. J. Exp. Psychol. Hum. Percept. Perform., 1992, 18: 217-232. Pivik, R.T., Broughton, R.J., Coppola, R., Davidson, R.J., Fox, N. and Nuwer, M.R. Guidelines for the recording and quantitative analysis of electroencephaiographic activity in research contexts. Psychophysiology, 1993, 30: 547-558. Posner, M.I. Orienting of attention. Q. J. Exp. Psychol., 1980, 32: 3-25. Requin, J., Riehle, A. and Seal, J. Neural activity and information processing in motor control: from stages to continuos flow. Biol. Psychol., 1988, 26: 179-198. Rizzolatti, G., Riggio, L., Dascola, 1. and Umilt& C. Reorienting attention across the horizontal and vertical meridian: evidence in favor of a premotor theory of attention. Neuropsychologia, 1987, 25:3140. Simon, J.R. The effects of an irrelevant directional cue on human information processing. In: R.W. Proctor and T.G. Reeve (Eds.), Stimulus-Response Compatibility. Elsevier, Amsterdam, 1990, pp. 31-86. Smid, H.G.O.M., Lamain, W., Hogeboom, M.M., Mulder, G. and Mulder, L.J.M. Psychophysiological evidence for continuous information transmission between visual search and response processes. J. Exp. Psychol. Hum. Percept. Perform., 1991, 17: 696-714. Smid, H.G.O.M., Mulder, G., Mulder, L.J.M. and Brands, G.J. A psychophysiological study of the use of partial information in stimulusresponse translation. J. Exp. Psychol. Hum. Percept. Perform., 1992, 18: 1101-1119. Umilt~, C. and Nicoletti, R. An integrated model of the Simon effect. In: J. Alegria, D. Holender, J. Junta de Morais, and M. Radeau (Eds.), Analytic Approaches to Human Cognition. Elsevier, Amsterdam, 1992, pp. 331-350. Vaughan, H.G., Costa, L.D. and Ritter, W. Topography of the human motor potential. Electroenceph. clin. Neurophysiol., 1968, 25: 110. Yamaguchi, S., Tsuchiya, H. and Kobayashi, S. Electroencephalographic activity associated with shifts of visuospatial attention. Brain, 1994, 117: 526-553.