P3 latency shifts in the attentional blink: further evidence for second target processing postponement.

A rapid serial visual presentation technique was used to display sequentially two targets, T1 and T2, and monitor P3 amplitude and latency variations associated with the attentional blink (AB) effect. A red T1 digit was embedded on each trial in a sequence of black letters. T2 was either masked by a trailing stimulus or not masked. T1 had to be identified on a proportion of trials, or ignored in other trials. T2 was the black letter 'E' on 20% of the trials, or any other non-'E' black letter in the other 80% of the trials. A delayed 'E' detection task was required at the end of each trial. An AB was observed when T1 had to be reported and T2 was masked. The AB effect was associated with a sizable amplitude reduction of the P3 component time locked to T2 onset. When T2 was not masked, no AB or P3 amplitude variations were observed. When T1 had to be reported, a delayed P3 peak latency was observed at short compared to long T1-T2 intervals. No effect of T1-T2 interval was observed on the T2-locked P3 peak latency when T1 could be ignored. Taken together these findings provide converging evidence in support of temporal attention models bridging behavior and electrophysiology that postulate a direct link between the cause of the AB effect and the sources of both amplitude and latency variations in the T2-locked P3 component.

Validity and boundary conditions of automatic response activation in the Simon task.

Three experiments were conducted to determine whether spatial stimulus-response compatibility effects are caused by automatic response activation by stimulus properties or by interference between codes during translation of stimulus into response coordinates. The main evidence against activation has been that in a Simon task with hands crossed, responses are faster at the response location ipsilateral to the stimulus though manipulated by the hand contralateral to the stimulus. The experiments were conducted with hands in standard and in crossed positions and electroencephalogram measures showed coactivation of the motor cortex induced by stimulus position primarily during standard hand positions with visual stimuli. Only in this condition did the Simon effect decay with longer response times. The visual Simon effect appeared to be due to specific mechanisms of visuomotor information transmission that are not responsible for the effects obtained with crossed hands or auditory stimuli.

ERP correlates of associative learning.

We examined changes of event-related potentials (ERPs) while participants learned stimulus-to-stimulus relations in an S1-S2 task. The design allowed for separating processes of associative learning from nonspecific effects. Participants had to respond to S2 by a left or right key-press dependent on S2 identity (letter W or M). Preparation for S2 could be improved by using the associative information given by S1. The S1 was an arrow pointing to the left or right. In combination with its color, arrow direction was informative about location and identity of S2, but participants were not informed about the relevance of color. Arrows in two of the colors were fully predictive for the S2 whereas the third color gave no valid information. This third stimulus controlled for habituation and procedural learning. Six blocks with 200 trials each and all three S1 colors in random order were presented. Behavioral and ERP differences in each block between "learning" and control trials were used to identify processes of associative learning. Several effects of associative learning were identified indicating the involvement of specific stages of information processing: a continuous increase of P3 amplitude evoked by S1 was accompanied by a decrease of P3 evoked by S2. These changes reflected the modifications of stimulus weights for response selection and the strengthened association between the two stimulus complexes in the time course of learning. The related motor preparation benefited from learning too, expressed in a decrease of CNV amplitude and an increase of LRP amplitude. Finally a decrease of N1 amplitude evoked by S2 indicated the reduced need to allocate spatial attention to the S2 location according to the learned meaning of S1.

The influence of time pressure and cue validity on response force in an S1-S2 paradigm.

Hypotheses about variations of response force have emphasised the influences of arousal and of motor preparation. To study both types of influences in one experiment, the effects of time pressure and of validity of S1 were investigated in tasks wherein a first stimulus (S1) indicated the most probable response (80% valid) required after a second stimulus (S2). Under time pressure, responses were executed more forcefully while, as could be expected, response times were shorter and errors were more frequent. This pattern of results was not only obtained when time pressure was varied between blocks, but also when varied from trial to trial, by information given by S2. Also invalidly cued responses were executed more forcefully but, as could be expected, in contrast to time pressure, response times were longer and errors were more frequent. The results demonstrate that latency and force of responses may vary in different directions. Ways are outlined on how current hypotheses must be extended in order to account for these results.

Attentional bias toward low-intensity stimuli: an explanation for the intensity dissociation between reaction time and temporal order judgment?

If two stimuli need different times to be processed, this difference should in principle be reflected both by response times (RT) and by judgments of their temporal order (TOJ). However, several dissociations have been reported between RT and TOJ, e.g., RT is more affected than TOJ when stimulus intensity decreases. One account for these dissociations is to assume differences in the allocation of attention induced by the two tasks. To test this hypothesis, different distributions of attention were induced in the present study between two stimulus positions (above and below fixation). Only bright stimuli appeared in one position and either bright or dim stimuli in the other. In the two RT experiments, participants had to respond to every stimulus appearing in one of the two positions. Reaction times to bright stimuli were faster when they appeared in the position where dim stimuli were likely to occur. This finding suggests that the allocation of attention was adapted to the asymmetrical arrangement of stimuli, not suggested by explicit instruction. In the two TOJ experiments, the temporal order of stimuli appearing in the two positions had to be judged. Although bright stimuli appearing at the bright-and-dim location were judged to be earlier, this effect was small and insignificant. Further, the intensity dissociation between RT and TOJ was insensitive to random vs blockwise presentations of intensities, therefore was not modified by attentional preferences. Thus, asymmetrical arrangement of stimuli has an impact on the allocation of attention, but only in the RT task. Therefore dissociations between TOJ and response times cannot be accounted for by an attentional bias in the TOJ task but probably by different use of temporal information in the two tasks.

Dimensional overlap between arrows as cueing stimuli and responses?. Evidence from contra-ipsilateral differences in EEG potentials.

In the S1-S2 interval, 400 ms after an arrow as S1, an EEG-potential difference occurs between scalp sites contralateral and ipsilateral to arrow direction. Eimer [J. Exp. Psychol. Hum. Percept. Perform. 21 (1995) 837-854] interpreted this difference as a sign of automatic activation of the manual response, due to dimensional overlap of arrows and responses. However, according to Kornblum et al.'s [Psychol. Rev. 97 (1990) 253-270] notion of dimensional overlap, responses can only be automatically primed if they are included in the response set. Therefore, participants of the present study had to respond to S2 in separate blocks either by key-press, as in Eimer [J. Exp. Psychol. Hum. Percept. Perform. 21 (1995) 837-854], or by making saccades. In addition, contra-ipsilateral differences were recorded not only from central positions, overlying the hand-motor area, but across the whole scalp. Contralateral negativity at 400 ms after S1 was indeed found over the hand-motor area in the 'hand blocks'. However, this 'L-400' (=lateralization at 400 ms) was generally as large in the 'eye' blocks as in the 'hand' blocks. Therefore, L-400 does not reflect automatic activation of manual responses in the sense of Kornblum et al. [Psychol. Rev. 97 (1990) 253-270]. Further, its topographical maximum was more anterior than the hand-motor-related negativity that preceded the manual response ('LRP') with its maximum at central sites. Therefore, L-400 probably does not originate in the hand-motor cortex. Rather, it may be related to activity of the lateral premotor cortex found in fMRI studies of spatial orienting. The present EEG study extends these studies by delimiting the time period of this activity, suggesting that it reflects encoding of the spatial properties of the arrow for action.

CNV and temporal uncertainty with 'ageing' and 'non-ageing' S1-S2 intervals.

OBJECTIVES: Uncertainty about the timing of a known external event is an everyday phenomenon but has been rarely investigated with electrophysiological methods. We studied how the amplitude of the contingent negative variation (CNV) is affected by temporal variation of S2 presentation. Competing hypotheses about the development of CNV during the foreperiod until S2 presentation were that CNV either would follow a monotonic trend, be it increasing or decreasing, or alternatively that the time-course of CNV would be affected by the probability with which S2 was presented at each time-point in a given task. METHODS: The interval between cueing stimulus and imperative stimulus was randomly chosen from 3 different values between 1.3 and 2.6 s, using 3 different probability distributions in separate blocks: an 'ageing', a 'non-ageing' and a 'Gaussian' distribution. RESULTS: As previously shown, reaction times were determined by the probability of the imperative stimulus at the given length of the foreperiod. The same was found for CNV amplitude: the effects of temporal uncertainty on CNV mainly depended on the particular distribution of temporal probabilities used in a block. The relevant parameter was the a posteriori probability of event occurrence, very similar to the effects of this parameter on response times. In fact, the major part of the effect of a posteriori probability on CNV was common variation of CNV and response times. CONCLUSIONS: Thus, under temporal uncertainty the amplitude of CNV reflects the subjective expectancies for the occurrence of a given event, with this variation being related to variations in response times.

Lateralized EEG components with direction information for the preparation of saccades versus finger movements.

During preparation of horizontal saccades in humans, several lateralized (relative to saccade direction), event-related EEG components occur that have been interpreted as reflecting activity of frontal and parietal eye fields. We investigated to what degree these components are specific to saccade preparation. EEG lateralization was examined within the interval (1 s) between a first (S1) and a second (S2) stimulus, after which a response had to be made (look left or right, or press a button with the left or right index finger). The visual S1 indicated either the direction (left vs right) and/or the effector (eye vs finger), and S2 (visual/auditory in different blocks) added the information not given by S1. An occipital component (220 ms after S1) was effector-independent, probably reflecting processing of the direction code. The following parietotemporal component (320 ms after S1) was specific for direction information. This component seems more relevant for finger movements than for saccades and may reflect a link between visual perception to action. A later frontal component (480 ms after S1) was specific for direction information and may be related to the planning of a lateral movement. One component was entirely specific for the preparation of a finger movement (the lateralized readiness potential before S2). Thus, several different lateralized processes in the S1-S2 interval could be delineated, reflecting hand-specific preparation, processing of the direction code, and the coordination of perception and action, but no components were observed as being specific for saccade preparation.

An evaluation of methods for single-trial estimation of P3 latency.

This study investigated the validity of procedures for estimating the P3 complex in single trials. In "pseudo-real" simulations of the N1-P2 complex of the occipital visual-evoked potential, Möcks, Köhler, Gasser, and Pham (1988) had reported that their maximum-likelihood method (Pham, Möcks, Köhler, & Gasser, 1987) performed better than Woody's (1967) method. Using pseudo-real simulations of auditory oddball data, we wanted to know whether this finding also held true for the P3 complex. The performance of three methods was studied: peak picking, Woody's method, and Pham et al.'s method (as well as an extension of this latter method). Performance of all methods critically depended on the signal-to-noise ratio. There was some advantage for the more sophisticated methods, particularly when signal-to-noise ratios were realistic. "Good" trials may be selected by all methods, to improve the signal-to-noise ratio, but this selection entails the risk of bias. Further research should investigate whether these conclusions also hold true when the P3 complex consists of more than one component.

Consequences of altered cerebellar input for the cortical regulation of motor coordination, as reflected in EEG potentials.

The cerebellum is certainly involved in fine coordination of movements, but has no efferences of its own to the muscles. Thus, it can exert its influence only via other cerebral areas that have those efferences. This study investigated in patients with cerebellar atrophy how cortical motor areas are affected by dysfunction of the cerebellum. The main question was whether the patients' slow cortical electroencephalogram (EEG) potentials during key-press preparation and execution would be generally altered or would be specifically altered when fine coordination was needed. In the coordination task, right- and left-hand keys had to be pressed simultaneously with different forces, under visual feedback. Control tasks were to press with both hands equally or with one hand only. The patients indeed had a performance deficit in the coordination task. Their cortical EEG potentials were already drastically reduced in the simple tasks, but were enhanced by the same amount as in healthy subjects when more coordination was needed. These results suggest that the cerebellum is not exclusively active in fine coordination, but is generally involved in any kind of preparatory and executive activity, whereas the motor cortex becomes more active with fine coordination. The role of the cerebellum might be to provide the motor cortex with information needed for coordinating movements. In cerebellar atrophy, this altered input may be sufficient for the motor cortex in controlling simple tasks, but not for complex ones.

Amplitudes and latencies of single-trial ERP's estimated by a maximum-likelihood method.

The common approach in research on event-related electroencephalogram (EEG) potentials is to assume that the trigger-related signal is always the same and can be extracted from EEG background activity by simple averaging. To check the validity of this approach and to provide more exact results, latencies and amplitudes of components have to be estimated in single trials. Pham et al. applied a maximum-likelihood approach to solve the more general model which assumes that the signal hidden in EEG background activity has the same shape and amplitude but may vary in its latency from trial to trial. Extending their method we present a solution in which amplitude variability is also allowed. The utility of the solution to estimate the P3 component in single trials was investigated both by extensive pseudoreal simulations and in an application to real data. The simulations showed some advantage of the method over two other methods (Woody's method and peak-picking) commonly used in event-related potentials research. Application to real data provided a plausible description of single-trial sequential effects on the amplitude of the P3 component.

Slow EEG potentials (contingent negative variation and post-imperative negative variation) in schizophrenia: their association to the present state and to Parkinsonian medication effects.

Between warning signal (S1) and imperative signal (S2), the EEG shifts negatively (contingent negative variation, CNV) reflecting preparation and expectancy. Reduced CNV and continued negativity after S2 (post-imperative negative variation, PINV) have been repeatedly found in schizophrenic patients and have been interpreted as a deficit in attentional processes (CNV) and as uncertainty about the correctness of one's own response to the S2 (PINV). Recent studies obtained a CNV reduction specifically at central sites but not at frontal ones. The present study investigated whether these alterations of slow negative potentials depend on present state of symptoms, on the particular task used, and on neuroleptic medication. Therefore, out-patients and in-patients were studied, two different S1-S2 tasks were used, and the control groups were healthy subjects and patients with Parkinson's disease. The central CNV reduction was stable across tasks and across in-patients and outpatients. Frontal CNV was reduced in in-patients but in only one of the two tasks in outpatients. The schizophrenic patients' enhanced PINV was larger contralaterally than ipsilaterally to the responding hand, correlated with medication, and occurred in similar way in patients with Parkinson's disease. Thus, the PINV increase might reflect the Parkinsonian side effects of the anti-psychotic medication. In contrast, the central CNV reduction appears as a stable marker of schizophrenia, the frontal CNV reduction as a state-dependent effect. The central CNV reduction might reflect impairment in forming stable stimulus-response associations, the relative frontal enhancement might reflect the out-patients' attempt at compensating that impairment.

Selective attention is impaired in amyotrophic lateral sclerosis--a study of event-related EEG potentials.

In humans, selective attention is assumed to be under control of the frontal lobe. A significant proportion of patients with amyotrophic lateral sclerosis (ALS) shows impairments in various tasks touching frontal lobe function. We, therefore, undertook a study of event-related EEG potentials (ERPs) in eight non-demented ALS patients in order to investigate a possible deficit of auditory selective attention: tones were presented in random sequence to the left or right ear, one of which was to be attended. The negative shift of the ERPs evoked by attended tones in relation to unattended tones ('processing negativity': PN) was smaller in ALS patients than in age-matched healthy control persons. This was true for Fz and Cz and for both a slow and a fast presentation rate of the tones. In the patients, reduced PN amplitude correlated with functional motor impairment. The utility of ERP testing to assess impaired frontal lobe function is shown for the first time in ALS patients. The results of our study fit to recent positron emission tomography (PET) and fMRI data.

Spatial S-R compatibility with centrally presented stimuli. An event-related asymmetry study on dimensional overlap.

Lateral presentation of relevant information facilitates manual responses if the side of relevant information corresponds to the side of the response. Recently, temporally overlapping EEG asymmetries over the central motor cortex and posterior sites were reported as a possible correlate of the sensory-motor integration of spatial information. The present study investigated whether sensory-motor integration of spatial information can occur with symbolic spatial information the same way as with laterally presented stimuli. The task required participants to respond to arrows (target stimuli), which were "flanked" (from above and below) by neutral stimuli or by other arrows (compatible or not). In Experiment 1, this task was compared to the same task with letters as stimuli and to an incompatible task where participants had to respond "against" the arrow direction. The effect of the flankers on response times was largest if subjects had to respond to the arrows in the common way. This was also the only task of Experiment 1 for which marked EEG asymmetries related to the direction of the flankers were observed. In Experiment 2, the onsets of target stimulus and flankers differed in time. Event-related lateralizations of the EEG over sensory and primary motor areas--as a lateralized readiness potential--were always, apparently automatically, evoked by flanking arrows, indicating automatic response activation evoked by symbolic spatial information. In accordance to recent theories of temporally decaying response activation, manual responses were affected only if the target was either shortly preceded by or appeared simultaneously with the flankers. The temporal overlap of EEG asymmetries related to direction encoding, automatic response activation, and to response preparation indicated that a widespread cortical network is activated by a salient directional information that enables subjects to respond quickly if the directional code of the stimulus overlaps with the directional code of the response.

Lateralized human cortical activity for shifting visuospatial attention and initiating saccades.

Lateralized human cortical activity for shifting visuospatial attention and initiating saccades. J. Neurophysiol. 80: 2900-2910, 1998. The relation between shifts of visual attention and saccade preparation was investigated by studying their electrophysiological correlates in human scalp-recorded electroencephalogram (EEG). Participants had to make saccades either to a saliently colored or to a gray circle, simultaneously presented in opposite visual hemifields, under different task instructions. EEG was measured within the short interval between stimulus onset and saccade, focusing on lateralized activity, contralateral either to the side of the relevant stimulus or to the direction of the saccade. Three components of lateralization were found: 1) activity contralateral to the relevant stimulus irrespective of saccade direction, peaking 250 ms after stimulus onset, largest above lateral parietal sites, 2) activity contralateral to the relevant stimulus if the stimulus was also the target of the saccade, largest 330-480 ms after stimulus onset, widespread over the scalp but with a focus again above lateral parietal sites, and 3) activity contralateral to saccade direction, beginning about 100 ms before the saccade, largest above mesial parietal sites, with some task-dependent fronto-central contribution. Because of their sensitivity to task variables, component 1 is interpreted as the shifting of attention to the relevant stimulus, component 2 is interpreted as reflecting the enhancement of the attentional shift if the relevant stimulus is also the saccade target, and component 3 is interpreted as the triggering signal for saccade execution. Thus human neurophysiological data provided evidence both for independent and interdependent processes of saccade preparation and shifts of visual attention.

Responses to cued signals in Parkinson's disease. Distinguishing between disorders of cognition and of activation.

Impairment of movement execution in Parkinson's disease could be due to disorders of cognition and/or of activation. These two factors are hard to separate by measuring response times only. Therefore, in this study response force and event-related EEG potentials were measured continuously during tasks in which subjects had to respond to cued signals. Fifteen patients with Parkinson's disease and 15 healthy subjects were studied during two tasks: (i) the 'clock task', in which the signal's identity was fully precued but its presentation time was uncertain and (ii) the 'validity task' in which the cue did not always predict the response validly. Thus, the clock task required more sustained attention, and the validity task sometimes required fast switching. The patients generally responded slower than control subjects. In the clock task, the response times of both groups changed to the same extent with presentation time, whereas in the validity task the patients were additionally slower than the control group with invalidly cued signals. The patients generally had a weaker response force and a lower rate of force production. In the clock task, both force measures changed with presentation time in the control group only, whereas in the validity task, the two measures increased in both groups to the same extent with invalidly cued signals. The contingent negative variation amplitudes in the patients' event-related EEG potentials were reduced, reflecting reduced activation of movement preparation, whereas lateralization of the motor cortices (i.e. the lateralized readiness potential) did not differ significantly between groups, reflecting unimpaired response selection. Force and contingent negative variation were generally reduced in the patients showing that their general slowing is at least partially due to impaired activation. Task-specific problems added to the general activation deficit; the lack of modulation of response force by presentation time revealed pronounced deficits of activation in the monotonous clock task. The specific delay of responses with invalidly cued signals, unparalleled by activation measures, might suggest a problem of cognition. The task-specific deficits may reflect a basic dilemma for patients with Parkinson's disease: cognitive problems may arise in complex tasks but disorders of activation may become pronounced in more simple, monotonous tasks.

Shifting attention between global features and small details: an event-related potential study.

In two experiments, large letters H or Z composed of small letters (also H or Z) were presented. Subjects had to make a two-choice motor response (e.g. H--left key, Z--right key). A cue presented 500 ms before the letter indicated which level (global or local) was relevant. In Experiment I, a third letter (T) sometimes appeared either at the cued or the non-cued level; in the former case, subjects had to shift their attention and to respond to stimulus features located at the non-cued level. The interference effect (RT delay in response to incongruent stimuli as compared to congruent ones) was larger when the local, rather than global, level was cued. A slow anterior negativity preceding globally-cued stimuli and shorter N1 and P2 ERP component latencies to these stimuli indicated better preparation for processing of global, as compared to local, stimulus features. The shift from local to global focus yielded a larger increase of RT, error rate, and of the P600 latency than the global-to-local shift. The P600 latency changes were parallel to those of RT. In Experiment II, the attentional shift was provoked by stimulus color red-colored letters meant that the cue was invalid, and thus, subjects had to respond to the non-cued level. Neither the interference nor the attentional shift demonstrated any asymmetry between the global and local levels. ERPs also did not differ substantially after local and global cues. In the condition demanding a shift of focus (invalid cue, incongruent letter), a positive deflection of the lateralized readiness potential indicated the activation of the wrong response channel. The large RT increment in this condition was not accompanied by an increase of the P600 latency. Two possible mechanisms of attentional shift may be proposed, the first related to perceptual processes (e.g. an additional visual search), and the second, to the competition between two response intentions.

Event-related potentials suggest slowing of brain processes in generalized epilepsy and alterations of visual processing in patients with partial seizures.

Event-related potentials were recorded in two auditory tasks and in one visual task from 13 patients with partial seizures (PS), 12 patients with idiopathic generalized epilepsy (IGE), and healthy age-matched participants. IGE patients had delayed latencies in the auditory tasks, with the delay being reliable already at N1, and continuously increasing at N2b and at P3. The P3 delay correlated with duration of epilepsy both in IGE and in PS patients. In the visual task, the posterior N2 was delayed in PS patients, in particular in PS patients with temporal focus. Resembling the delays as found in healthy elderly adults, the delays with auditory stimuli might reflect a slowing of brain processes as it occurs in healthy aging. The isolated delay of posterior N2 in temporal PS patients might indicate a specific impairment of the occipito-temporal visual pathway. Taken together, event-related potentials prove to be a very sensitive instrument for measuring altered brain functioning in epileptic patients, when compared to measurement of overtly visible responses.