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.

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.

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.

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.

[Spinal metastases of malignant gliomas]. / Spinale Metastasierung bei malignen Gliomen. Zwei Fallbeschreibungen.

PURPOSE: Extracranial metastases of malignant gliomas are rare. We report 2 cases with spinal metastases in patients suffering from glioma. PATIENTS AND METHOD: Two patients (33 and 57 years old) developed spinal canal metastases of a glioblastoma multiforme and anaplastic astrocytoma Grade III respectively 25 and 9 months after surgical resection and radiotherapy. Both metastases were confirmed pathohistologically. RESULTS: Intraspinal metastases were irradiated with a total dose of 12.6 Gy and 50 Gy. Treatment withdrawal was necessary in one patient due to reduced clinical condition. Regression of neurological symptoms was observed in the second patient. CONCLUSIONS: Spinal spread of malignant glioma should be considered during care and follow-up in glioma patients with spinal symptoms.

Lateralised cortical activity due to preparation of saccades and finger movements: a comparative study.

The topography and time course of event-related asymmetries of the EEG associated with horizontal saccadic eye movements and finger movements was compared in a 4-choice response task, where the subjects had to respond to the imperative stimulus (S2) by moving the right or left index finger or by making a saccade to the right or the left. The cue stimulus (S1) contained full, partial, or no information about the direction and the effector. In case of finger movements 3 distinct lateralisations were found: (1) increased negativity over the motor cortex contralateral to the future movement direction, (2) increased contralateral negativity at temporoparietal sites beginning 200 ms after delivery of the directional information, and (3) increased ipsilateral negativity at temporo-parietal sites beginning 350-500 ms after delivered direction and effector information. The early temporo-parietal lateralisation was also visible in case of saccadic eye movements and in case of effector-unspecific directional information. Before saccadic eye movements no other distinct lateralisation could be observed at any recording site. In sum, lateralised cortical activities due to preparation processes for finger movements and due to effector-unspecific processing of directional information for motor preparation by the posterior parietal cortex could be demonstrated, whereas no distinct lateralisation due to preparation for saccadic eye movements was visible.

[EEG correlates of different methods of information processing in response selection]. / EEG-Korrelate unterschiedlicher Informationsverarbeitung zur Reaktionsauswahl.

The lateralized readiness potential as a measure of response tendencies has become a relevant method in experimental psychology within the last decade. This measure delivers information about response preparation long before the overt response. Applying the method of measuring event-related lateralizations (ERL) of the EEG to posterior sites, correlates of processing of spatial information and of response selection have been found. The present study investigated the topographical distribution of asymmetries at the moment of response selection. Response selection was assumed to be reflected in a discrete peak of parietal asymmetries. At this time point two distinct topographies were found that might reflect two different types of information processing. If response selection was based on spatial information a parieto-occipital maximum was found. In contrast, if centrally presented symbolic stimuli were used for response selection, lateralizations were largest at central (motor areas) and at parietal sites.

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

The present study focused on the relationship between movement- and stimulus-related asymmetries of the electroencephalogram (EEG). In seven tasks the same 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.

Preparation for action: an ERP study about two tasks provoking variability in response speed.

This study focused on the covariation of response speed and event-related potentials during response preparation and on whether these variations can be brought under experimental control. Two S1-S2 choice response tasks with temporal uncertainty were conducted. In Experiment 1, S1 was 100% informative. Fast subjects showed larger P3s with S1 than slow subjects. The terminal CNV (tCNV) increased intraindividually with response speed. In Experiment 2, 50% of S1s were uninformative and the visual display was designed to attract more attention. Effects of information were found on P3 amplitude, on the topography of tCNV, and on the temporal distribution of response times. Interindividual differences disappeared in Experiment 2. The results suggest that group differences in Experiment 1 were due to different strategies of allocating visual attention. Interindividual variations of strategy showed a pattern of effects different from intraindividual variations of efficiency.

The presaccadic cortical negativity prior to self-paced saccades with and without visual guidance.

The presaccadic negativity (PSN) of the scalp EEG potential prior to self-initiated saccades aimed either at a visual target or at the remembered position of that target in total darkness was analysed in 10 normal subjects. Under both conditions a PSN with a negligible EOG contamination was found, showing 4 characteristics: (1) In both conditions, the PSN maximum is localized at the vertex, probably containing the activity of the supplementary motor area. (2) At an electrode placed over the frontal eye field (FEF) contralateral to the saccade direction, there is a temporary, circumscribed maximum prior to saccades to the visual target, thus probably reflecting activity of the FEF. (3) Prior to saccades to the visual target, there is a statistically significant interhemispheric difference of the PSN over the parietal cortex with a larger amplitude over the hemisphere contralateral to the saccade direction; this might be attributed to directed visual attention. (4) Prior to saccades without visual guidance in darkness there is a statistically significant interhemispheric difference of the PSN over the frontal cortex with a larger amplitude over the hemisphere contralateral to the saccade direction. The amplitude of the PSN decreased in the course of the experiment, probably due to psychological factors such as attention and motivation. Our results suggest that the PSN is a readiness potential preceding voluntary saccades, containing activity related both to unspecific psychological processes and to specific movement preparation in the frontal and parietal ocular motor areas.

Suspense and surprise: on the relationship between expectancies and P3.

Few theories of the P3 component have emphasized the distinction between its parietal and frontocentral parts. This study used a new paradigm for testing the predictions that the parietal P3 is evoked by awaited stimuli (suspense) and the frontal P3 by unexpected stimuli (surprise). Subjects had to make simple responses whenever a yellow ring appeared. This signal appeared on the screen within a clock, most frequently when the pointer was at 12 o'clock (every 6 s) but sometimes also at other times. The suspense process should therefore have its minimum shortly after 12 o'clock and then steadily increase until 12 o'clock, and the parietal P3 should accordingly be smallest with stimuli shortly after 12 o'clock, then gradually increase and be largest with 12 o'clock stimuli. Further, the stimuli presented at times other than 12 o'clock should evoke large frontal P3s because they were unexpected. The results confirmed parts of these predictions. A frontocentral and a parietocentral component could indeed be discerned. The frontal P3 was largest with non-12 o'clock stimuli, whereas the parietal P3 was large with all stimuli. The parietal result was not predicted, but these results taken together pose more problems for the usual view, which assumes that the parietal P3 is evoked by unexpected stimuli, than for our assumption that the parietal P3 reflects suspense, and the frontocentral P3 reflects surprise. Generalizing to other paradigms, we assume that different topographies of P3 in different paradigms or in different groups of subjects might be due to different mixtures of these two components.