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Human evoked potentials as a function of stimulus information content in an expectancy-reaction time situation
FREE COMMUNICATIONSIN EEG generator mechanism and that of locally generated rhythmic potentials. Average evoked potentials (AEPs) and alpha rhythms were recorded from electrodes placed bilaterally over occipital and parietal areas referenced to the ear lobes. Flashes were presented to the right and/or left visual fields of one or both eyes and could be delivered either in or out of phase. Evoked potentials were obtained by averaging from the onset of flashes; alpha rhythms were studied by averaging unfiltered EEG activity for 250-1000 msec epochs before and during repetitive stimulation. Alpha averaging was triggered by an amplitude criterion individually determined for each subject. The alpha averages and the AEPs to repetitive stimulation produced relatively simple sinusoidal wave forms whose amplitude was largest when stimuli were presented at 10 c/sec and decreased when the stimulus frequency was varied either above or below 10 c/sec. The amplitude of alpha averages was considerably greater than that of the rhythmic AEPs recorded from the same electrode. Alpha amplitude was smallest at the inion and largest over more anterior occipital and parietal areas. The amplitude of AEPs was maximal at the inion and was reduced at more anterior recording sites. As frequency of photic stimulation varied, frequency of alpha remained constant while frequency of AEPs followed the rate of stimulation. Out of phase flashes presented to the two eyes or to the two visual fields produced amplitude reductions and wave form changes in the AEPs at all stimulus frequencies, but produced only amplitude reductions in the alpha averages when the repetitive stimulation was in the neighborhood of 10 c/sec. These results provide a basis for the differentiation of diffusely distributed alpha activity, presumably controlled by a nonspecific thalamic generator, and regionally confined rhythmic potentials which closely follow the frequency of the repetitive stimulation and apparently reflect the influence of local generators associated with specific thalamic control centers. 144. Inter- and intra-hemispheric interactions of average visual evoked potentials.-J. D. Wicke (Los Angeles, Calif., U.S.A.). Repetitive flashes of 5-20 c/sec were presented binocularly to the left and/or right visual fields to investigate inter-hemispheric relationships, and monocularly to one visual field of each eye to investigate intra-hemispheric relationships. Flashes were either in phase or 180° out of phase with respect to the left and right visual fields, or with
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respect to the two eyes. In order to insure that each cerebral hemisphere received information from only one visual field, the flashes to each of the visual fields were separated by a continuously illuminated central adapting field. Average evoked potentials (AEPs) were obtained from scalp electrodes placed bilaterally over the occipitals (AEPs) were obtained from scalp electrodes placed bilaterally over t h e occipital and parietal areas and were referenced to the ear lobes, The AEPs were relatively simple sinusoids which followed the frequency of the flash trains. With respect to inter-hemispheral comparisons the following results were obtained. 1. Simultaneous stimulation of the right and left visual fields of both eyes produced larger AEPs than when only homonymous half fields of both eyes were stimulated. 2. AEPs recorded bilaterally were similar whether flashes were presented to the right or left half of the visual fields. 3. Stimulation of the same visual fields of the two eyes 180 ° out of phase caused a marked reduction in the amplitude and a modification of the wave form of the AEPs. Intra-hemispheral comparisons provided the following results. 1. Bilateral stimulation of homonymous half fields produced larger AEPs than unilateral stimulation. 2. Stimulation of the same visual half field of either eye separately produced similar AEPs bilaterally. 3. Stimulation of the same visual half field of both eyes with flashes 180 ° out of phase caused interaction and cancellation of AEPs. 145. Human evoked potentials as a function of stimulus i n f o r m a t i o n content in an expectancy-reaction time situation.-M. D. Buck (Los Angeles, Calif., U.S.A.). The purpose of this study was to devise a stimulus-response situation which would allow three stimuli presented 1 sec apart and preceding the response to define the limits of the periods during which anticipation or expectancy could be established, and endow the second stimulus with more or less information content. Given these conditions, the goal was to determine the effect of them upon the average evoked potentials. The response itself was a simple finger press upon a right or left key, depending upon the nature of the information contained in the third stimulus. The first stimulus, a tone-pip of medium pitch, was always a Warning Stimulus (Sw); the second stimulus, a tone-pip of either high or low pitch, served as an information carrying stimulus, and is referred to as a Discriminative Stimulus (Sd); the third stimulus, which consisted of a change in a visual fixation figure, indicated which of two keys to press and is called the Imperative Stimulus (Si). The fixation figure, presented in a
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INTERNATIONAL FEDERATION - 7TH CONGRESS
three-field tachistoscope was observed throughout with fixation maintained upon the center of a cross inscribed in a circle. The change in the figure, which constituted the Si, was effected by having an open sector pointing right or left. In the information situation the S d of high or low pitch carried information agreeing with that in the Si and thus allowed the subject to prepare for the correct response following Si. In the noinformation situation the Sd was always of high pitch and simply marked time until the Si. Instructions to the subject determined whether Sd would be informational or not. Cortical evoked potentials were recorded from somatosensory and vertex scalp electrodes, using long time-constant amplifiers. Averages were obtained by means of a CAT computer. Reaction times were measured from the Si. Results show that there are significant differences in the nature of the CNV under the different conditions.
146. Relationships between motor potentials and CNV.-K. Syndulko (Los Angeles, Calif., U.S.A.). The purpose of this experiment was to determine what relationships exist between the motor potential (MP) and the contingent negative variation (CNV). Each human subject was studied under three different conditions. Condition I was a choice reaction time task in which the subject was instructed to make a right or left hand movement, after presentation of one of two visual patterns. Condition I! involved the same stimuli and responses, but included a tone stimulus preceding both visual stimuli by 1 sec. In Condition III the tone was high or low indicating which of the two visual patterns was to follow and allowing the subject to anticipate the specific responses he was to make. The visual stimuli were presented via 2 channels of a 3-channel tachistoscope. The third channel was used to display a continuously present fixation point. Subjects were given practice trials before each of the three conditions. Steady potential changes were recorded at sites presumed to overlie the motor hand area of each hemisphere as well as from points anterior and posterior to the motor sites, from the vertex and from a site on the forehead to monitor eye movements. EMG activity was recorded from forearm muscles involved in the response. Both MPs and CNVs were averaged either backward from the response or foreward from the warning stimulus. The plan of the experiment permitted comparison of motor potentials which are assumed to arise from a specific locus, and CNVs, which are
assumed to have a different distribution. Condition I provides an imperative stimulus to a motor response but without warning and anticipation, Condition II provides only general expectancy and a pre-imperative stimulus period favoring CNV, and Condition III provides a specific and directed motor expectancy favoring the MP.
147. Behavioral and electrophysioiogical responses in a two flash discrimination task in the cat.-C, K. Peck (Los Angeles, Calif., U.S.A.). The purpose of this study was to train cats to discriminate two brief flashes presented as a pair from a single flash and then to reduce the temporal interval between the paired flashes until discrimination of two from one was no longer possible. Electrodes implanted in the optic tract, lateral geniculate body and the visual cortex permitted concurrent recording of responses to the visual stimuli. The behavioral task associated with this visual temporal discrimination involved a "go/no-go" procedure. On each trial the stimulus was either a single flash or a pair of flashes separated by a variable interval. During initial training the interflash interval was 250 msec but was subsequently reduced to a value as low as 20 msec which was well below the behavioral threshold at approximately 75 msec. Stimuli were presented only when the cat faced a transparent plastic panel upon which the flashes appeared. On any one trial the stimulus was repeated at 1.5 sec intervals until 5 stimuli had been presented or until the cat pressed the panel. A press following a paired flash provided a milk reward; a press following a single flash delayed the start of the next trial 15 sec. Fairly stable behavioral thresholds have been obtained and data will be presented concerning these and associated electrical responses to the flashes at each of the levels of the visual system sampled. Of particular interest is the question whether separation of the responses to the two flashes occurs when the behavioral discrimination is just possible as opposed to when it is not.
148. Polygraphy and its applications in the study of stabilized medullary lesions.-A. Heilporn (Brussels, Belgium). The author presents findings from 500 cases of medullary lesions all with chronic and severe sequelae. Special emphasis is laid on changes in the vasomotor and thermoregulatory functions, in blood pressure and cardiac rhythm, in their relation to type and level of the lesion. Polygraphic recordings were made simultaneously from regions above and below the level of the lesion under conditions of rest and of varying body positions and body temperature. Emphasis is
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respect to the two eyes. In order to insure that each cerebral hemisphere received information from only one visual field, the flashes to each of the visual fields were separated by a continuously illuminated central adapting field. Average evoked potentials (AEPs) were obtained from scalp electrodes placed bilaterally over the occipitals (AEPs) were obtained from scalp electrodes placed bilaterally over t h e occipital and parietal areas and were referenced to the ear lobes, The AEPs were relatively simple sinusoids which followed the frequency of the flash trains. With respect to inter-hemispheral comparisons the following results were obtained. 1. Simultaneous stimulation of the right and left visual fields of both eyes produced larger AEPs than when only homonymous half fields of both eyes were stimulated. 2. AEPs recorded bilaterally were similar whether flashes were presented to the right or left half of the visual fields. 3. Stimulation of the same visual fields of the two eyes 180 ° out of phase caused a marked reduction in the amplitude and a modification of the wave form of the AEPs. Intra-hemispheral comparisons provided the following results. 1. Bilateral stimulation of homonymous half fields produced larger AEPs than unilateral stimulation. 2. Stimulation of the same visual half field of either eye separately produced similar AEPs bilaterally. 3. Stimulation of the same visual half field of both eyes with flashes 180 ° out of phase caused interaction and cancellation of AEPs. 145. Human evoked potentials as a function of stimulus i n f o r m a t i o n content in an expectancy-reaction time situation.-M. D. Buck (Los Angeles, Calif., U.S.A.). The purpose of this study was to devise a stimulus-response situation which would allow three stimuli presented 1 sec apart and preceding the response to define the limits of the periods during which anticipation or expectancy could be established, and endow the second stimulus with more or less information content. Given these conditions, the goal was to determine the effect of them upon the average evoked potentials. The response itself was a simple finger press upon a right or left key, depending upon the nature of the information contained in the third stimulus. The first stimulus, a tone-pip of medium pitch, was always a Warning Stimulus (Sw); the second stimulus, a tone-pip of either high or low pitch, served as an information carrying stimulus, and is referred to as a Discriminative Stimulus (Sd); the third stimulus, which consisted of a change in a visual fixation figure, indicated which of two keys to press and is called the Imperative Stimulus (Si). The fixation figure, presented in a
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INTERNATIONAL FEDERATION - 7TH CONGRESS
three-field tachistoscope was observed throughout with fixation maintained upon the center of a cross inscribed in a circle. The change in the figure, which constituted the Si, was effected by having an open sector pointing right or left. In the information situation the S d of high or low pitch carried information agreeing with that in the Si and thus allowed the subject to prepare for the correct response following Si. In the noinformation situation the Sd was always of high pitch and simply marked time until the Si. Instructions to the subject determined whether Sd would be informational or not. Cortical evoked potentials were recorded from somatosensory and vertex scalp electrodes, using long time-constant amplifiers. Averages were obtained by means of a CAT computer. Reaction times were measured from the Si. Results show that there are significant differences in the nature of the CNV under the different conditions.
146. Relationships between motor potentials and CNV.-K. Syndulko (Los Angeles, Calif., U.S.A.). The purpose of this experiment was to determine what relationships exist between the motor potential (MP) and the contingent negative variation (CNV). Each human subject was studied under three different conditions. Condition I was a choice reaction time task in which the subject was instructed to make a right or left hand movement, after presentation of one of two visual patterns. Condition I! involved the same stimuli and responses, but included a tone stimulus preceding both visual stimuli by 1 sec. In Condition III the tone was high or low indicating which of the two visual patterns was to follow and allowing the subject to anticipate the specific responses he was to make. The visual stimuli were presented via 2 channels of a 3-channel tachistoscope. The third channel was used to display a continuously present fixation point. Subjects were given practice trials before each of the three conditions. Steady potential changes were recorded at sites presumed to overlie the motor hand area of each hemisphere as well as from points anterior and posterior to the motor sites, from the vertex and from a site on the forehead to monitor eye movements. EMG activity was recorded from forearm muscles involved in the response. Both MPs and CNVs were averaged either backward from the response or foreward from the warning stimulus. The plan of the experiment permitted comparison of motor potentials which are assumed to arise from a specific locus, and CNVs, which are
assumed to have a different distribution. Condition I provides an imperative stimulus to a motor response but without warning and anticipation, Condition II provides only general expectancy and a pre-imperative stimulus period favoring CNV, and Condition III provides a specific and directed motor expectancy favoring the MP.
147. Behavioral and electrophysioiogical responses in a two flash discrimination task in the cat.-C, K. Peck (Los Angeles, Calif., U.S.A.). The purpose of this study was to train cats to discriminate two brief flashes presented as a pair from a single flash and then to reduce the temporal interval between the paired flashes until discrimination of two from one was no longer possible. Electrodes implanted in the optic tract, lateral geniculate body and the visual cortex permitted concurrent recording of responses to the visual stimuli. The behavioral task associated with this visual temporal discrimination involved a "go/no-go" procedure. On each trial the stimulus was either a single flash or a pair of flashes separated by a variable interval. During initial training the interflash interval was 250 msec but was subsequently reduced to a value as low as 20 msec which was well below the behavioral threshold at approximately 75 msec. Stimuli were presented only when the cat faced a transparent plastic panel upon which the flashes appeared. On any one trial the stimulus was repeated at 1.5 sec intervals until 5 stimuli had been presented or until the cat pressed the panel. A press following a paired flash provided a milk reward; a press following a single flash delayed the start of the next trial 15 sec. Fairly stable behavioral thresholds have been obtained and data will be presented concerning these and associated electrical responses to the flashes at each of the levels of the visual system sampled. Of particular interest is the question whether separation of the responses to the two flashes occurs when the behavioral discrimination is just possible as opposed to when it is not.
148. Polygraphy and its applications in the study of stabilized medullary lesions.-A. Heilporn (Brussels, Belgium). The author presents findings from 500 cases of medullary lesions all with chronic and severe sequelae. Special emphasis is laid on changes in the vasomotor and thermoregulatory functions, in blood pressure and cardiac rhythm, in their relation to type and level of the lesion. Polygraphic recordings were made simultaneously from regions above and below the level of the lesion under conditions of rest and of varying body positions and body temperature. Emphasis is