EVOKED RESPONSES AND PERCEPTION: STIMULUS CONTENT VERSUS STIMULUS STRUCTURE

These experiments tested whether waveform changes in evoked responses to visual stimuli could be produced by changes in stimulus content independent of changes in stimulus structure. Ambiguous figures were employed to manipulate content without changing structure. To manipulate structure without changing content, stimuli were presented in original and mirror image versions or versions varying in size. The results indicated that evoked responses can be modified by content differences independent of any structural changes, but it was suggested that content related changes might reflect the operation of more general psychological processes such as attention and emotion.     

The Effect of Predictability on Evoked Response Enhancement in Intramodal Selective Attention

We investigated the relationship between the attention enhancement of the visual average evoked response (AER) and the S's ability to predict the presentation of the attended stimulus. Twelve students were presented with sequences of two distinct visual stimuli while DC EEG and electro-oculogram (EOG) were simultaneously recorded. Stimuli were either regularly alternated (predictable) or randomly intermixed (nonpredictable). Verbal instructions directed S's attention and push button response to either one or both of the two stimuli. Interstimulus interval was held constant to permit computer averaging of the AER and the contingent negative variation. Selective attention enhanced the late positive component of the AER equally in the two conditions of predictability. Thus, it seems that the AER enhancement with intramodal selective attention does not depend on the S's ability to predict the presentation of the attended stimulus or to differentially arouse prior to its presentation. In the high predictability condition, the baseline EEG potential fluctuated with attention such that the AERs to the attended stimuli were negative relative to those to the ignored stimuli.     

Retinal versus central processes in determining averaged evoked response waveforms

Random dot stereograms permit the separation of central, perceptual, from peripheral, retinal effects of the stimulus. Anaglyph versions of such stereograms were used to reassess whether perception of stimulus content alters the waveform of averaged evoked brain responses to photic stimuli in human subjects. The evoked responses were shown to be sensitive to stimulus color, microstructure, and the perception of three-dimensional stimuli, but not to changes in the figural content of the stimuli.     

Expectation of an important event affects responses to irrelevant stimuli of different modalities

Periodic alterations of event-related potentials (ERPs) were studied during "oddball" tasks. Sequences of randomly intermixed frequent (non-target) and rare (target) stimuli were presented. In visual experiments, these were flashes of light of two different colors. In auditory tests there were two tones of different frequencies. The instruction was to keep a mental count of each target stimulus. To study the alterations of the "state of the brain" produced by target detection, responses to non-targets immediately following targets were compared with responses to an eighth subsequent non-target stimulus. To evaluate the effect of such "brain states" on responses to stimuli of a different modality, additional visual stimuli (probes) were delivered after both auditory and visual "oddball" stimuli. It was found that responses to the eighth presentation of non-target stimulus were preceded by significant negative shift of recorded potential. This shift was smaller before the responses to non-targets immediately following the presentation of target stimuli. The difference was significant both in auditory and visual tests. Responses to "oddball" stimuli were little affected: only the reduction of P200 peaks in "after target" responses was significant in visual tests. Responses to probes showed stronger effects: when visual probes followed visual "oddball" stimuli, all three components measured (N100, P130 and P200) were shifted positively in responses to eighth presentations of non-targets. When visual probes were presented in auditory tests, only the amplitude of the N100 component was significantly affected.     

Does the Averaged Evoked Response Encode Subliminal Perception?

In a series of studies, Shevrin and his colleagues have reported that the effects of subliminal perception are encoded in the average evoked response. Our experiment was a more stringent test in that we (1) collected both physiological and behavioral data in the same trials, (2) attempted to minimize criterial differences in the employment of physiological and behavioral responses, and (3) behaviorally verified conditions designed to be subliminal. Two stimuli were presented tachistoscopically in a given trial, separated by 1 sec; over blocks of trials, exposure duration for the stimuli was 3, 7, 15, and 30 msec. At 3 msec exposure, all subjects detected the stimuli but could not discriminate between them; discrimination increased with increasing exposure duration. But at no exposure duration did average evoked response measures discriminate between the stimuli-the only changes in evoked response measures were those due to increasing stimulus energy. Thus, there was no evidence for either subliminal or supraliminal discrimination of stimulus content by the AER.     

ELEMENTARY FUNCTIONAL CORRELATES OF THE TRANSCEPHALIC DC CIRCUIT

A brief general theory is proposed linking the transcephalic DC potential, which is recorded on the midline surface of the head over the frontal and occipital emissary veins, with a certain functional organization of the brain. From this it is proposed that interoceptive or protopathic afferents—carotid sinus, carotid body, vestibular, and visceral—should produce a positive frontal deflection, while exteroceptive or epicritic afferents—tactile, auditory, proprioceptive, gustatory, visual, and olfactory—should produce a negative frontal DC shift. It is proposed that hunger promotes a negative frontal base line, satiety a positive one, and anxiety increases the base-line variance of a group. A relationship is proposed between frontal negativity and body metabolic peaks. It is proposed that a noxious exteroceptive stimulus produces a positive frontal shift, despite its cortical projection. The hypotheses were tested using normal adult male Ss and good agreement between results and predictions was found.     

Magnetic brain activity evoked and induced by visually presented words and nonverbal stimuli

Evoked and induced magnetic brain activity measured over the left hemisphere were tested for their specificity to language-related processing. Induced activity refers to oscillatory alterations time locked but not phase locked to the stimulus. Words, false font stimuli, and two types of nonverbal patterns were presented visually while subjects performed a nonlinguistic visual feature detection task. The comparison of evoked and induced brain activity around 200 ms after stimulus onset revealed differential sensitivity to the stimuli. The M180 component of the evoked magnetic field was larger at the processing of words and false font stimuli compared with nonverbal stimuli. The induced magnetic brain activity in the 60-Hz band at a compatible latency range was correlated with the familiarity of the visual Gestalt. Sensitivity to language-specific information processing can be concluded if a parameter differentiates the word condition from the nonlexical conditions. Such a difference was observed at sensors located over the frontal-temporal scalp regions for induced but not evoked magnetic brain activity. Thus, evoked and induced magnetic brain activity revealed a differential sensitivity to elements of cognitive processing during the given task.     

Selective listening modifies activity of the human auditory cortex

Summary We have studied the effect of selective listening on the neuromagnetic evoked activity of the human auditory cortex. In the word categorization experiment the stimuli were 5-letter words, each beginning with /k/. Half of them were targets, i.e., names of animals or plants, and half other meaningful Finnish words. In the duration discrimination experiment equiprobable tones of 425 ms (targets) or 600 ms duration were presented. In both experiments the interstimulus interval (ISI) was 2.3 s and the stimuli of the two classes were presented randomly. Subjects either ignored the stimuli (reading condition) or counted the number of targets (listening condition). The magnetic field over the head was measured with a 7-channel 1st-order SQUID-gradiometer. The stimuli evoked a transient response followed by a sustained field. The transient response did not differ between the two conditions but the sustained field was significantly larger in the listening than reading condition; the increase began 120–200 ms after stimulus onset and continued for several hundred milliseconds. The equivalent source locations of both transient and sustained responses agreed with activation of the supratemporal auditory cortex. In the dichotic listening experiment 25-ms square-wave stimuli were presented randomly and equiprobably either to the left or to the right ear at an ISI of 0.8–1 s, either alone or in presence of a speech masker. Counting the stimuli of either ear resulted in differences between responses to relevant and irrelevant sounds. The difference began 140–150 ms after stimulus onset and peaked at 200–240 ms. During monaural speech masking, N100m was larger for attended than ignored stimuli. The results suggest that neural mechanisms underlying direction of attention include modification of the activity of the auditory cortex and that the mechanisms are similar for words and tones.     

EVOKED CORTICAL RESPONSES TO AFFECTIVE VISUAL STIMULI

The influence of affective meaning on visual evoked responses was investigated in male college students. By utilization of conditioning procedures, previously meaningless figures (CS) acquired affective loadings, e.g., positive, negative, and neutral. The semantic differential scale and critical flicker interval (CFI) were used as indices of conditioning. Although conditioning occurred without awareness, both measures of conditioning yielded results in the same direction, and all three affective conditions differed significantly from one another. Averaged evoked responses were obtained for each affective stimulus. Amplitudes were found to differ significantly from one another in all three conditions, the unpleasant stimulus eliciting the lowest evoked response, and the neutral condition evoking the highest amplitude. Significantly shorter latencies were obtained for the unpleasant stimulus, whereas the latencies of the positive and neutral stimuli did not differ significantly from each other. The results suggest a direct influence of the emotional centers (limbic system) on the visual perception of affective stimuli. Further studies were suggested to clarify the role of awareness and the specific modalities involved in conditioning.     

The Effect of Stimulation Frequency and Retinal Stimulus Location on Visual Evoked Potential Topography

The activity of cortical neurons is influenced by retinal stimulus location and temporal modulation. We investigated how reversal frequency of black-and-white checkerboard patterns presented in different parts of the visual field affects evoked potential topography. Visual evoked potentials were recorded from an array of 16 electrodes over the occipital cortex in 12 healthy adults. A checkerboard reversal stimulus (40′ check size) was presented with frequencies between 1.95 reversals/s and 7.81 reversals/s in the center or in the left or right hemiretina. Evoked potential fields displayed the well-known components of pattern reversal evoked activity. Computation of FFT and wavelets displayed electrical brain responses directly related to stimulation frequency. Further analysis showed that both retinal stimulus location and stimulation frequency affected visual evoked activity. Field strength as well as scalp field topography changed significantly with different reversal frequency. In addition, the pattern of lateralization of components also depended on temporal frequency of stimulation.Electrical brain activity elicited by visual stimuli shows globally similar features which are modulated by stimulus location and frequency. Our results indicate that—at least partly—different neuronal assemblies are activated by stimuli of different temporal characteristics.     

Perceptual learning: psychophysical thresholds and electrical brain topography.

We studied perceptual learning by determining psychophysical discrimination thresholds for visual hyper acuity targets (vernier stimuli) as a function of stimulus orientation. One aim was to relate perceptual improvements to changes of electrophysiological activity of the human brain. A group of 43 healthy adults participated in a psychophysical experiment where vernier thresholds for vertical and horizontal vernier targets were compared. In 16 subjects thresholds were measured for each orientation twice at an interval of 25 min. Between threshold estimations, evoked brain activity was recorded from 30 electrodes over the occipital brain areas while the subjects observed appearance and disappearance of supra-threshold vernier offsets. Mean evoked potentials were computed for the first and second 600 stimulus presentations, and the scalp topography of electrical brain activity was analyzed. Vertically oriented stimuli yielded significantly better performance than horizontal targets, and thresholds were significantly lower in the second half of the experiment, i.e. after prolonged viewing of stimuli. The improvements in discrimination performance were specific for stimulus orientation and did not generalize. Learning effects were also observed with electrical brain activity, and field strength of the potentials increased significantly as a function of time. Scalp topography of the evoked components was significantly affected indicating a shift of activation between different neuronal elements induced by perceptual learning.     

THE AUDITORY EVOKED RESPONSE TO REPEATED STIMULI DURING A VIGILANCE TASK

The auditory evoked response was measured between the vertex and left ear in 9 Ss. In an extension of the idea of the recovery function, series of tones, including sets of 5 tone pips with ½, 1, or 2 sec between them, and 11 sec between sets, were presented, while Ss were instructed to press a button whenever there was a change in tone length. Because of the slow recovery of this response, no subtraction procedure was necessary. There were 6 runs representing 6 experimental conditions for each S. Habituation within runs was not observed, but there was significant habituation between rims in spite of a constant vigilance level. The evoked responses to the second and subsequent stimuli of each set were much smaller than to the first, depending on the interstimulus interval within sets. Evoked responses to 65 db stimuli were less than to 85 db, but showed proportionally the same changes with successive stimuli.     

Variability of cortical evoked responses in man related to slow wave activity

Summary Acoustically evoked responses were recorded from subdural and intracerebral electrodes chronically implanted in man. it was found that during slow wave activity especially the negative component at about 100 ms latency varies in form and amplitude. If the stimuli were presented during increasing cortical negativity, the amplitude of the 100 ms component of the evoked responses was generally larger than in the case of stimuli presented during decreasing negativity.Supported by the Fonds zur Förderung der wissenschaftlichen Forschung in Österreich, Project 1429, and the British Council.     

The Effects of Stimulus Expectancy on Evoked Brain Potentials

The effects of self-generated expectancy of stimulus content on the visual evoked potential to physically identical stimuli were studied in college students. The subject set up his own internal expectant; by choosing to see either a bright or dim Hash. When a bright or dim Hash was anticipated, the potentials evoked by u medium stimulus intensity resembled the responses elicited by an actual bright or dim flash, respectively. Significant differences in visual evoked potential amplitude were obtained between identical medium intensity stimuli depending on the stimulus intensity expected, despite (he constant physical properties of the stimulus. 1 In1 results suggest that a subject's expectancy of certain physical parameters of a stimulus are as important In determining (he resultant visual evoked potential as the actual physical features of the stimulus.     

Comparative Study of Conditioning of Averaged Evoked Responses By Coupling Sound and Light in Normal and Autistic Children

This study compared the conditioning of averaged evoked potentials in 11 psychotic and 11 normal children (57 experiments). Evoked potentials, like the blocking of alpha rhythm, can be conditioned by coupling a short sound of low intensity with an intense light flash. In normal children, the amplitude of the potential evoked by sound at the occipital region is small and quite variable. Its amplitude increases and its variability diminishes after coupling of sound and light. Also its form may resemble the potential evoked by light, especially in its rhythmic after-effects. In psychotic children auditory evoked responses are variable and small and remain so during the coupling of sound and light. Instead, these patients respond to the sound with a late slow wave, which is strong and generalized. It is particularly prominent during conditioning and resembles the slow waves which follow the visual evoked potentials in these autistic children. This slow wave can be experimentally reproduced in normal Ss by Psilocybine. It also resembles in certain ways the slow waves provoked by movement or its anticipation in normal Ss but it appears here during sensorial conditioning which requires no movement. We suspect that this slow wave may reflect a diffuse motor component in the perceptual and associative processes of these autistic children.     

Visual Evoked Responses (VERs) in Normal and Disabled Readers

The present study investigated the possibility that reading disabled children exhibit abnormal visual evoked responses (VERs). Comparisons were made between a group of disabled children and two control groups of normals, one group matched on age and IQ, and the other on reading level and IQ. VERs were obtained for flashes of light and word stimuli. Results indicated that the reading disabled children showed a significantly smaller amplitude in the negative wave at 180 msec following stimulus onset, for an electrode placed in the region of the left angular gyrus, compared to the two controls. All three groups showed significantly greater negative amplitudes at this latency for the word stimulus compared to the light flash. The results confirm some earlier findings and suggest that a neurological origin of reading disability is likely.     

Visually triggered K-complexes: a study in New Zealand rabbits

K-complexes are the EEG elements recorded during the state of developing sleep and during slow wave sleep. They are the only EEG components which can be elicited by sensory stimulation during sleep. The peculiarity of New Zealand rabbits to sleep with their eyes open allows the use of visual stimuli to elicit K-complexes. Experiments were performed with three rabbits. For visual stimulation, an elongated screen illuminated by LED flashes was attached to an implant on the animal’s skull. The screen covered 20–120° of the visual field of one eye, and moved with the head during animal motion. One-millisecond flashes (15-s interval) were used during daytime in an illuminated room. Flashes elicited evoked responses, which, during the first stages of sleep, were often accompanied by K-complexes. The induced K-complexes were recorded from electrodes located both above visual and somatosensory areas. Evoked responses to visual stimuli were also recorded from both pairs of electrodes, although they were generated exclusively in the visual cortex. Correlation analysis showed that visual evoked responses and K-complexes induced by this stimulation were generated in visual cortex, and passively spread to the electrodes above the somatosensory area. Investigation of the latencies of induced K-complexes revealed two time windows when these complexes could be seen. Within each window there was no correlation between latency and amplitude of K-complexes. There was also no correlation between amplitudes of the visual evoked responses and K-complexes elicited by these responses. We propose that visual stimulation in light sleep temporarily opens a gate for some independent external signals, which evoke activation of the visual cortex, reflected in K-complexes.     

The "novelty response" in an electric fish: response properties and habituation

The electromotor behavior evoked by novel sensory stimuli in the electrogenic teleost fish Gnathonemus petersii was examined. Novelty responses (NRs) consisted of a transient accelerations of the rate of electric organ discharges following a change in sensory input. NRs were basically similar in nontreated and in immobilized (treated with curare) fish. NRs could be evoked reliably by brief novel stimuli of all four sensory modalities (acoustic, visual, electrical. electrolocation) used in this study. Stimuli of a duration longer than 5 s caused an on- and off-response. A sudden change in the quality of an ongoing sensory stimulus also evoked novelty responses. NR properties depended on the stimulus modality, stimulus intensity, stimulus duration, and on the prior stimulus history. Habituation of several response parameters of the NR (latency, duration, maximal amplitude, response probability) occurred within a series of repetitive stimuli of a given sensory modality. Each modality appeared to habituate separately. Rate of habituation depended on stimulus intensity and on interstimulus interval. A strong disruptive stimulus of another modality lead to dishabituation. The novelty response evoked by stimuli of low or medium intensities resembled an "orienting response" as described by Sokolov.     

Multiple temporal components of optic flow responses in MST neurons

 Neurons in monkey medial superior temporal cortex (MST) respond to optic flow stimuli with early phasic, tonic, and after-phasic response components. In these experiments we characterized each response component to compare its potential contributions to visual motion processing. The early responses begin 60–100 ms after stimulus onset and last between 100 and 250 ms, the tonic responses begin 100–300 ms after stimulus onset and last for as long as the evoking stimulus persists, and the after-responses begin about 60 ms after the stimulus goes off and last for 100–350 ms. A neuron’s tonic responses were evoked by specific optic flow stimuli: over two-thirds of the 264 neurons showed tonic responses evoked by two to five stimuli, whereas only 15% responded to either all or none of the stimuli. The tonic responses continued with stimulus presentations as long as 15 s, with their directional preferences being maintained throughout stimulation. However, the tonic response to a given stimulus was seen to change in amplitude when it was presented in random sequence with different sets of other stimuli. Thus, the tonic responses might convey substantial information about optic flow patterns, which continue with prolonged stimulation, but can be modified by the visual context created by other visual motion stimuli. Only about one-third of the 264 neurons had early responses that were selective for specific stimuli. In neurons yielding at least one early response, that neuron was most often activated by all the visual motion stimuli. After-rsponses occurred in only half the neurons, but they were more often specifically related to particular optic flow stimuli, regardless of whether those stimuli had evoked tonic excitatory or tonic inhibitory responses. The presence of early and after-responses complicates the interpretation of activity evoked when one stimulus immediately follows another. However, under those conditions, early responses and after-responses might contribute to signaling changes in the ongoing pattern of optic flow. We conclude that several components of MST responses should be recognized and that they potentially play different roles in the cortical analysis of optic flow. Tonic responses show the greatest specificity for particular optic flow stimuli, and possess characteristics which make them suitable neuronal participants in self-movement perception. Received: 1 April 1996 / Accepted: 1 October 1996     

Increased motor excitability produced by alerting stimuli

The proposal that increased excitability of sensory and motor systems occurs in response to dishabituated sensory stimuli was supported by experiments with cats in which excitability was measured by the amplitude of the response evoked by an electrical test stimulus. Evoked EMG, evoked head movement and the postsynaptic visual cortex response were briefly augmented following such stimuli, with a latency less than 20 msec for auditory and less than 50 msec for visual stimuli. Evoked EMG responses were rare during habituation, but even faint stimuli (40 db (SPL) tone pips) were effective during conditioning procedures. The proposed organization of the brain was thought to have adaptive significance, helping the subject to cope with emergency situations by enhancing the processing of sensory and motor information.