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PubMed2005

EEG gamma-band activity in rapid serial visual presentation.

Evidence is available that oscillatory activity in the gamma frequency range (>30 Hz) might be related to the attentional selection of target items. Rapid serial visual presentation (RSVP) paradigms are instrumental in addressing cognitive functions such as visual attention, and they are increasingly combined with the measurement of electrical brain activity. In the present study, gamma-band responses for target and standard stimuli were investigated in an RSVP oddball paradigm. In a first study, stimuli were presented at a frequency of 10 Hz, the stimulus sequence consisted of rare colored letters (targets) and frequent black letters (standards). In addition, stimulus size was varied across experimental blocks. Significant target modulations were observed for the P3 ERP and induced (i.e., not phase-locked) gamma-band responses. Besides this late activation, no further gamma-band responses were observed. A second study aimed at replicating these findings by employing a reduced stimulus presentation rate of 7.1 Hz. Again, besides the P3 ERP a late increase in induced gamma-band activity was observed. However, as compared to Study 1, this induced response was less pronounced. The induced gamma-band response observed in the present studies might reflect utilization of information derived from previous processing steps for behavioral performance or memory storage as suggested in the 'match-and-utilization-model' of gamma activity.

Adult↗
PubMed2006

Limited plasticity of difference neurons in the visual cortex and hippocampus in rabbits during the oddball (random substitutions) test.

The activity of 41 visual cortex neurons and 20 hippocampal field CA1 neurons was studied in rabbits during application of the oddball stimulation paradigm using color stimuli of different intensities. Among these cells, about one third were plastic cells (34% of cortical cells and 37% of hippocampal cells). These neurons showed significant increases in late responses, at times 200-500 and 200-1000 msec for visual cortex neurons and 300-550 msec for hippocampal neurons, to rare deviant stimuli of lesser intensity as compared with responses to the frequent standard stimuli of greater intensity. The initial peak of the response (40-120 msec), the "difference discharge," remained stable in responses to deviant and standard stimuli throughout the experiment. It is suggested that the strengthening of the late components of neuron responses to rare deviant stimuli (limited plasticity) reflects inclusion of the mechanisms of the orientational reflex.

Animals↗
PubMed1992

Effect of subject's family name on visual event-related potential in schizophrenia.

Visual event-related potentials (ERPs) were recorded from 15 schizophrenics and 15 age-matched and gender-matched controls, while they performed a modified version of the oddball paradigm. Each subject was required to detect target stimuli among a random sequence of stimuli under two conditions, name and color. In the name condition the stimulus sequence consisted of the subject's family name (deviant 18%), four other family names (standard 73%), and a city name (target 9%). In the color condition the respective stimuli were a pair of solid red circles, four white paired-arrows, and a pair of white plus and minus signs. ERPs elicited by stimuli contained triphasic potentials of P2, N2, and P3. In controls these waves were selectively enhanced for the subject's family name as compared with standards, whereas in schizophrenics no significant difference between the subject's family name and other names was observed. In contrast, selective enhancement for color deviants was observed in both subject groups. These results suggest that impairment of involuntary attention, especially for familiar and significant stimuli such as names in daily life, may underlie disturbances of attentionally controlled central processing in schizophrenia.

Adult↗
PubMed1981

Prefrontal unit activity during delayed conditional discriminations in the monkey.

Single unit activity was recorded from the prefrontal cortex (principalis, arcuate and inferior convexity areas) of the monkey while the animal was performing a delayed conditional discrimination task. Sequential events of the task were as follows: instructional cue presentation, 1st delay, presentation of two pattern stimuli on left and right windows respectively as a discriminative cue, 2nd delay and choice response to the left or right window. The positive pattern was dependent on the instructional cue. In total, 424 units obtained from two monkeys showed a correlation with some aspects of the task. Of these 424 task-related units, 169 differential between the instructional cues and/or the discriminative cues. The majority of these differential units (n = 123) were found to be related to spatial information processing (related to the side of the response) while 19 differential units were related to non-spatial information processing (related to the color or pattern configuration of the cue). The activity of the remaining 27 differential units was considered to be related to both spatial and non-spatial information processing (related to both the instructional cue and the side of the response) and this type of unit was shown to be involved in conditional information processing. The results indicate that prefrontal units may be related to the meaning of the stimulus independent of its physical properties.

Animals↗
PubMed1984

Sensitivity of complex cells in cat striate cortex to relative motion.

Sensitivity of 95 complex cells to relative motion between oriented bars and textured backgrounds was investigated monocularly in the striate cortex of lightly anesthetized, paralyzed cats. Cells were classified conventionally. Those in deep layers were either direction-selective, or strongly preferred one direction of motion, and responded well to background texture motion alone: backgrounds potentiated the response to the bar in the cell's preferred direction when moved in phase, or in the opposite direction when moved in antiphase; other combinations depressed the level of response compared with that for the bar alone. The majority of direction-selective or strongly direction-biased cells in superficial layers behaved similarly. The most interesting superficial-layer cells were bidirectional or weakly direction-biased, and recorded closer to the cortical surface than the direction-selective neurons. A majority showed preference for relative motion, some for antiphase, others for in-phase motion, regardless of the absolute direction of motion across the receptive field, which could not be accounted for on the basis of separate responses to bars and backgrounds alone. Three of the superficial-layer direction-selective cells also showed preference for antiphase relative motion. In a few complex cells from superficial laminae, backgrounds were either without influence on responses to oriented stimuli, or purely suppressive. Visual backgrounds against which objects are perceived are usually neither featureless nor motionless: the results suggest that most complex cells in striate cortex are sensitive to the context in which objects are seen and susceptible to relationships between objects and their backgrounds in relative motion.

Animals↗
PubMed1979

Modifications of the pattern-evoked potential (PEP) in relation to the stimulated part of the visual field (clues for the most probable origin of each component).

A spatio-temporal analysis of the successive and simultaneous components of the pattern-evoked potential recorded on the scalp, and of their modifications according to which part of the visual field is stimulated, was carried out with 20 'normal' subjects, in order to shed some light on their most probable sites of origin. The stimulus consisted in the onset of a 20 degree checkerboard presented in runs of 75 stimuli each. Its duration was 750 msec and its frequency of occurrence was random (about 1 every 1500 msec). Twelve different visual field situations were recorded: whole field, half fields and quadrants and stimuli limited to the fovea, to the macula and to extramacular areas. Data were collected from 9 active electrodes (in line, forming a cross montage), and various reference electrodes (ear lobes, Fz, non-cephalic). Eye movements were simultaneously recorded. The electrophysiological data were digitized on line and processed by computer in the form of averaged spatio-temporal maps. In addition to the classical posterior components which peak on the midline (N 60, N 140, and P 200) or less than 4 cm away on both sides (P 90), a late negative wave (LN 210) was differentiated which peaked lower than the inion and more than 8 cm away from the midline on both hemispheres. The large inter-individual variability of the spatio-temporal organization of these components under the same conditions, as well as its very good intra-individual reproducibility, were emphasized. Interpreted on the basis of a simple dipole sheet model of the visual cortex, the changes observed for each component in the 12 experimental situations led to the following suggestions: only the first component N 60 could reflect the activity of the part of area 17 emerging on the convexity, whereas P 90 (Jeffreys' CI) is more likely to originate in area 19 and the midline components N 140 and P 200 in area 18. The topography and reactivity of LN 210 could fit with the hypothesis that it reflects activity of the infero-temporal cortex.

Adult↗
PubMed1983

Changes in the visual evoked potential to pattern reversal with lithium medication.

Previous reports in the literature have suggested that lithium medication does not affect the VEP to flash stimulation. It was predicted that this would not be true for pattern reversal stimulation. Seven patients had their pattern evoked potential measured using a 42' check to fractional pattern displacement of 1/4, 1/2, 3/4 and full square. The VEP was measured before, 1 week after and 5 weeks after the commencement of lithium medication. The results show that there is a significant increase in both the N65-P95 and P95-N125 amplitudes when the 'before lithium' sessions are compared to 'after lithium.' Seven normal subjects matched for the age and sex of the patient group were tested twice, once as a 'control' and a further 5 weeks after this. No significant differences were found in the 'control' sessions between patient and normal groups although a significant difference in both the N65-P95 and P95-N125 amplitude after the treatment of the patient group with lithium was found.

Adult↗
PubMed1982

Differences among humans in steady-state evoked potentials: evaluation of alpha activity, attentiveness and cognitive awareness of perceptual effectiveness.

We examined some of the variables that were possible sources of the wide variability among and within human subjects in their steady-state visual evoked potentials (VEP) in response to continuously counter-phased visual stimuli (vertical bars). We found that within a given subject the magnitude of VEP was reasonably consistent during replicate trials under comparable conditions. However, across subjects there were enormous differences (as much as 17-fold) in the VEP magnitude (i.e. in the spectral power developed at the stimulus reversal frequency). These differences could not be explained by differences among subjects in arousal (alpha activity before or during stimulation), attentiveness (as indicated by reaction times to random cueing), or by a person's subjective impression of his responsiveness to stimulation.

Adult↗
PubMed1991

Stereopsis in the cat: behavioral demonstration and underlying mechanisms.

The neural substrates subserving stereopsis were investigated behaviorally and electrophysiologically in the cat. In one set of studies, we examined behaviorally the ability of normal cats to perceive depth on the sole basis of spatial disparity using random-dot stereograms. Results showed that the animals were able to carry out this discrimination. We then evaluated the contribution of the optic chiasm, the corpus callosum and the primary visual cortex to this function. Results indicated that: (1) chiasma transection drastically reduced the ability of the animals to solve the random-dot problem; (2) a callosal split had little or no effect on their ability to relearn the same discrimination; (3) a section of both the corpus callosum and optic chiasm abolished this ability; and (4) bilateral lesions of areas 17-18 also abolished it. In another set of studies, we examined electrophysiologically the properties of neurons in the various visual cortical areas where disparity-based depth discrimination processes are presumed to take place. We recorded from areas 17, 18 and 19 of normal and split-chiasm cats. Results showed that: (1) the primary visual cortex of the normal cat contained cells sensitive to stimulus disparity; (2) these disparity sensitive neurons were also present in area 19 although in a much lower proportion and were more widely tuned than those in areas 17-18; and (3) following the section of the optic chiasm, there was a significant decrease in the number of disparity sensitive cells in areas 17-18, whereas in area 19 they were nearly completely absent. The results obtained from the lesion studies and from the single unit recording experiments indicate that stereoscopic depth perception is highly dependent in the cat upon the integrity of the through-the-chiasm geniculo-striate pathway and its target primary visual cortex.

Animals↗
PubMed1993

Brain potentials in a phonological matching task using Chinese characters.

In readers of English, involved in a rhyme judgement task, mismatch trials are associated with an enhanced N450 component of the Event Related Potentials (ERPs). It has been suggested that N450 is related to orthographic or phonological priming. In this paper ERPs were recorded during a phonological matching task, using pairs of logographically dissimilar Chinese characters. A pair was considered to match if they sounded alike with identical phoneme sequences. The subjects (native Chinese speakers) were instructed to ignore vowel-inflections, which in Chinese have lexical status. Since sublexical assembly of phonology is not used in reading Chinese characters, and the members of each pair were logographically dissimilar, match and mismatch trials did not suffer in the amount of orthographic or sublexical phonological priming. An enhanced negative component (latency near 400 msec), was observed in ERPs elicited by the second character in non-matching pairs. The negativity could be similar to N450. If this were so, then N450 could not be associated with orthographic priming, nor with sublexical phonology, but would probably be associated with postlexical processing. Also, in both readers of Chinese and English, the negativity enhanced in non-match trials is larger over the right side of the scalp, suggesting a similar brain lateralization of the underlying processes.

Adolescent↗
PubMed1982

Pattern visual evoked potentials in the rat.

Pattern and flash visual evoked potentials (VEPs) were obtained in 25 adult male hooded rats. Different electrode placements were used to detect the origin of the VEPs. The spatial frequency and temporal modulation of the stimulating pattern and the refractive state of the eye were varied to investigate the specificity of these factors in pattern VEPs. Variations in latency, amplitude, and trace morphology were found which were consistent with the characteristics of the visual system of the rat.

Animals↗
PubMed1982

The effects of contrast and length of gratings on the visual evoked potential.

Visual evoked potentials (VEP's) were obtained to grating stimuli modulated in phase at 8 Hz. The amplitude of the VEP was linearly related to the length of a centrally-fixated grating up to a critical length value equivalent to 9-16 grating cycles, for a range of spatial frequencies; thereafter amplitude first decreased then increased with increasing length. VEP amplitude was proportional to log. contrast as well as to length, and the intercept of the regression line with the contrast axis coincided with the psychophysical threshold for gratings of different lengths. More sensitive measurements with low-contrast stimuli around threshold indicated that in this range VEP amplitude is linearly related to contrast, and there is no threshold nonlinearity. The effects of visual field location and adaptation on the VEP to gratings of different lengths and spatial frequencies were also studied. The relationship of these findings to psychophysical, single-cell and transient VEP data is discussed.

Adaptation, Ocular↗
PubMed1983

The impulse response of a movement-detecting neuron and its interpretation.

When a visual pattern undergoes a brief displacement, the response of the neuron depends primarily upon the size of the displacement. One measures a tradeoff between velocity and duration of motion, analogous to the reciprocal relationship between intensity and duration in the perception of a brief flash of light (Bloch's law). Characterizing the movement detector by its "impulse response" could be useful in predicting the response to arbitrary motions.

Animals↗
PubMed1983

Spatial frequency and the pattern onset-offset response.

Visually evoked responses (VERs) were recorded with vertical sinusoidal gratings presented in the on-off mode at rates of 0.5 and 1 Hz. Stimulus spatial frequency ranged from 0.5 to 16 c/deg and its contrast varied from near-threshold to the value of 0.3. At low spatial frequencies, 0.5 and 1 c/deg, the onset and offset VERs were of similar shape, magnitude and contrast dependence. At spatial frequencies higher than 2 c/deg the onset VERs were usually larger and persisted at lower contrast levels than the offset VERs. The results are consistent with the hypothesis that a gradual transition from the operation of transient channels to the operation of sustained channels takes place on increasing stimulus spatial frequency and that within a wide spatial frequency range (at least from 2 to 8 c/deg) suprathreshold gratings effectively stimulate both types of channels. It is confirmed that both onset and offset VERs are delayed at high spatial frequencies. In addition, a longer delay of the late onset waves was found in comparison with the delay of the early onset waves.

Adolescent↗