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At least 91 records · Page 5Linked to original sources
PubMed1991

Luminance-unbalanced pattern onset-offset electroretinogram and visual evoked cortical potential.

The effects of a luminance-unbalanced pattern onset-offset mode of stimulation on electroretinograms and visual evoked cortical potentials were investigated. With the use of originally devised software, only offset luminance was varied from 14.7 to 62.3 cd/m2. A vertical grating pattern (1.5 c/deg, 38.8 cd/m2 mean luminance, 0.95 contrast) was presented for 260 ms and was absent for 260 ms to normal subjects. With an increase in the luminance level of the offset pattern from the lowest level, the amplitude of the onset electroretinogram increased by degrees, while that of the offset electroretinogram gradually decreased. Conversely, onset visual evoked cortical potential responses decreased gradually, and offset visual evoked cortical potentials increased correspondingly. Furthermore, the spatial tuning of the onset pattern electroretinogram was already ambiguous, even when there was only a 4-cd/m2 difference between onset and offset pattern luminances. Thus, luminance control is indispensable for pattern onset-offset stimulation.

Adult↗
PubMed1992

The application of Laplacian analysis in the recording of half-field pattern-onset evoked potentials.

The Laplacian operator in electroencephalographic measurements consists of a mathematical combination of the responses from a number of electrodes (e.g., five in a crosswise montage). It enhances activity from sources lying underneath the area covered, relative to activity from outside this area. Thus, by appropriate positioning, the contributions of extrastriate and striate sources can be recorded selectively. To quantify the contribution of each hemisphere to half-field onset evoked potentials, the responses in two Laplacian operators, one over each hemisphere, were analyzed and compared to monopolar derivations and a bipolar derivation between the two hemispheres. Both the Laplacian and bipolar analyses were helpful in interpretation of the responses.

Electrodes↗
PubMed1991

Influence of a twofold voluntary hyperventilation on visually evoked cortical potentials and human pupillogram.

We studied the direct and aftereffects of twofold hyperventilation (HV) on pattern reversing VEPs and pupillograms (PGs) of 19 healthy volunteers. The VEP-N80 and P100 latencies increased during HV. Both peak times were maintained for a longer period, up to 20 minutes after HV-2 ended. In addition, the PG-latency time during HV and the PG-construction time during and after HV were increased. The results indicated a temporary delay of neural afferent transmission in the visual system during and after HV. A similar delay of the nervous transmission appeared in the efferent part of the system regulating the pupillary movements after HV ended. The observed changes of the VEP and PG parameters most probably resulted from the hypocapnia cased by HV and its effect on the brain vessels, although other explanations for the changes of the VEP- and PG-parameters may have been possible.

Adult↗
PubMed1991

Topographic mapping and source localization of the pattern reversal visual evoked magnetic response.

The topography of the visual evoked magnetic response (VEMR) to pattern reversal stimulation was studied in four normal subjects using a single channel BTI magnetometer. VEMRs were recorded from 20 locations over the occipital scalp and the topographic distribution of the most consistent component (P100M) studied. A single dipole in a sphere model was fitted to the data. Topographic maps were similar when recorded two months apart on the same subject to the same stimulus. Half field (HF) stimulation elicited responses from sources on the medial surface of the calcarine fissure mainly in the contralateral hemisphere as predicted by the cruciform model. The full field (FF) responses to large checks were approximately the sum of the HF responses. However, with small checks, FF stimulation appeared to activate a different combination of sources than the two HFs. In addition, HF topography was more consistent between subjects than FF for small check sizes. Topographic studies of the VEMR may help to explain the analogous visual evoked electrical response and will be essential to define optimal recording positions for clinical applications.

Adult↗
PubMed1988

The pattern electroretinogram and visual-evoked potential in glaucoma.

The pattern electroretinogram (PERG) may reflect ganglion cell or inner retinal layer activity. The most sensitive spatial and temporal variables for testing patients with glaucoma have not yet been identified. Fifty-two glaucoma suspects, 51 glaucoma patients, and 28 normal subjects were studied with the PERG and VEP, using three repetition rates and three spatial frequencies. Fast Fourier transforms were calculated at each spatial frequency and reversal rate. An analysis of variance revealed that normals could be differentiated from ocular hypertension and glaucoma patients using the amplitude of the PERG (second and fourth harmonic). Abnormalities in phase of the PERG between groups were also detected. A discriminant analysis of all amplitude and phase data revealed that the phase shift of the response of the second harmonic at 11 alternations/s (15-min checks) and at 5.5 alternations/s (15-min checks) correctly identified 81% of the normal and 75% of the glaucoma patients. The phase shift determinations of the VEP revealed significant abnormalities using 2 and 1/2 standard deviation confidence limits. There was significant overlap in the pattern ERG amplitude and phase shift in all three groups.

Analysis of Variance↗
PubMed1988

Neuronal responsiveness in area 21a of the cat.

Photic responsiveness was studied in cells of area 21a which was identified as a region containing few cells projecting to area 17, and was bounded to two other visual areas (posteromedial lateral suprasylvian area and area 19) providing abundant efferent projection to area 17. Area 21a cells were characterized by strong orientation, but demonstrated poor direction and end-stop selectivity, in contrast to strong direction or end-stop selectivity of cells in another two visual areas.

Animals↗
PubMed1985

Electrophysiological evidence for the existence of coarse and fine disparity mechanisms in human.

Visual sensitivity to stereoscopic disparity changes was measured both psychophysically and by means of evoked potentials. The binocular disparity of a dynamic random-dot stereogram portraying a single flat plane alternated between two values symmetrical about the plane of fixation. The threshold for disparity alternation of the stereoscopic plane was determined at alternation rates between 4 and 12 depth reversals per second (rps). Evoked potential and forced-choice psychophysical estimates of stereoscopic threshold at each reversal frequency agreed, with a mean discrepancy of only +/- 0.1 log units. Evoked potential amplitude was a linear function of log disparity up to about 15 arc min peak to peak disparity. For larger disparities, the evoked potential amplitude versus log disparity function was found to be nonmonotonic with a dip occurring at approximately 26 arc min disparity. Responses to fine disparities of less than 20 arc min lay close to one temporal phase while those evoked by coarse disparities greater than 40 arc min lay near a different phase. The data suggest that disparity processing mechanisms either undergo dynamic changes as disparity increases or that processing shifts between at least two independent mechanisms.

Depth Perception↗
PubMed1987

Representation of edges of variable blur by neuronal responses in the lateral geniculate body and the visual cortex of cats: limits of linear prediction.

We have measured the responses of cells in the cats lateral geniculate body and the visual cortex to edges which were blurred to various degrees (cosinusoidal blur). For the same cells also the responses were determined to sinusoidal gratings of various fundamental frequency and to slits of various blur and width. All stimuli were moved across the receptive fields at various speeds. The responses of most cells increased with increasing edge sharpness, but usually reached a maximum at a blur corresponding to a high frequency cutoff at 0.6-1.2 c/deg. The responses to the sharpest edges were usually smaller than those to a blurred edge (up to -50% in individual cells and -15% in the average). After normalization, the responses predicted from the spatial frequency tuning curves and the Fourier transform of the edge stimuli corresponded well to the measured blur functions up to the maximum of the edge response which varied considerably between cells, however. At edge sharpness beyond that maximum, the predicted curves rose up to edge sharpness with high frequency cutoff 1.6-1.8 times above that which produced the experimental neuronal response maximum. On the other hand, responses could increase with edge sharpening in spatial frequency regions, in which no or only small responses were seen with sinusoidal gratings (e.g. at lower spatial frequencies in "band pass neurons"). Geniculate X- and cortical simple cells as well as those geniculate Y-cells which showed phase locked grating responses behaved similarly in all respects. We concluded that edge sharpness is not represented by response amplitude of individual neurons but by the spatial distribution of excitatory peaks across the representation of the retinotopic cortical map. Our findings further indicate that spatial models of receptive fields assuming linear signal summation have only a limited value for predicting edge sharpness.

Animals↗
PubMed1990

Responses of visual cortical neurons to curved stimuli and chevrons.

Single cells were recorded in area 17 of anaesthetized and paralyzed cats and their responses to curved stimuli and chevrons compared. Striate cells exhibited three different response patterns. A first group responded optimally to a straight line (i.e. zero curvature) and responded similarly to chevrons and to curved lines. A second group responded to all curvatures and was broadly tuned for the straight line when tested with chevrons. A third group responded only to large curvatures, many (2/3) to both signs of curvature and a number (1/3) to only one sign. Cells in this group responded differently to chevrons and curved lines. Cells in these three classes differed both in length-response curve and in width of orientation tuning. Laminar analysis revealed that the three classes are distributed differently across cortical layers. These data shed new light on the finding of Malpeli and coworkers that orientation is extracted at least twice in a cortical column.

Animals↗
PubMed1990

The two-dimensional spatial structure of nonlinear subunits in the receptive fields of complex cells.

We have estimated the second-order response properties of complex cells in two spatial dimensions by cross-correlating their spike trains with a binary approximation of a Gaussian white noise stimulus ensemble. Wiener-like kernels were computed and generally consisted of two or three parallel, elongated subregions alternating between augmented and suppressed response. These subunits were scattered across the receptive fields of complex cells and their axes of elongation agreed with the optimal orientation determined with drifting gratings.

Animals↗
PubMed1990

The visual acuity of the lynx.

Visual evoked potentials were recorded from the occipital scalp of two anaesthetized Lynx (Lynx europea) in response to alternating gratings of various spatial frequencies and contrasts. The visual acuity of the Lynx was found to be around 5-6 c/deg, i.e. very close to the visual acuity of the cat and by far inferior to human acuity.

Animals↗
PubMed1990

Influence of spatial frequency on tuning and bias for orientation and direction in the cat's striate cortex.

Directionality, orientation and spatial frequency tuning were determined for 108 neurones recorded extracellularly from the striate cortex of anaesthetized cats. Significant sharpening of orientation selectivity with increasing spatial frequency was seen in all simple neurones and the overwhelming majority of complex neurones. Orientation selectivity sharpened in 90 and broadened in only 10 of 100 fully characterized neurones. At least four distinct classes of neurone could be characterized on the basis of their directionality at optimal spatial frequency, and the presence or absence of changes in directionality over a range of spatial frequencies: in two classes, directionality was spatial-frequency dependent; in the remaining two it was invariant. With two exceptions Type A neurones (23 cells) were direction-selective; they were narrowly tuned for orientation and spatial frequency, and their directionality was invariant with spatial-frequency. The majority of neurones (52 cells) were Type B, most of which were direction-biased; their bias for direction varied systematically with spatial frequency. Type C were direction-biased and spatial-frequency selective (5 cells), but showed a clear reversal of bias with change in spatial frequency. Type D, a subset of direction-biased cells, were bidirectional and spatial-frequency invariant (8 cells), with comparable response strengths to motion in two opposing directions at all spatial frequencies. These response types crossed traditional boundaries between categories of simple and complex neurones, assigned on the basis of spatial summation, presence or absence of end-inhibition, and receptive field size.

Animals↗
PubMed1991

Classifying simple and complex cells on the basis of response modulation.

Hubel and Wiesel (1962; Journal of Physiology, London, 160, 106-154) introduced the classification of cortical neurons as simple and complex on the basis of four tests of their receptive field structure. These tests are partly subjective and no one of them unequivocally places neurons into distinct classes. A simple, objective classification criterion based on the form of the response to drifting sinusoidal gratings has been used by several laboratories, although it has been criticized by others. We review published and unpublished evidence which indicates that this simple and objective criterion reliability divides neurons of the striate cortex in both cats and monkeys into two groups that correspond closely to the classically-described simple and complex classes.

Animals↗
PubMed1991

A programmable digital video pattern generator controlled by microprocessor for basic research and clinical applications.

A programmable digital video pattern generator controlled by a microprocessor is described as a part of any commercial or specific visual evoked potential analysis system. The generator handles all kinds of patterns (uniform, orthogonal and radial) with all known presentation modes (onset, offset and reversal) and resolution 640 x 200 square pixels. The frame rate of the generator is adjusted to be compatible to either low cost 50 Hz video monitor or expensive ones of higher frame rate. The operation parameters defined by software are: pattern selection, trigger mode, field selection and stimulation rate. The development of software applications for special research demands is desirable because of the control of the generator by a microprocessor.

Computer Graphics↗
PubMed1992

Texture segregation is processed by primary visual cortex in man and monkey. Evidence from VEP experiments.

We investigated whether the process of texture segregation can be allocated to a specific visual cortical area. We designed a stimulus to reveal the presence of a mechanism, which is specifically sensitive to a checkerboard, that is solely defined by textures segregating due to orientation differences of the constituting line segments. We recorded evoked potentials to this stimulus in man and awake monkey. A difference component, signalling texture segregation sensitivity, could be recorded from both types of subjects. Its presence depended on the spatial extent of the textures, in a manner correlating with the perceptibility of the checkerboard. This difference response could be localized in primary visual cortex by means of equivalent dipole estimations.

Animals↗
PubMed1992

Orientation selectivity of 3-month-old infants.

A modification of the visual evoked potential (VEP) technique, first employed by Braddick, Wattam-Bell and Atkinson [(1986) Nature, London, 320, 617-619] was used to estimate the orientation selectivity of 3-month-old infants. The orientation-selective VEP was recorded in response to various changes in the orientation of a square-wave grating. The magnitude (the square root of the power of the FFT) at the frequency of orientation change was assumed to represent the response to the change in grating orientation. Orientation sensitivity was then estimated by plotting the magnitude of the FFT at the frequency of orientation change as a function of the log of the orientation change in degrees. For each individual, the data were fit by nonlinear regression and threshold was defined as the largest orientation angle for which the magnitude of the FFT was zero. The results suggest that the orientation selectivity of 3-month-old infants (1.33 deg) is similar to that of adults (1.13 deg) tested with the same stimulus parameters (1 c/deg, 9 Hz). However, when adults are tested with stimulus parameters selected to optimize their VEP response (4 c/deg, 18 Hz) instead of those which optimize the infant's response, the orientation discrimination of adults improves by a factor of 2 (0.53 deg). The results obtained from adults under optimum stimulus conditions (4 c/deg, 18 Hz) approach the estimates reported in the literature for static stimuli.

Adult↗
PubMed1992

Cortical neurons: isolation of contrast gain control.

The selectivity of cortical neurons remains invariant with contrast, even though the contrast-response function saturates. Both the invariance and the saturation might be due to a contrast-gain control mechanism. To test this hypothesis, a drifting grafting was used to measure the contrast-response function, while a counterphase grating was simultaneously presented at the null position of the receptive field (where it evokes no response at any contrast). When the contrast of the counterphase grating increased, the contrast-response function shifted primarily to the right. This result is consistent with the hypothesis that there is a fast-acting gain-control mechanism which effectively scales the input contrast by the average local contrast.

Adaptation, Ocular↗