Search PubMed⌕ Search

SEARCH · Search PubMed

Results for “Pattern Recognition, Visual”

Search indexed PubMed citations on genomics, clinical trials, systematic reviews and public health. Explore titles, authors and supplied subject terms, then open the PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 163 records · Page 9Linked to original sources
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↗
PubMed1994

Short-term changes in the response characteristics of the human visual evoked potential.

The present study examined how the response characteristics of the visual evoked potential (VEP) varied during the course of trials using a sinusoidal grating stimulus that reversed contrast in a square-wave manner. To accomplish this, amplitude and phase values were derived in short segments during the course of continuous stimulation for three subjects. When stimulus spatial frequencies of 0.77 or 1.55 c/deg were used, VEP amplitude remained at a stable value throughout the trial. At 3.1 c/deg, 6-12 sec were required for VEP amplitude to increase to a stable value, which was on average 204% greater than the value noted during the first few seconds of the trial. At 6.2 and 12.4 c/deg, VEP amplitude changes were more complex, first increasing and then decreasing substantially, to levels that were on average 63.8% and 38% of the peak reached earlier in the trial. In all cases, VEP phase decreased during the trial. The magnitude of this decrease ranged up to 50 deg, corresponding to an approx. 10.5 msec delay for the 6.65 Hz stimulation rate used. Prior exposure to an adapting grating diminished the changes in VEP amplitude and advanced the phase changes. Therefore, these changes appear to represent a form of contrast adaptation that is restricted to responses to high spatial frequencies. In addition, the present results provide evidence against a fundamental assumption of signal averaging--that an invariant stimulus will evoke an invariant response.

Adaptation, Ocular↗
PubMed1982

Binocular deprivation can erase the effects of preceding monocular or binocular vision in kitten cortex.

Kittens were given visual experience through one or both eyes for two weeks around the peak of the sensitive period. Subsequently they were binocularly deprived for at least one year. This period of pattern deprivation erased completely the effects of the preceding temporary experience. Ocular dominance distribution, orientation selectivity and response quality of the cortical units resembled those obtained from kittens which are contour-deprived throughout their early postnatal development. This suggests that the effect of visual experience is not to engrave irreversibly certain features of the early visual world, but to adapt the cortex continuously and in an integrative fashion to features which are prominent throughout the sensitive period.

Animals↗
PubMed1984

Combined electrophysiological assessment of the visual system in central serous retinopathy.

Six subjects suffering from idiopathic central serous retinopathy were examined during the acute phase using a battery of neurophysiological tests. The records (using skin electrodes) included the electro-oculogram (EOG), the white light electroretinogram (ERG) during various stages of dark and light adaptation, and the flash and pattern reversal visual evoked potentials. Results obtained from the affected eye were compared with those from the healthy eye. The values of the healthy eye were not significantly different from our normal control group. The EOGs of the affected eyes were not significantly different from those of the healthy eyes. The 'a' wave of the ERG during light adaptation was significantly (P less than 0.05) smaller in the affected eye. The amplitudes of the other components and the latencies of all components were not affected. The latency of the PR-VEP of the affected eye in all subjects was prolonged in comparison with the unaffected eye. In 3 subjects the latency was prolonged by more than 2 S.D.s compared with our control group. The amplitude of the PR-VEP in 5 of our 6 subjects was slightly but significantly (P less than 0.02) smaller in the affected eye. There was no correlation between amplitude and latency changes.

Adaptation, Physiological↗
PubMed1994

Differences in the pattern visual evoked potential between pregnant and non-pregnant women.

It has been proposed that latencies of some components of the pattern-reversal visual evoked potential (PRVEP) are shorter in women than in men because of differences in levels of circulating sex steroids. Pregnancy is a time when serum levels of oestrogen and progestogen are considerably greater than in the non-pregnant state. Whole and half field PRVEP latencies and amplitudes have been compared in 16 pregnant and 38 healthy non-pregnant women. The mean P100 latencies for all responses were shorter in the pregnant women, with statistically significant differences for the left eye whole field latency (P < 0.05) and the left eye right and left half field latencies (P < 0.005 and P < 0.05, respectively) and the right eye right half field latency (P < 0.05). The latencies in women in the pregnant group showed a negative correlation with gestation, which reached statistical significance for the REWF (r = -0.55, P < 0.05). These observed differences in PRVEP latencies in pregnant and non-pregnant women and the association between latency and gestation are likely to be due to differences in circulating sex steroids, and this effect may be the principal reason for latency differences between the sexes.

Adolescent↗
PubMed1988

Separate brain potential characteristics in children with reading disability and attention deficit disorder: relevance-independent effects.

The effects of reading disability (RD) and attention deficit disorder (ADD) on event-related potentials (ERPs) were investigated in 52 boys between 8 and 12 years of age. There were four groups: 25 children without RD divided into two groups (17 without ADD and 8 with ADD) and 27 children with RD divided into two groups (11 without ADD and 16 with ADD). The children participated in three tasks ("games") designed to assess selective attention to the color black (vs white), to any of two letters (vs. two patterns), and to any of 12 letters (vs. 12 patterns). ERPs were recorded over the left and right occipital, central, and frontal hemispheres. Children with RD as compared to those without RD were characterized by the following ERPs: (1) a smaller amplitude positive wave at about 240 msec (P240) over the left central hemisphere: (2) a smaller amplitude positive wave at about 500 msec (P500) over the right central hemisphere: (3) a larger P500 over the right central hemisphere: (3) a larger P500 over both left and right occipital hemispheres: and (4) smaller within-subject-and-condition variability of ERP waveform. These effects did not differ significantly for the three games and, therefore, were not specific to letter processing per se. The effects of RD did not interact with the effects of ADD for the most part. The ERPs clearly indicated these two disorders, in part, involve different underlying brain processes.

Arousal↗
PubMedDate not supplied

Rat and human visual-evoked potentials recorded under comparable conditions: a preliminary analysis to address the issue of predicting human neurotoxic effects from rat data.

Pattern-onset visual-evoked potentials (VEPs) were recorded from rats and humans in order to perform cross-species comparison of neuronal functional properties reflected by the early VEP components. The spatial frequency of a sinusoidal test grating was varied in Experiment 1. For both species, amplitude of the first positive VEP component was larger at low spatial frequency and decreased as spatial frequency increased. The immediately succeeding negative component was small at low spatial frequency and was of maximal amplitude at moderate spatial frequency. The effects of stationary pattern adaptation on these components were investigated in Experiment 2. Subjects viewed either a blank field or the test grating prior to recording VEPs. For both species, adaptation had no effect on the positive component but strongly attenuated the negative component. Experiment 3, in which only humans were tested, indicated that the negative component was of cortical origin. Only cortical neurons are known to be orientation selective, and the effect of adaptation diminished as the orientation difference between the adaptation and test gratings increased. These results suggest that the early positive and negative components arise from parallel visual pathways, and that the rat components may reflect visual processes qualitatively similar to those of humans.

Adaptation, Physiological↗
PubMed1988

Asymmetries in ON and OFF visual pathways of humans revealed using contrast-evoked cortical potentials.

Positive- and negative-contrast stimuli yield the perceptions of brightness and darkness, respectively, and are processed separately by ON and OFF neural pathways. The properties of these morphologically and pharmacologically distinct subsystems were measured in humans by recording visual evoked potentials (VEPs). These electrical responses from the visual cortex were elicited by novel positive- and negative-contrast stimuli, designed to emphasize, selectively, contributions from ON and OFF pathways. Results revealed differential processing of the two types of contrast information, suggesting asymmetries in ON and OFF subsystems; OFF subsystems have finer spatial tuning and greater contrast gain than ON subsystems. These VEPs may be useful in diagnosing neurological disorders that involve primarily one subsystem.

Adult↗
PubMed1992

Electroretinograms (ERGs) and visual-evoked potentials (VEPs) elicited by pattern displacement.

The relation between the amplitude of visual responses to a checkerboard stimulus and the degree of lateral displacement of the checks was examined across different check sizes with simultaneously recorded electroretinograms (ERGs) and visual-evoked potentials (VEPs). The amplitudes of both the b-wave and the after-potential of the ERG increase linearly with pattern displacement. However, the major components of the VEP (N70 and P100) were smaller than expected from linearity for both small checks with small displacements (thresholding) and for large checks with large displacements (saturation). These results suggest that the ERG is proportional to the number of receptors stimulated, but the VEP reflects neural processes influenced by the spatial structure of the stimulus.

Adult↗
PubMed1992

Cortical potentials evoked by short wavelength patterned light.

Pattern-evoked cortical potentials (PECPs) were recorded in response to the onset of blue and black, square wave gratings superimposed on a bright orange background. Several lines of evidence, including spectral sensitivity and the absence of response in a tritanopic subject, indicate that the measured cortical potentials reflect input from S cones. Spatial and temporal tuning of this response are comparable to psychophysical measures of the S-pathway. In agreement with previous studies of achromatic and chromatic spatial processing, a comparison of the response to patterns of different complexity indicates that spatial processing in the S-pathway can be described by linear systems analysis. An oblique effect for the S-pathway is demonstrated. Additional findings which bear on the nature of postreceptoral processing in the S-pathway are considered. Included are measurements from a patient with diabetic retinopathy. This study underscores the potential importance of the S-pathway for spatial information processing.

Adult↗