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Results for “Pattern Recognition, Visual”

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PubMed1984

Spatiotemporal conditions which elicit or abolish the oblique effect in man: direct measurement with swept evoked potential.

Reversing sine wave gratings were electronically swept in spatial frequency and contrast. The acuity limits and contrast thresholds of 4 observers were inferred from evoked potential stimulus-response functions elicited by these stimuli and retrieved with a quadrature lock-in amplifier. The evoked potential functions, linearized in the case of contrast by increasing contrast logarithmically with time, were extrapolated to the point of zero response. This point provides an electrophysiologically defined threshold value for acuity and for contrast. An oblique effect (superior sensitivity for HV-oriented gratings) could reliably be demonstrated in both acuity and contrast threshold performance. This oblique effect could readily be abolished under low spatial/high temporal frequency conditions. The findings are discussed in terms of shifting relative strengths of X and Y contributions to the steady-state evoked potential.

Adult↗

PubMed1986

The relationship between log-complex transforms of stimuli and the cortical responses they evoke.

Two stimuli whose log-complex transform images in striate cortex are orthogonal square-wave gratings cause responses of equal magnitude in striate cortex but of unequal magnitude in extra-striate cortex. However, two other stimuli, orthogonal square-wave gratings in visual space, whose log transform images are identical except for phase shift, cause responses of equal magnitude in both striate and extra-striate cortex. A phase shift in the log-complex transform image represents a change either in the size or orientation of a form in visual space, parameters that do not affect its intrinsic shape. A change in the orientation of a contour line in the log-complex image represents either a change in the intrinsic shape of a form or its translation in visual space. Thus the results suggest that the extrastriate response may be related to the shape of an object in visual space.

Cerebral Cortex↗

PubMed1987

Visual resolution and sensitivity in a nocturnal primate (galago) measured with visual evoked potentials.

Visual resolution and contrast sensitivity were examined in anesthetized, paralyzed galagos using visual evoked potentials (VEPs) resulting from stimulation with phase-reversed sinewave gratings. Spatial frequency vs contrast response functions were band-pass with peak sensitivity at 0.2-0.4 c/deg and a high frequency cut-off between 1.6 and 3 c/deg. Peak contrast sensitivities (estimated from extrapolation of contrast response functions) varied across animals from 10 to 170. Variation of the stimulus modulation rate showed that best responses occurred at 1 Hz with an upper limit of 6-16 Hz. As in other primates, an oblique effect was seen in 6 of 8 animals. The contrast sensitivity function (CSF) determined from cortical VEPs agrees well with the CSFs of cells in the lateral geniculate nucleus, but peak sensitivity and spatial frequency are slightly lower than found for the behavioral CSF. Overall visual performance resembled closely that of another nocturnal species, the cat.

Animals↗

PubMed1987

Nonlinearity in human visual responses to two-dimensional patterns, and a limitation of Fourier methods.

Subjects viewed a pattern consisting of two superimposed gratings: a vertical grating that was counterphase-modulated at Fl Hz and a variable-orientation grafting modulated at F2 Hz. A nonlinear orientation-tuned cross-modulation term of frequency (2F1 + 2K2)Hz in the evoked potential was large when the gratings were parallel and had a half-height full bandwidth of about 12 deg. But a strong (2F1 + 2F2) term was also produced by orthogonal gratings. The application of Fourier methods to analyzing human visual processing of patterns modulated in two dimensions assumes that there is no nonlinear interaction between visual responses to orthogonal gratings. The existence of a strong cross-modulation term in the orthogonal-grating response violates this crucial requirement. Our findings could not result from the stimulation of independent, linear, orientation-selective mechanisms.

Evoked Potentials, Visual↗

PubMed1987

Principal components analysis for source localization of VEPs in man.

This study defines and compares the topologies of the visual evoked potentials to various stimuli such as pattern onset/offset, pattern reversal, pattern motion and high frequency luminance flicker. The responses recorded from 24 occipital derivations were examined using a three sphere conductance model to represent the head, with the assumption that activity from an underlying cortical source is equivalent to a single dipole. Principal components analysis was used to find the dimensionality of the data space. From this analysis could be concluded that all stimuli evoked responses in the primary visual cortex. Only pattern onset, and to a lesser degree pattern offset and pattern reversal, yielded activity in higher visual areas. In particular it has been shown that the CI, CII interval of the pattern onset response has its origins in two different cortical regions. A fast positive (CI)-negative (part of the CII) component arises from area 18 (or 19), a slower negative (initial part of CII) component comes from area 17.

Evoked Potentials, Visual↗

PubMed1988

Spatial and temporal frequency selectivity of neurons in visual cortical area V3A of the macaque monkey.

Response properties of neurons in V3A were studied at a retinal eccentricity of 2-4 deg. The distributions of spatial frequency bandwidths and orientation bandwidths were similar to those of neurons in V1. Peaks of spatial frequency tuning curves ranged from 0.35 to 8.0 c/deg with a mean of 1.75 c/deg. Most V3A cells showed lowpass or, less often, broad bandpass temporal frequency selectivity. The mean direction selectivity index was 0.41. The response properties of cells in V3A differed most from those in V1 with respect to the larger receptive field widths in V3A averaging about 4 deg, the consequent larger number of cycles of the preferred grating that fall within the receptive field, and the previously reported profound response suppression incurred when patches of the preferred grating are extended both within and beyond the classical receptive field. The response properties of cells in V3A differed most from those in V3 in that V3A neurons are much less selective to the speed and direction of stimulus motion than are neurons in V3. The overall response properties of cells in V3A are consistent with anatomical evidence that places this cortical area in the visual pathway from V3A to V4 and then to IT.

Animals↗

PubMed1991

Striate cortical contribution to the surface-recorded pattern-reversal VEP in the alert monkey.

The striate cortical contribution to the surface pattern-reversal visual evoked potential (VEP) was investigated in awake monkeys during performance of a visual fixation task, by examining laminar profiles of VEP, current source density (CSD) and concomitant multiunit activity (MUA) in Area 17, recorded simultaneously at incremental depths using multicontact electrodes. Stimuli were black/white bar gratings centered on the fixation point. The typical surface pattern-reversal VEP over striate cortex consists of a prominent positivity peaking at 50-70 msec (P60), followed by a large negativity peaking at approx. 80 msec (N80), and then by a late broad positivity, peaking between 120 and 150 msec (P125). P60 is often preceded by a small negativity peaking at 45-55 msec (N50), and on rare occasions a small positivity (P40) is also observed. N50 is generated primarily by current sinks in Lamina 4C. P60 arises from large current sources in the supragranular laminae. N80 and P125 appear to be composite waveforms reflecting complex contributions from local activity and from activity occurring outside of the foveal/immediate parafoveal representation in Area 17. The basic physiologic sequence elicited by patterned stimulation is similar to that elicited by diffuse luminance or by electrical stimulation, but is characterized by more prominent supra- and infragranular activation. It is consistent with the cellular and synaptic anatomy of Area 17: initial activation of the thalamorecipient subdivisions of Lamina 4C, followed by activation of mid/upper Lamina 4 and of supra- and infragranular laminae. Our results suggest the possibility of differentiating synaptic stages and cellular processes reflected in the human VEP, based on homologies with simian VEP components.

Animals↗

PubMed1991

Spatial-frequency-tuned attenuation and enhancement of the steady-state VEP by grating adaptation.

Steady-state visual evoked potentials (VEPs) were recorded from adults using 10% C fast spatial frequency (SF) sweeps of horizontal gratings under two conditions: (a) after exposure to a 40% C grating of 6 or 4 c/deg, and (b) after exposure to a blank screen equalling the adapting gratings in space-averaged luminance. SF adaptation attenuated VEP amplitude near the adapting SF, but maximum attenuation was displaced from the adapting SF for 6 c/deg adaptation. Small displacements in maximum attenuation would be expected if underlying neural subunits are tuned to a small number of different center SFs. In addition, SF adaptation caused amplitude enhancement 1.0-2.0 octaves below the adapting SF, providing electrophysiological evidence in humans for coinhibitory relationships among neural mechanisms that have been postulated on the basis of analogous psychophysical findings. The results are consistent with coinhibition between SF-tuned subunits and between transient and sustained mechanisms.

Adaptation, Ocular↗

PubMed1992

Development of temporal properties of pattern electroretinogram and visual evoked potentials in infants.

The postnatal development of the temporal properties of the responses to pattern contrast reversal has been studied by recording simultaneously the pattern electroretinogram (PERG) and visual evoked potentials (PVEP) in infants 3-22 weeks old. The stimulus grating (0.5 c/deg) was either reversed in contrast sinusoidally at frequencies 4-10.5 Hz to study the temporal frequency function of steady-state responses, or square-wave reversed at 1 Hz to evaluate the peak latency of transient responses. Developmental changes of the shape and bandwidth of the temporal frequency function of both PERG and PVEP occur post-natally and are particularly pronounced between 13 and 20 weeks from birth, possibly indicating deferred maturation of classes of retinal and central neurons with higher temporal resolution. The peak latency of the PERG decreases during the age period tested to approach adult values towards the end of the fifth month. The rate of decrease of the peak latency of the PERG differs from that of the PVEP, indicating that post-retinal factors contribute largely to the maturation of the latter, especially in the earliest life period.

Adult↗

PubMed1993

Does chromatic sensitivity develop more slowly than luminance sensitivity?

Chromatic sensitivity is very low in humans during the first few months of life. We examined whether low chromatic sensitivity reflects a deficiency among chromatic mechanisms or whether it is simply a manifestation of poor visual sensitivity in general. The sweep VEP was used to measure contrast sensitivity to gratings varying in the mixture of red and green components. For infants from 2 to 8 weeks of age, sensitivity to all mixtures was lower than color-normal adults' sensitivity, but infant and adult ratios of luminance/chromatic sensitivity were similar. This finding is consistent with the hypothesis that infants have functional MWS and LWS cones and the requisite post-receptor chromatic mechanisms to compare their signals.

Age Factors↗

PubMed1995

Contrast dependence of motion-onset and pattern-reversal evoked potentials.

This study deals with the effect of stimulus contrast, between 1.3% and 96%, on the visual evoked potentials (VEPs) for onset of motion and for pattern reversal of checkerboard stimuli. The VEPs for pattern reversal and for the onset of motion both contain an initial positive peak (P1; peak latency about 120 msec) followed by a later negative peak (N2; peak latency 160-200 msec). However the P1 peak dominates the pattern-reversal VEP when recorded from the midline occipital lead, where it is maximal, while the N2 peak is larger in the motion-onset VEP, especially when recorded from unipolar lateral occipital leads. Whereas the amplitude of the P1 peak in both the pattern-reversal VEP and the motion-onset VEP decreases with decreasing contrast (becoming undetectable at a contrast of about 2% for the motion-onset VEP), the amplitude of the N2 peak in both types of VEP does not vary significantly with contrast, above a contrast of 1.3%. The increase in peak latency with decreasing contrast is also more pronounced for the positive than the negative peaks of both types of VEP. Taking into account the high contrast sensitivity of the magnocellular system (thought to be involved in the processing of motion) compared with the parvocellular system (probably more concerned with the processing of form), our findings suggest that for both motion-onset and pattern-reversal VEPs the negative peak is attributable to the motion-processing magnocellular pathway and the positive peak to the form-processing parvocellular system.

Adult↗

PubMed1994

Effect of orientation on spatiotemporal contrast sensitivity in multiple sclerosis.

Spatiotemporal contrast sensitivity at three orientations, vertical, horizontal and oblique, was studied in 18 patients with clinically definite and laboratory-confirmed definite multiple sclerosis (MS). Nineteen age-matched control subjects were also studied under identical experimental conditions. Contrast thresholds for detecting steady and counterphase modulated (5 Hz) gratings ranging in spatial frequency from 0.5 to 12 c/deg were measured by a modified psychophysical method of limits. With the exception of two patients (three eyes) whose Snellen acuity scores were 20/70, all observers had acuity scores of 20/30 or better. All subjects were corrected for astigmatism. Orientation, spatial frequency and temporal frequency interacted differently in determining contrast sensitivity in the two groups of observers. For the controls, an oblique effect was present for both the steady and counterphase modulated gratings of high spatial frequencies, and there was no orientation-dependent loss of sensitivity for low spatial frequencies. For the observers with MS, there was no oblique effect, but sensitivity was dependent on orientation for the low spatial frequencies. Most patients with MS had reduced contrast sensitivity, compared to the controls, at one or more orientations. Counterphase modulation increased sensitivity to the low spatial frequencies and decreased sensitivity to the high spatial frequencies for both normal controls and patients with MS. In patients with MS this effect of temporal modulation on contrast sensitivity was markedly enhanced.

Adult↗

PubMed1994

Effect of kainic acid and NMDA on the pattern electroretinogram, the scotopic threshold response, the oscillatory potentials and the electroretinogram in the urethane anaesthetized cat.

Kainic acid (KA, 12.5-100 nmol) or N-methyl-D-aspartate (NMDA 25-250 nmol) was injected into the vitreous of one eye of urethane anaesthetized cats. Pattern electroretinograms (PERGs) were recorded to transient contrast reversing bars. Scotopic luminance electroretinograms (ERGs) were recorded to blue flashes. All doses of KA reduced the oscillatory potentials (OPs), PERG and focal ERG (FERG). At 50 nmol KA, the b-wave and scoptic threshold response (STR) were normal. At 100 nmol KA, the STR was absent and the b-wave reduced by over 50%. OPs and STRs were reduced in all NMDA injected eyes. NMDA at 25 nmol enhanced the FERG, PERG, and b-wave and high doses (above 150 nmol) reduced them. Light microscopic examination of retinas showed 25 nmol KA only damaged dendrites of ganglion cells. NMDA damage was slight with < 200 nmol. These data show that the cat PERG has a proximal component which is very sensitive to low doses of KA; the PERG and FERG are very similar; the STR and PERG are generated by different structures and that the OPs and the FERG and PERG are all generated close to the ganglion cell layer, proximal to the STR.

Animals↗

PubMed1987

Neural response to the motion of textures in the lateral suprasylvian area of cats.

Neural response to the motion of visual texture was studied in the lateral suprasylvian area of anesthetized and immobilized cats. We used two types of texture: random noise (a Julesz pattern) and random stripes (parallel bars with random spacings). Among 152 neurons studied 73% responded to the motion of texture, and among these neurons 61% preferred the noise to the stripes. Textures with pixels between 1.6 degree and 4.8 degrees were most effective. Directional tuning of each neuron was usually wider to the noise than to the stripes. The neurons preferred lateral-downward directions of stimulation more often than the other directions. Possible involvement of the neurons in visually guided behaviors, such as optokinetic nystagmus, was discussed.

Animals↗

PubMed1994

Neuronal responses to photographs in the superior temporal sulcus of the rhesus monkey.

The activity of a total of 174 visual neurons in the superior temporal sulcus (mostly area TE) of rhesus monkeys was examined quantitatively using complex visual stimuli. Colored photographs of human faces (n = 411), monkeys (n = 308), and non-face objects (n = 35), as well as computer-generated graphics were presented as visual stimuli during a performance of a visual discrimination task. All neurons responded to a limited number of photographs. We quantified these stimulus-selective responses, using two kinds of selectivity indices. With the first index (SI1), we attempted to estimate how many stimuli produced a significant response. The mean value of SI1 was 0.63. About 31% of neurons had values of SI1 greater than 0.8. With the second index (SI2), we attempted to estimate how many stimuli could be distinguished from the stimulus that elicited the strongest activity. The mean value of the SI2 was 0.71. About 51% of neurons had values of SI2 greater than 0.8. About 66% of neurons had values of either SI1 or SI2 that were greater than 0.8, a value that corresponds to a selectivity of one out of five stimuli. We designated these neurons as stimulus-selective (SS) neurons. Of these SS neurons, 45% showed the best response to human faces. Similarly, 28.8% showed the best response to a photograph of monkeys. 7.5% to food, 8.8% to non-food and 10% to simple graphics, such as, a colored square or a circle.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1987

Hemispheric asymmetry of pattern reversal visual evoked potentials in healthy subjects.

Pattern reversal visual evoked potentials were recorded from temporal leads of both hemispheres in 3 right-handed and 3 left-handed subjects. The stimuli subtended 6 degrees of visual field (full field condition), or 1 degree of visual field (foveal condition), or were restricted to the peripheral portion of the visual field (annular condition). Check sizes varied from 16 to 2.8 min of arc. The stimuli were phase-reversed at 1 or 8 Hz. Remarkable hemispheric asymmetries were found, depending on the portion of the visual field and temporal frequency. In right-handed subjects the amplitude of visual evoked potentials recorded over the left hemisphere was larger for stimuli phase-reversed at 8 Hz in full-field and annular conditions, whereas the amplitude of visual evoked potentials recorded over the right hemisphere was larger for stimuli phase-reversed at 1 Hz in the foveal condition. Different patterns of hemispheric asymmetries were observed in left-handed subjects.

Arousal↗

PubMed1994

Pattern reversal visual evoked potentials in retinitis pigmentosa.

Pattern reversal visual evoked potentials (PR-VEPs) of 31 patients with retinitis pigmentosa (RP) and 20 normal control subjects were recorded and compared. Recordings were taken from Oz referred to Fz, by stimulating with squares subtending 60 min, presented at a visual field of 30 degrees and 98% contrast. The mean latency of the main positive component P100 of the PR-VEP of the RP patients was 116.1 msec (SD 12.71) and 114.84 after stimulation of the left and right eye respectively. These values are significantly different (p < 0.001) from the equivalent values of the normal controls (102.5 ms (SD 5.1) and 100.6 ms (SD 4.6) for the left and right eye, respectively). Fourteen patients had a P100 latency more than 3 SDs from the normal mean value. Subsequently the patients were classified into three groups (for each eye separately) according to visual acuity (VA) and comparison for PR-VEP amplitude and latency was performed. In group A patients with VA 6/6, in B with 6/9 and in G with 6/12 or more were included. Decrease of VA was significantly associated with increased latency. The amplitude was not significantly affected except for group C. All 3 groups were not significantly different in age. The P100 in group A was consistently prolonged, compared to normals. It seems that RP can introduce significant increase in the PR-VEP latency associated with decreased VA. This may have diagnostic and prognostic implications.

Adolescent↗

PubMed1996

The scotopic electroretinogram to blue flashes and pattern reversal visual evoked potentials in insulin dependent diabetes.

Pattern reversal visual evoked responses (PR-VEPs), the electroretinogram (ERG) to blue flashes of light in dark adaptation, the steady state ERG to 40 Hz flicker, and alterations in pupil diameter following dark adaptation were studied in 56 juvenile onset diabetics, 34 of whom had no ophthalmoscopic or photographic evidence of diabetic retinopathy (DR-group). The remaining 22 had mild background retinopathy (DR+group). Normal data was obtained from 24 subjects matched for age and sex with diabetics. Skin electrodes were used for all recordings. The scotopic 'b' wave of the ERG was significantly lower in amplitude in the DR- diabetics and it was even more reduced in the DR+group. The PR-VEP was significantly delayed in diabetics but there was no difference between the two groups. The steady state ERG was not significantly different between normals and diabetics. These findings indicate that retinal and more central abnormalities develop early in diabetics. Detection of objective electrophysiological abnormalities may be used to identify persons at risk of developing retinopathy and to monitor the effects of treatment.

Adolescent↗