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At least 721 records · Page 40Linked to original sources
PubMed1988

Objective evidence for phase-independent spatial frequency analysis in the human visual pathway.

Electrophysiological responses in human index an interaction between responses to two gratings that is relatively independent of the distribution of light in the retinal image. Two 5 cycle/deg sinewave gratings were superimposed, one counterphase-modulated at F1 Hz and the other at F2 Hz. Nonlinear interaction terms of frequency (nF1 +/- mF2) were recorded that could not be produced by superimposing the F1 Hz grating on blank-field 7 Hz flicker. A local luminance origin could be excluded for the (2F1 + 2F2) term and for the suppression of 2F1 and 4F1, but not for the (F1 + F2), (F1 + 3F2) and (5F2 - F1) terms. The relative spatial phase of the two gratings was varied, thus altering the light distribution in the retinal image without changing its spatial power spectrum. The (2F1 + 2F2) Hz contrast-specific grating-grating interaction term was almost unaffected by these substantial changes in retinal image light distribution providing that the spatial frequency power spectrum of the retinal image was held constant. The (2F1 + 2F2) term and the suppression of 2F1 were both tuned to spatial frequency.

Evoked Potentials, Visual↗
PubMed1988

Luminance-contrast evoked responses and color-contrast evoked responses in the human electroretinogram.

Pattern electroretinograms to onset-offset stimuli were studied in response to luminance-contrast (e.g. red-black or green-black) and color-contrast (e.g. red-green) stripe patterns of equal luminance. Onset responses to color-contrast patterns show no spatial selectivity and a constant peak latency at all spatial frequencies, a behavior different from that of luminance-contrast evoked responses (spatial selectivity and increasing peak latency with spatial frequency). These results are tentatively related to the physiology of tonic and phasic primate retinal ganglion cells and to the spatially selective and non-selective human contrast sensitivity to respectively luminance-contrast and color-contrast gratings.

Color Perception↗
PubMed1987

Electrophysiological evidence for the oblique effect in human infants.

Visually-evoked potentials (VEPs) were recorded from infants between the ages of 2 and 11 months in response to 2.5 c/deg main axis and oblique square wave gratings. The oblique effect first appears at 3 months of age; some infants showed smaller VEP amplitude and/or longer VEP latency for obliquely oriented gratings. Regarding the age of onset of the oblique effect, VEP data from this study agree with the results obtained with preferential looking (PL) studies which have directly paired vertical and oblique gratings, but the current study found that fewer infants show an oblique effect by VEP than by PL.

Aging↗
PubMed1986

Tissue responses of the monkey retina: tuning and dependence on inner layer integrity.

Recent technical and conceptual advances have made it possible to experiment with models of local human inner retinal disease and changes in very small, tissue-specific signals. Local retrograde degeneration of the ganglion cells was induced in four rhesus monkeys by 160 degrees microdiathermy fiber layer burns at the nasal or temporal edges of the optic disc. There were no abnormalities of the classical electroretinograms (ERGs) during the following 210 days. With nasal lesions, pattern-evoked retinal (PERR) and cortical responses over a range of grating contrasts and spatial frequencies were largely normal. The authors found a cortical spatial tuning peak near 0.5 cycles/deg (cpd) and a retinal peak at 0.25-0.3 cpd. With temporal lesions, the retinal signals to high frequency stimuli (greater than 1.0 cpd) approached zero between 20-60 days. The cortical evoked signal declined with a course similar to the retinal components. Histological evidence was found for extensive loss of ganglion cells and fibers in a central 30-40 degrees temporal area, including the macula, 210 days after the temporal lesions. This is strong evidence that local ganglion cell-dependent electrical potentials, bearing little relation to the ERG, can be measured in response to selected stimuli.

Animals↗
PubMed1983

[Changes in the electric organization of the brain in visual search and discrimination processes of different degrees of difficulty].

EEG activity during visual search processes with different degrees of discrimination difficulty was investigated in 13 male subjects. In accordance with the paradigm used by Neisser, 17 corresponding lists of rounded or angular letters were displayed. The task was to select the target letter Z from each list. The discrimination of the letter Z out of the lists of angular letters is the more difficult task. Spectral analysis shows a significant change in alpha-frequency structure during the more difficult search process. There is a shift of the dominant alpha-frequency from 11.5 to 10 Hz with a significant increase and concentration of power in the dominant frequency. This concentration of alpha-power around 10 Hz is more pronounced in the right occipital region than in the left. Concomitantly, there is a significant increase of power in subharmonic theta-frequencies. By consecutive principal analysis of spectral data these results were further differentiated and confirmed. The changes in the alpha- and theta-band during the more difficult search process are the expression of a complex transformation of EEG-frequency structure and patterning which cannot be interpreted by the usual concepts based on the theory of activation. Moreover, they seem to be the neurophysiological correlate of the accommodation (Piaget) of the neurodynamic processes to the altered perceptual conditions and to their effects on the internal matching process.

Adult↗
PubMed1994

[Clinical application of LED pattern visual evoked potential].

The light emitting diode (LED) stimulator was used to record PVEP from 59 eyes (38 patients) with optic neuropathy and 46 eyes (23 patients) with cataract. The amplitude and the latency of P100 in LED PVEP were comparable to those in TV PVEP with insignificant differences (P > 0.05), suggesting that LED PVEP may be used in the evaluation of visual function and prognosis of patients with optic neuropathy or cataract.

Cataract↗
PubMed1996

Contextual modulation in primary visual cortex.

We studied extra-receptive field contextual modulation in area V1 of awake, behaving macaque monkeys. Contextual modulation was studied using texture displays in which texture covering the receptive field (RF) was the same in all trials, but the perceptual context of this texture could vary depending on the configuration of extra-RF texture elements. We found robust contextual modulation when disparity, color, luminance, and orientation cues variously defined a textured figure centered on the RF of V1 neurons. We found contextual modulation to have a spatial extent of approximately 8 to 10 degrees diameter parafoveally. Contextual modulation correlated with perceptual experience of both binocularly rivalrous texture displays and of displays with a simple example of surface occlusion. We found contextual modulation in V1 to have a characteristic latency of 80-100 msec after stimulus onset, potentially allowing feedback from extrastriate areas to underlie to this effect.

Animals↗
PubMed1998

Visuotopic organisation and neuronal response selectivity for direction of motion in visual areas of the caudal temporal lobe of the marmoset monkey (Callithrix jacchus): middle temporal area, middle temporal crescent, and surrounding cortex.

On the basis of extracellular recordings in marmoset monkeys, we report on the organisation of the middle temporal area (MT) and the surrounding middle temporal crescent (MTc). Area MT is approximately 5-mm long and 2-mm wide, whereas the MTc forms a crescent-shaped band of cortex 1-mm wide. Neurones in area MT form a first-order representation of the contralateral hemifield, whereas those in the MTc form a second-order representation with a field discontinuity near the horizontal meridian. The representation of the vertical meridian forms the border between area MT and the MTc. In both areas, the fovea is represented ventrocaudally, and the visual field periphery is represented dorsorostrally. Analysis of single units revealed that 86% of cells in area MT show a strong selectivity for the direction of motion of visual stimuli. The proportion of direction-selective cells in the MTc (53%), whereas lower than that in area MT, is much higher than that observed in most other visual areas. Neurones in the cortex immediately rostral to area MT and the MTc are direction selective, with receptive fields predominantly located in the visual field periphery. In contrast, only a minority of the cells in the cortex ventral to the MTc are direction selective, and their receptive fields emphasise central vision. The results suggest that the MTc is functionally closely related to area MT, and distinct from the areas forming the dorsolateral complex. The MTc may have a role in combining information about motion in the visual field, processed by area MT, with information about stimulus shape, processed by the dorsolateral complex.

Animals↗
PubMed2006

Imprinting modulates processing of visual information in the visual wulst of chicks.

BACKGROUND: Imprinting behavior is one form of learning and memory in precocial birds. With the aim of elucidating of the neural basis for visual imprinting, we focused on visual information processing. RESULTS: A lesion in the visual wulst, which is similar functionally to the mammalian visual cortex, caused anterograde amnesia in visual imprinting behavior. Since the color of an object was one of the important cues for imprinting, we investigated color information processing in the visual wulst. Intrinsic optical signals from the visual wulst were detected in the early posthatch period and the peak regions of responses to red, green, and blue were spatially organized from the caudal to the nasal regions in dark-reared chicks. This spatial representation of color recognition showed plastic changes, and the response pattern along the antero-posterior axis of the visual wulst altered according to the color the chick was imprinted to. CONCLUSION: These results indicate that the thalamofugal pathway is critical for learning the imprinting stimulus and that the visual wulst shows learning-related plasticity and may relay processed visual information to indicate the color of the imprint stimulus to the memory storage region, e.g., the intermediate medial mesopallium.

Animals↗
PubMed2000

Activation timecourse of ventral visual stream object-recognition areas: high density electrical mapping of perceptual closure processes.

Object recognition is achieved even in circumstances when only partial information is available to the observer. Perceptual closure processes are essential in enabling such recognitions to occur. We presented successively less fragmented images while recording high-density event-related potentials (ERPs), which permitted us to monitor brain activity during the perceptual closure processes leading up to object recognition. We reveal a bilateral ERP component (N(cl)) that tracks these processes (onsets approximately 230 msec, maximal at approximately 290 msec). Scalp-current density mapping of the N(cl) revealed bilateral occipito-temporal scalp foci, which are consistent with generators in the human ventral visual stream, and specifically the lateral-occipital or LO complex as defined by hemodynamic studies of object recognition.

Adolescent↗
PubMedDate not supplied

Computations in the early visual cortex.

This paper reviews some of the recent neurophysiological studies that explore the variety of visual computations in the early visual cortex in relation to geometric inference, i.e. the inference of contours, surfaces and shapes. It attempts to draw connections between ideas from computational vision and findings from awake primate electrophysiology. In the classical feed-forward, modular view of visual processing, the early visual areas (LGN, V1 and V2) are modules that serve to extract local features, while higher extrastriate areas are responsible for shape inference and invariant object recognition. However, recent findings in primate early visual systems reveal that the computations in the early visual cortex are rather complex and dynamic, as well as interactive and plastic, subject to influence from global context, higher order perceptual inference, task requirement and behavioral experience. The evidence argues that the early visual cortex does not merely participate in the first stage of visual processing, but is involved in many levels of visual computation.

Animals↗
PubMed2004

Familiar-face recognition and comparison: source analysis of scalp-recorded event-related potentials.

OBJECTIVE: We studied the event-related potentials elicited by categorical matching of faces. The purpose was to find cortical sources responsible for face recognition and comparison. METHODS: Nineteen healthy volunteers participated in the study. Each trial began with one of the two cues (S1) followed by consecutive pictures (S2 and S3). Each picture was a photograph of a familiar face with a superimposed abstract dot pattern. One cue directed attention to compare faces and another to compare patterns. 128-channel electroencephalogram was recorded. Spatio-temporal multiple dipole source models were generated using Brain Electromagnetic Source Analysis 2000, for the window of 80-600 ms from S3 onset. RESULTS: The obtained model for face recognition and comparison contained 8 dipoles explaining 97% of grand average and about 90% of individual data and showing temporal and spatial separation of sources: in the frontal region, in the occipital cortex, and in the bilateral medial temporal and inferotemporal regions. Different faces elicited larger components than same person's faces around 400 ms, mainly explained by frontal dipoles. CONCLUSIONS: The sources in our models estimate the activity common for both Face task conditions (the recognition of a familiar person) and also differential activity, related to the match/mismatch item processing.

Adult↗
PubMed2000

Sensory and cognitive contributions of color to the recognition of natural scenes.

Although color plays a prominent part in our subjective experience of the visual world, the evolutionary advantage of color vision is still unclear [1] [2], with most current answers pointing towards specialized uses, for example to detect ripe fruit amongst foliage [3] [4] [5] [6]. We investigated whether color has a more general role in visual recognition by looking at the contribution of color to the encoding and retrieval processes involved in pattern recognition [7] [8] [9]. Recognition accuracy was higher for color images of natural scenes than for luminance-matched black and white images, and color information contributed to both components of the recognition process. Initially, color leads to an image-coding advantage at the very early stages of sensory processing, most probably by easing the image-segmentation task. Later, color leads to an advantage in retrieval, presumably as the result of an enhanced image representation in memory due to the additional attribute. Our results ascribe color vision a general role in the processing of visual form, starting at the very earliest stages of analysis: color helps us to recognize things faster and to remember them better.

Cognition↗