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At least 289 records · Page 16Linked to original sources
PubMed1997

Examining the right temporal lobe's role in nonverbal memory.

Tests of facial recognition and spatial learning were administered to presurgical patients with unilateral temporal lobe EEG foci. Right temporal lobe patients obtained lower facial recognition scores than left temporal lobe patients. The groups performed equally on the spatial learning test. A factor analysis revealed two independent factors: a general visuospatial factor and a more specific facial identification factor. The findings provide support for the existence of two dissociable visual processing systems. Memory impairments associated with right temporal lobe dysfunction may be characterized as an impairment in a ventral visual processing system responsible for facial memory and pattern recognition.

Adolescent↗
PubMed1999

Prosopagnosia and structural encoding of faces: evidence from event-related potentials.

Event-related brain potentials (ERPs) were recorded in response to unfamiliar faces and to houses from a severely prosopagnosic patient (PHD) and 24 control subjects. For all control subjects, faces elicited an enhanced negativity at lateral temporal electrodes (N170). This component was absent for PHD. Comparable results were obtained in response to inverted faces and houses. A selective deficit in face recognition is therefore reflected by abnormalities in ERP components specific to faces. As PHD was shown to have substantial deficits on tasks requiring the structural analysis of faces, these findings are consistent with the view that the N170 reflects processes involved in the structural encoding of faces, and may be a measure of selective impairments in the analysis of face components.

Adult↗
PubMed1999

Effects of target area and letter complexity on event-related potentials and reaction time.

The idea that an area of the visual field stimulated by a recognizable image activates a corresponding area of neural tissue that generates the recognition potential (RP) was examined. Sixteen subjects detected targets in a stream of non letter character arrays. The targets were one or five rows of a repeated letter (O or G). RT was less for the larger targets and less for O than for G. RP latency differences agreed with the RT differences. RP amplitude was substantially greater for large than for small targets and moderately larger for G than for O. P3 amplitude showed a different relationship. The observed amplitude-latency relationships indicated that differences in stimulus strength were not responsible for the greater RP evoked by the larger targets. The results strengthened the neural area explanation for RP amplitude modulation by the area of the visual fields impinged on by recognizable images.

Adult↗
PubMed1985

Contour, color and shape analysis beyond the striate cortex.

The corticocortical pathway from striate cortex into the temporal lobe plays a crucial role in the visual recognition of objects. Anatomical studies indicate that this pathway is mainly organized as a serial hierarchy of multiple visual areas, including V1, V2, V3, V4, and inferior temporal cortex (IT). As expected from the anatomy, we have found that neurons in V4 and IT, like those in V1 and V2, are sensitive to many kinds of information relevant to object recognition. In the spatial domain, many V4 cells exhibit length, width, orientation, direction of motion and spatial frequency selectivity. In the spectral domain, many V4 cells are also tuned to wavelength. Thus, V4 is not specialized to analyze one particular attribute of a visual stimulus; rather, V4 appears to process both spatial and spectral information in parallel. A special contribution of V4 neurons to visual processing may lie in specific spatial and spectral interactions between their small excitatory receptive fields and large silent suppressive surrounds. Thus, although the excitatory receptive fields of V4 neurons are small, the responses of V4 neurons are influenced by stimuli throughout a much larger portion of the visual field. In IT, neurons also appear to process both spatial and spectral information throughout a large portion of the visual field. However, unlike V4 neurons, the excitatory receptive fields of IT neurons are very large. Many IT neurons, for example, are selective for the overall shape, color, or texture of a stimulus, anywhere within the central visual field. Together, these results suggest that within the areas of the occipito-temporal pathway, many different stimulus qualities are processed in parallel, but the type of analysis may become more global at each stage of processing.

Animals↗
PubMed1993

Benefit from visual cues in auditory-visual speech recognition by middle-aged and elderly persons.

The benefit derived from visual cues in auditory-visual speech recognition and patterns of auditory and visual consonant confusions were compared for 20 middle-aged and 20 elderly men who were moderately to severely hearing impaired. Consonant-vowel nonsense syllables and CID sentences were presented to the subjects under auditory-only, visual-only, and auditory-visual test conditions. Benefit was defined as the difference between the scores in the auditory-only and auditory-visual conditions. The results revealed that the middle-aged and elderly subjects obtained similar benefit from visual cues in auditory-visual speech recognition. Further, patterns of consonant confusions were similar for the two groups.

Acoustic Stimulation↗
PubMed1996

The recognition potential and conscious awareness.

The idea that conscious awareness of a recognizable image is necessary for it to evoke the recognition potential (RP) was tested by asking bilingual subjects to selectively attend to superimposed English and Chinese word images. The subjects detected most of the words in the attended language, but were largely oblivious of words in the non-attended language. Attended word images evoked the RP. Non-attended words did not. RP latency was less for Chinese than for English words. This provided a basis for inferring which language a subject was trying to read when valid English and Chinese words were both present. A subject was looking for Chinese if the latency was short and for English if it was long. The results showed that selective attention had a powerful effect on the RP. They supported the idea that conscious awareness is necessary for evoking it, though they did not rule out the theoretical possibility that some method not yet tested could be found that would block conscious awareness without blocking the RP. The sensitivity of the RP to what a subject is trying to see and its low variance seem to provide advantages for studying visual perception. It provides a short latency indicator of image processing that merits further investigation. Use of it may lead to a better understanding of visual perceptual processes.

Adult↗
PubMed1985

Visual evoked response abnormality in myoclonus epilepsy with large pupils. Occurrence in a family with acorpuscular myoclonus epilepsy.

Flash and pattern evoked responses were recorded from three siblings with myoclonus epilepsy who all had strikingly large pupils in daylight. Comparison with the visual evoked responses (VERs) of 15 normal and eight epileptic control subjects (including one with myoclonus epilepsy but normal pupils) disclosed a substantial reduction of the amplitude of the flash response as compared with the normal pattern response in these three patients. It is suggested that the VER constellation and the pupillary abnormality, together with the normal electroretinogram and diffusely distributed relative scotomas, were due to a ganglion cell loss in the retina.

Adult↗
PubMed1993

Neuroelectric concepts: form-color classification.

Visual event-related potentials (VEPs) were recorded from the scalp of human observers who viewed an orthogonal stimulus set, consisting of four stimuli, each of which had two attributes: a form (circle or triangle) and a color (green or red). The stimulus set was represented by an a priori stimulus classification model, defined by positions (i.e., degrees of arc) on a unit circle that specified the relationships among the form and color features. An analysis of VEP deviation waveshapes (delta VEP: deviations around average VEP for each electrode) showed that the a priori unit circle model predicted morphologies of the delta VEP waveshapes, as well as the overall relationships between waveshapes obtained for the form and the color attributes. Further analyses demonstrated that individual delta VEP waveshapes for color and for form were located on the circumference of a unit circle at the positions (angle) specified by the a priori model. The studies show that formal modeling of the way humans classify stimulus attributes provides a quantitative and predictive model of the way VEPs become classified and organized according to psychological principles.

Adult↗
PubMed1990

The human motion VEP as a function of size and eccentricity of the stimulation field.

A 'motion onset VEP' was elicited by the onset of a pattern drift. The amplitude of the most distinct wave (AN200) was determined on the following stimulation conditions: eccentricity, 0 to 23 deg; velocity. 1.5 to 16 deg/s; spatial frequency, 0.19 to 2.1 c/deg; and stimulation field size, 0.2 to 160 deg2, AN200 remained constant at any degree of eccentricity if stimulation field size, velocity, and spatial frequency were M-scaled according to Rovamo-Virsu's M-equations. AN200 decreased as a function of eccentricity if field size and velocity were kept constant (spatial frequency had minimal effect). The size of the cortical representation field (Sc) in this case varied with change in eccentricity (stimulation field size constant). In another experiment, it varied by change in stimulation field size (eccentricity constant). For both conditions, AN200 was proportional to log Sc.

Electrodes↗
PubMed1989

Retinal and retinocortical times to pattern stimulation in amblyopic children.

In order to determine whether in amblyopes retinal conduction delays contribute to the cortical measureable delays in the visual evoked cortical potential (VECP), peak latencies of the pattern electroretinogram (ERG) are measured in amblyopic children. The results are compared with those of the normal fellow eyes and those of a healthy control group. Simultaneously the latencies in the VECP are recorded and the determination of the retinocortical times is performed. Statistically retinal b-wave (Q) and a-wave (P) of the pattern ERG of amblyopic eyes do not show significant delays of peak latency. In retincortical times, however, there are significant prolongations. During occlusion therapy retinocortical values of normal fellow eyes are also delayed in comparison with the control group. A pathological conduction delay of visual information on the retinal level up to the generators of the pattern ERG can thus be excluded in amblyopia. The total latency delay in the VECP of amblyopes consists solely in a prolongation of retinocortical times.

Adolescent↗
PubMed1992

Detection duration thresholds and evoked potential measures of stereosensitivity.

Visual evoked potentials have been proposed by some researchers to be more useful than behavioral techniques to evaluate stereo performance in children and certain clinical populations. Stimulus duration detection thresholds, visual evoked potentials, and scalp electrical potential distribution maps to dynamic random dot stereograms were studied. A high degree of correspondence was found between visual evoked potential amplitudes and behaviorally determined detection thresholds. Upper field stimuli had higher detection thresholds and generated lower-amplitude visual evoked potential responses than did centrally presented stimuli. For the most eccentrically presented stimuli, lower detection thresholds were found for stimuli presented in the right visual field than the left visual field. This finding was consistent with the pattern of VEP responses to be lateralized, with higher-amplitude responses recorded over left-hemisphere sites. The study examined a proposal that the major negative component of the stereoscopic visual evoked potential originates in cortical area V1. The results failed to support the proposal and were consistent with the main negative component of the VEP being generated in V2, rather than V1.

Adult↗
PubMed1992

The effect of spatial frequency and contrast on the latency in the visual evoked potential.

The latency in the visual evoked potential was measured at spatial frequencies of 2-12 c/deg in 10 subjects. The contrast levels of the sinuosoidal grating patterns were set at 1.5, 1.75, 2.0, 2.25, 2.5, 2.75 and 3.0 log units above each subject's contrast sensitivity threshold. Two factors were shown to influence the latency: suprathreshold contrast and, to a lesser extent, spatial frequency. The visual evoked potential latencies at contrast sensitivity threshold were extrapolated. These threshold latencies showed considerable variation with spatial frequency and between subjects. Therefore, the visual evoked potential latency cannot be considered a useful tool for estimating the contrast sensitivity function.

Adult↗
PubMed1992

Visual evoked cortical potentials from transient dark and bright stimuli. Selective 'on' and 'off-pathway' testing?

The superior hemifields of five normal left eyes were stimulated by novel equal and opposite contrast pattern onset stimuli which were generated on a cathode ray tube. Patterns consisted of 144 discs, each subtending 60 min arc at the viewing distance of 40 cm and were separated by a distance equal to their diameter. Equal and opposite light changes were created by presenting the disc patterns with different luminance values on a uniform constant background (20 cd/m2). Transient visual evoked cortical potentials to the appearance and disappearance of the patterns were recorded separately and analysed. Significant amplitude differences between the responses to bright and dark stimuli were observed with light increment responses being 36-53% larger than the light decrement responses for pattern on-set and 54-80% larger for pattern off-set respectively. This finding is attributed to the difference in the input to the ON and OFF Parallel Pathways which are known to carry light increment and decrement information respectively, as well as differences in the metabolic and discharge rates of these pathways.

Adult↗