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P A Apkarian

Publications and source records attributed to P A Apkarian.

5 recordsLinked to original sources
PubMed1985

Effects of contrast, orientation and binocularity in the pattern evoked potential.

Monocular and binocular visual evoked potentials were studied as a function of modulation depth (contrast) of a counterphase sinusoidal grating stimulus. A range of spatial and temporal frequencies of stimulation were used. The contrast functions showed many different forms, and were in some cases nonmonotonic. The binocular response usually had a steeper slope than the monocular response. Extrapolating the slope to the zero intercept gave electrophysiological thresholds which did not correspond with psychophysical threshold at any spatial or temporal frequency used. The binocular response was reduced to the monocular level when the orientation difference between the two eyes exceeded about 20 degrees. The data show that the pattern evoked potential is highly specific to many stimulus variables.

Depth Perception↗
PubMed1983

Visual training after long term deprivation: a case report.

During evaluation of an optical to tactile conversion device, the Tactile Vision Substitution System (TVSS), a training program was developed to teach blind subjects to perceive visual information via the tactile modality. Tactile training paradigms which compensate for the limited resolution of the skin sense were applied to visual training procedures in a case of visual restoration following long term deprivation. Visual training in the visual restoration case also included electro-oculography biofeedback to facilitate oculomotor control and visual evoked potential evaluation for the objective assessment of visual function. The development of functional vision over an 11 month training period of a young girl who was form deprived form 3 months to 12 years is discussed.

Blindness↗
PubMed1981

Binocularity in the human visual evoked potential: facilitation, summation and suppression.

The electrophysiology of normal binocular function was investigated by studying the binocular interactions from monocular and binocular visual evoked potentials (VEPs) recorded under a range of stimulus conditions. The amplitude and phase of the steady-state VEPs, which were obtained with sinusoidal gratings temporally modulated in counterphase, were measured with synchronous narrow-band filtering techniques. Binocular interactions were investigated as functions of spatial frequency, temporal frequency and contrast. Detailed sampling and testing within each stimulus domain revealed an unprecedented degree of VEP specificity in the extent of binocular interaction, which varied from zero summation to pronounced facilitation. Binocular facilitation was explored in terms of its relation to the neural mechanisms subserving binocular function. VEP correlates of rivalry, fusion and stereopsis were obtained. The facilitatory binocular interactions, revealed by careful spatial and temporal stimulus manipulations, were akin more to the neurophysiological responses of single neurons than to previously reported evoked potential work. The relationship between the specificity and narrow tuning of the binocular interactions recorded with VEP techniques and those recorded with single unit techniques is discussed.

Brain↗