PubMed Health⌕ Search

Biomedical subjects

A Vassilev

Publications and source records attributed to A Vassilev.

9 recordsLinked to original sources
PubMed1991

Effect of light adaptation on the orientation acuity of human cone vision.

At the detection threshold, a line orientation is identified by the accuracy of 15-20 deg. Deviations from this rule are observed for short lines presented on a dim background. We checked up for one possible source of deviation: the substitution for foveal cone vision by peripheral rod vision. Long wavelength (red) light was used for the test line. In a control experiment such light did not stimulate rods. The test line was 4 min of arc long. It was presented for 20 ms on a white background of 1 or 1000 trolands and was viewed foveally. In a 2-alternative forced-choice experiment the psychometric functions for detection and orientation identification (vertical v. horizontal) were compared. In another experiment, the line was presented randomly in one of 18 orientations in the range 0-180 deg and the subject was asked to determine the orientation. In both experiments orientation perception was impaired at the dim background in comparison with the results at the bright background. This suggests that light-adaptational changes of the mechanisms responsible for orientation perception exist within the photopic vision.

Adaptation, Ocular↗
PubMed1991

Hemisphere asymmetry of the visually evoked potentials elicited by gratings of varying spatial frequency.

Visually evoked potentials (VEP) were recorded upon hemifield stimulation with sinusoidal gratings of varying spatial frequency (SF). Recording was bipolar from 0.1-0.2. The gratings were presented randomly in the left, in the right or in both visual hemifields. No systematic VEP asymmetry was observed at low SF. At SFs above 1.5-2 cpd, however, the early wave peaking at about 100 msec after grating onset was usually of greater amplitude when the grating was presented in the left visual field. In previous experiments of our, contrast sensitivity was almost the same in both hemifields. Thus, the VEP data suggest right-hemisphere specialization in processing high SFs and their comparison with the contrast sensitivity data suggests that this specialization occurs at a level higher than stimulus detection or is evident at suprathreshold contrast levels only.

Electroencephalography↗
PubMed1990

Hemisphere asymmetry in the detection of gratings of varying spatial frequency.

Visual field symmetry in contrast sensitivity was investigated under conditions of spatial uncertainty. The stimuli were sinusoidal gratings of varying spatial frequency (SF). They were presented randomly in the left, right or in both visual hemifields. The percent detected gratings were slightly higher than the gratings in the left visual field except for gratings of low SF (0.24-0.48 c/deg). The results suggest the existence of a slight left-field (right-hemisphere) superiority in detecting stimuli of medium and high SF. The asymmetry is too slight to allow any significant difference in the SF-content of the neural representation of the left and right halves of the images; neither might it provide an explanation of some recent data on hemispheric specialization, in particular visual tasks.

Brain↗
PubMed1989

Hemispheric asymmetry of the harmonic components ot the visual potentials evoked by the onset-offset of spatially periodic stimuli.

The distribution on the scalp of the first (H1) and second (H2) harmonic components of the steady-state visual evoked potential was studied. The stimulus was the onset-offset of a sinusoidal grating with spatial frequency of 4 c/deg, temporal frequency of 7.1 or 16.7 Hz and contrast of 0.2. The visual evoked potentials were recorded monopolarly from symmetric points at distances of 3, 6 and 9 cm laterally from Oz. It was found that while in the averaged data for all investigated subjects there were no differences in the distributions of H1 and H2 to the right and to the left of Oz, the different individuals had a characteristic asymmetry in the amplitude of the response. The statistical processing of the data revealed a significant prevalence of the second harmonic in the left hemisphere upon stimulation at 16.7 Hz for right-handed subjects. The hypothesis about the dominating role of the left cerebral hemisphere in the processing of signals with high temporal frequency was supported for this group of subjects.

Brain↗
PubMed1986

The effect of standing contrast and pattern adaptation on the human visually evoked potential.

Human visually evoked potentials (VEPs) were recorded with sinusoidal gratings of low (0.5 c/deg), medium (4 c/deg) and high (16 c/deg) spatial frequency (SF). Simultaneous recording from Oz-A1, O2-A1 and Oz-O2 allowed the separation of the early and late VEP waves for most subjects. The effect of standing contrast and pattern adaptation on these waves was studied. At low SF both early and late waves were not affected by standing contrast or adaptation with motionless grating, while adaptation with a drifting grating reduced them. At medium and high SFs both principal negative waves. N1 and N2, were reduced by standing contrast and pattern adaptation. The amplitude of N2 saturated at low contrast level (0.1) and was reduced after adaptation regardless of the SF of test and adapting stimuli, while the amplitude of N1 did not saturate up to the contrast level of 0.3 and exhibited SF-selective adaptation. The adapted onset-VEP was similar to the offset-VEP. The data suggest that: the early wave is generated by multiple narrowly tuned SF-selective structures that are medially positioned while the generators of the late wave are not SF-selective and occupy a wider area centered laterally; the effects of standing contrast and pattern adaptation on VEPs, as well as the relationship of onset and offset VEPs, reflect the time-course of neural activity evoked by long-lasting stimuli.

Adaptation, Ocular↗
PubMed1978

Perception of orientation of short lines: its dependence on line's length and intensity.

The threshold intensity necessary for correct identification of orientation of a briefly presented line was measured as a function of line's lenght and number of possible orientations (2 orientations differing by 90 degrees, 4 differing by 45 degrees, and 8 differing by 22.5 degrees). The thresholds were the same for 2, 4 and 8 orientations when the lines were longer than 20 min of arc. On the contrary, in the case of short lines, the correct report of 8 orientations required much more intensive stimulation than the report of 2 orientations. The discrimination of small differences in orientation of short lines involves additional processes to those for their detection and coarse perception of their orientation.

Discrimination, Psychological↗