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A Vassilev

Publications and source records attributed to A Vassilev.

At least 55 records · Page 3Linked 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

Length and width summation in human vision at different background levels.

The effect of background intensity on the spatial summation of rectangular stimuli of varying length and width was studied in human psychophysical experiments and compared with the known effects of light adaptation on the spatial summation of circular stimuli. Both the detection threshold and the threshold of orientation identification were measured. In agreement with previous data, summation was more efficient along the line stimulus than across it. This asymmetry was found to exist at all adaptation levels studied (1-1000 trolands). The adaptation level affected both length and width summation; the change in the length of summation was twice as great as the change in the diameter of summation with circular stimuli. Orientation selectivity was reduced for short lines presented on a dim background. The results suggest the existence of mechanisms of light adaptation at the level of cortical orientation-selective units.

Adaptation, Physiological↗
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↗
PubMed1988

Application of the equiweight analysis method for the evaluation of the origin of the N1 and N2 waves of visual evoked potentials.

The applicability of the equiweight analysis method was studied with a view to evaluating the position in depth of the two dominating waves of the visual evoked potentials, recorded from the scalp of human subjects upon the appearance of a sinusoidal grating. The stimulus spatial frequency was 2, 4, 8 or 16 c/deg and its contrast was 0.2 or 0.3. The data of the topographic studies confirm earlier observations that the N1 wave with latency of about 100 ms was localized medially with maximum in Oz, while the N2 wave with latency of about 150 ms was spread over a wider area of the scalp. The results of the equiweight analysis corresponded to the evidence of cortical origin of N1. The wide propagation of the N2 wave proved to be an obstacle for the applicability of the method, although it seems promising when the source of the visual evoked potential is limited in area.

Electrodes↗
PubMed1988

On the processing rate of gratings superimposed on gratings.

Psychophysical and electrophysiological experiments suggest that visual image processing is effectuated successively as regard as its spatial frequency (SF) content, low SF-information being processed faster than high SF-information. In an investigation of the effect of exposure time on the detection threshold of a test grating superimposed on a high-contrast masking grating Mezrich (1978) has obtained data which he interprets as suggesting the reverse spatio-temporal relationship in processing of suprathreshold masking stimuli, namely, that processing latency decreases with increasing SF. The experiments described in the present paper confirmed his data. It was, however, shown that his results are not related to the processing rate of the masking stimulus and can be explained by an impaired temporal summation due to reduced signal-to-noise ratio or to interchannel interaction.

Humans↗
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↗
PubMed1986

The effect of grating contrast on the human visually evoked potential.

Visually evoked potentials (VEPs) were recorded from the human occiput at the appearance of sinusoidal gratings. Grating spatial frequency was 4 c/deg, at which both the early and late negative waves, N1 and N2 were present in VEP. Increasing grating contrast from a near-threshold value, 0.006, to the value of 0.3, the maximum in the linear range of the device, resulted in a peak-latency shortening and an amplitude increase of both N1 and N2. While the latencies changed in parallel, the amplitudes did not. The N1 amplitude increased almost linearly with log contrast in the whole range tested. The amplitude of N2 was large at the lowest contrast level, increased by a factor of about 2 and saturated at the contrast of 0.05. The results suggest a greater contrast sensitivity of the generator of N2 than of the generator of N1 and support the assumption that these generators belong to structures with different properties and functions.

Electroencephalography↗
PubMed1986

Reaction time and critical duration for lines' detection and identification of their orientation.

Reaction time (RT) and duration of complete temporal summation (tc) were studied for both detection of lines and identification of their orientation. The RT was studied as a function of stimulus intensity, number of orientations and angular difference between them. The temporal summation was studied for detection and identification at several angular differences. It was found that the intensity-dependent part of RT for identification at angular difference of 22 degrees equaled that for detection but considerably lengthened at smaller angular differences (10 degrees). The intensity-independent part of RT was minimal for detection and maximal for identification with eight alternatives. The tc increased from 30 msec for detection and identification with the accuracy of 22 degrees to 100 msec for identification with the accuracy of 5 degrees. The results support the conclusion that detection and coarse identification differ only in their later stages and that accurate identification involves additional processes with longer time-constants compared to coarse identification.

Humans↗
PubMed1985

Perception of line orientation in the center and periphery of the visual field.

Perception of orientation in the center and periphery of the visual field were compared. The stimuli were bright lines 1 degree long. They were presented equiprobably in 18 orientations from 0 degree to 180 degrees at the center or at 20 degrees in the periphery of the visual field. Stimulus duration was 50 msec and its intensity was twice above the detection threshold for each localization. Percentage of errors, response standard deviation, stimulus standard deviation from the orientation assigned and information transmitted were studied. No scale or standard stimulus were presented. At both stimulus localizations the accuracy of orientation was high and the errors larger than 20 degrees were less than 10% of all responses. There was a small but significant foveal superiority in accuracy and information transmitted. For central vision, the percentage of errors, response standard deviation and stimulus standard deviation all showed an oblique effect, i. e. a lower performance with oblique lines than with vertical or horizontal. For the peripheral vision, some measures (response standard deviation) showed an oblique effect and others (stimulus standard deviation and percentage of errors) did not. This makes difficult the conclusions about the presence of an oblique effect at the periphery. Further studies by methods free of response bias are necessary.

Fovea Centralis↗
PubMed1983

Spatial frequency and the pattern onset-offset response.

Visually evoked responses (VERs) were recorded with vertical sinusoidal gratings presented in the on-off mode at rates of 0.5 and 1 Hz. Stimulus spatial frequency ranged from 0.5 to 16 c/deg and its contrast varied from near-threshold to the value of 0.3. At low spatial frequencies, 0.5 and 1 c/deg, the onset and offset VERs were of similar shape, magnitude and contrast dependence. At spatial frequencies higher than 2 c/deg the onset VERs were usually larger and persisted at lower contrast levels than the offset VERs. The results are consistent with the hypothesis that a gradual transition from the operation of transient channels to the operation of sustained channels takes place on increasing stimulus spatial frequency and that within a wide spatial frequency range (at least from 2 to 8 c/deg) suprathreshold gratings effectively stimulate both types of channels. It is confirmed that both onset and offset VERs are delayed at high spatial frequencies. In addition, a longer delay of the late onset waves was found in comparison with the delay of the early onset waves.

Adolescent↗
PubMed1980

Temporal characteristics of the recognition of short lines orientation.

The influence of the number of orientations (3 and 8) and of the magnitude of the angle for identification (45 and 22.5 degrees) on the temporal characteristics of orientation recognition with test lines of different lengths (5.5, 10 and 22') was studied. The time for recognition was shorter for longer stimuli. Both the number of stimuli and the magnitude of the angle influenced the percentage of correct responses. The effects of the number of orientations were weaker than expected for a sequential type of recognition. Therefore, it is assumed that the mechanism of orientation recognition is basically of the detector type even when there is influence of the angle, i.e. under conditions of insufficient orientational selectivity. The distribution of the responses was asymmetric. The degree of asymmetry of the frequencies of the errors depended on the length of the test stimuli and on the number of orientations.

Form Perception↗
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↗