Biomedical subjects

V Manahilov

Publications and source records attributed to V Manahilov.

8 recordsLinked to original sources

PubMed1992

The laplacian analysis of the pattern onset response in man.

The pattern onset VEP described by means of principal component analysis was shown to be composed of at least two overlapping time components, of which one has a striate and the other an extrastriate origin. This analysis was based on a multi (24)-electrode registration. We looked for a technique that can distinguish, with a limited number of electrodes, between the striate and extrastriate components and can be applied in the clinic. The calculation of a 5-point 'Laplacian' operator over the striate area enhances the contribution of the striate source relative to the contribution of the extrastriate source that lies outside the area of the operator, and vice versa. We applied two 5-point operators, one centered on Oz and the other 6 cm left of Oz. This montage allowed for the selective recording of the striate and extrastriate activities separately. This was proved by means of an adaptation paradigm that selectively reduced the striate contribution. Application of the laplacian operator for clinical practice is exemplified by means of the VEPs of a patient suspected of psychogenic hemianopsia.

Brain↗

PubMed1991

Grating detection and identification: comparison of postadaptation reaction times.

The effect of pattern adaptation on grating detection and grating identification was investigated by the method of reaction time. The test and adapting stimuli were sinusoidal vertical gratings. Adaptation caused a spatial frequency-selective increase of the mean detection reaction times. However, the mean identification reaction times were delayed regardless of the adapting spatial frequency. The similarity of the present results with some data about visually evoked potentials is discussed.

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↗

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↗

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↗

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↗