Localization of human brain areas activated for chaotic and ordered pattern perception

The aim of our work was to localize cortical areas involved in the processing of incomplete figures using functional MRI (fMRI) for 8 healthy volunteers (18-30 year old) with the did of anatomical and fMRI fast imaging technique: echo planar imaging (EPI), whole brain scan (36 slices) matrix 64 x 64, 3.7 second. We used 1.5 T MR-scanner and BOLD-method (Blood Oxygenation Level Dependent), based on distinctions of magnetic properties of hemoglobin. Fast imaging technique on modern MR-scanners with > or = 1.5 T provides precise statistical maps of oxygenation increase with high spatial resolution. For test stimuli we used matrix of Gabor grating. We used two types of 10 x 10 matrices with chaotic and ordered orientation of Gabor gratings. The size, brightness and contrast of the stimuli were identical. The chaotic and ordered patterns activated different brain areas. We establish that ordered patterns activated only primary visual cortex - V1 and V2, (BA17-18), wheareas chaotic patterns activated in addition primary visual cortex, the V3,V4,V5 (BA19) of the occipital cortex and the area 7 of parietal area (BA7) classification. Decision making for that task is localized in prefrontal and frontal cortex, including (BA 6, 9, 10).     

Visual mechanisms of recognition of vanishing figures

The acuity test Landolt C's with a complex contour imitating various kinds of high-pass spatial frequency filtering generated as a printed visual acuity charts with a grey background were studied with regard to recognition thresholds. Comparisons were made for recognition thresholds with different complex contours. When the vanishing optotypes are out of focus, their image on the retina fades rapidly into the grey background, rendering them invisible rather than merely blurred as in the standard chart. The data obtained confirm that the recognition distance depends on physical property of complex contour profile and show the importance of the Landolt C spatial frequency spectrum for recognition of different types of the complex contour.     

Latency of evoked potentials in the tasks involving classification of images after wavelet filtration

We studied the characteristics of evoked potentials recorded during the recognition test based on four types of series of images subjected to the wavelet filtration: images of living objects containing either low frequency or high frequency portion of the spatial frequency spectrum, and imaging of non-living objects in the same two spatial frequency bands. Each subject had to classify the image either by its semantic feature (living–non-living), or by its physical feature (low frequency–high frequency). The purpose of this study was to compare the time characteristics of evoked potentials in these two types of tasks, which provides information on the time characteristics of categorization mechanisms of visual images. Analysis of the latent periods and amplitudes of the components of evoked potentials allowed us to detect the occipital areas of the leads where the early components (up to 170 ms) are associated with spatial and frequency characteristics of the image, the frontal and temporal areas where the components of 170–200 ms correspond to the process of categorization, and the later frontal, central, and parietal areas (300–500 ms) correspond to the process of error detection and the organization of motor response.     

Human magno- and parvo-systems and selective disruptions in their function

In psychophysical and electrophysiological investigations, we tested the hypothesis that in patients with multiple sclerosis at its early stage either phasic cells of the magnocellular pathway or tonic cells of the parvocellular pathway are disturbed. Healthy humans and patients with early stage of diseases have a contrast spatial-frequency sensitivity measure. The decrease in sensitivity at low, medium and high spatial frequency indicates changes in parvo- and magno-system activity. The data are separated into two clusters. Psychophysical data is in good accordance with electrophysiological data of visual evoked potentials (VEPs). First group of patients showed changes of the amplitude of positive wave P100 whereas in second group of patients changes of negative wave N80 dominated comparative to healthy subjects. According to some literature data these changes were specific for M-pathway and for P-pathway.     

Features of perception of length of segments under conditions of ponzo and Müller-Lyer illusions in schizophrenia

In order to better appreciate the neurophysiologic mechanisms of perception of length under conditions of geometrical visual illusions, we studied sensitivity of mentally healthy subjects and schizophrenic patients to Ponzo and Müller-Lyer illusion. Patients with schizophrenia estimated length of segments of Müller-Lyer figure less precisely. Accuracy of perception of length of segments in Ponzo figure was ambiguously connected with the duration of the disease. Persons suffering from schizophrenia for a short time were less inclined to Ponzo illusion than mentally healthy subjects. On the contrary, patients who suffered from schizophrenia for a long time were more sensitive to this illusion. Ponzo illusion can be used as a marker of schizophrenia which is found out during the specific period of development of the disease. High sensitivity of patients with schizophrenia to Müller-Lyer and Ponzo illusions supports a hypothesis about the role of the global analysis of an image during processing of its low-frequency component in formation of the illusions under study.     

Spatial frequency characteristics of receptive fields of neurons in the lateral suprasylvian area of the cat cortex

Two-dimensional spatial frequency characteristics of receptive fields of 46 neurons in the lateral suprasylvian area of the cat cortex were obtained. These receptive fields possessed orientation anisotropy. Peak frequencies lay in the frequency region below 1.5 cycles/deg. The transmission band width was measured during optimal orientation of test gratings in 21 neurons. It averaged 1.47±0.6 octave. In the remaining neurons the lower boundary frequency was shifted into the region of spatial frequencies below the range used. During nonoptimal orientation of test gratings, inhibition of the discharge was observed in 17 neurons. The inhibitory spatial frequency characteristics of six neurons were of the narrow band type, and averaged 1.1±0.6 octave.I. P. Pavlov Institute of Physiology, Academy of Sciences of the USSR, Leningrad. Translated from Neirofiziologiya, Vol. 14, No. 6, pp. 608–614, November–December, 1982.     

Visual evoked potentials in response to dichoptic presentation of sinusoidal grating and noise background

Dichoptic stimulation was used in comparison of visual evoked potentials (VEPs) with those obtained with monocular stimulation (recordings made from the occipital area). 16 subjects viewed sinusoidal gratings with the right eye while a visual noise was added via a mirror for the left eye. In presence of the noise, amplitude of the early VEP components' N1, P1b, and the late component P2 decreased, P1a is not changed in presence of the noise, and the late negative wave N2 increased for all spatial frequencies. The effect of noise on the amplitude of VEPs obtained for monocular and dichoptic stimulation was similar. The data suggest that external noise is filtered by the V1 cortical neurons--matched filters for the gratings.     

Electrophysiological and psychophysical studies of the effect of duration of presenting textures on the recognition threshold

We studied the dependence of correct response number, response time, and the main characteristics of visual evoked potentials on the time of the stimuli presenting. The coincidence of the recognition temporal thresholds and occurrence of differences in the visual EPs suggests that the temporal characteristics of these processes differ in different subjects. Two groups of observers were distinguished with different temporal characteristics of perception and the EP development.