It is well known that rhythmic light stimulation can alter the electrical activity of the human and animal brain. Moreover, the brain response to certain flicker frequencies significantly exceeds the responses to neighboring frequencies. This phenomenon is thought to be related with the effect of resonance, as evidenced by the coincidence of one of the maxima in the profile of the response to flashes with the frequency of the alpha rhythm. However, other frequencies that cause an increased response to flashes are not reflected in electroencephalogram (EEG) as dominant oscillations. The goal of this study was to reveal the relationship between local maxima in the profile of the responses to flashes of different frequencies and dominant brain oscillations recorded in electrocorticogram (ECoG) of rhesus monkeys without stimulation. The study was carried out on four male Macaca mulatta individuals. In three animals, peak responses were elicited by flickers at 8 and 16 Hz, while one monkey showed a second peak in the 22−30 Hz range. The first maximum (8−10 Hz) in the profile of the response to rhythmic photostimulation coincided with the dominant rhythm recorded in the occipital and parietal regions at rest. The second maximum at 16 Hz coincided with the dominant ECoG rhythm in one of the primates when it was in the state of emotional arousal, which may account for the resonant origin of the increase in responses in this frequency range. The data obtained indicate that dominant brain rhythms, including latent rhythms revealed only by rhythmic photostimulation, can coincide in frequency in monkeys and humans. Neuronal mechanisms of selective sensitivity of neural networks to different frequencies of photostimulation are discussed.

This study is devoted to the search for electrophysiological indicators of the human perception of a threat in a complex background target environment. It is shown that the human visual system can distinguish between threatening and nonthreatening stimuli at an unconscious level, and the primary stages of image processing are similar to those during conscious perception. An increase in the amplitude of the positive wave of the evoked potential with a latency period of 320 ms and a decrease in the amplitude of the electroencephalogram rhythms at a frequency of approximately 12 Hz in the range of 350–750 ms after presentation of a threatening stimulus can serve as electrophysiological indicators of unconscious perception. The results obtained are considered from the perspective of the matched filtration model.

This paper proposes a principle for synthesizing a complex target environment and identifies the optical masking characteristics for unconscious perception of a signal. A signal hidden by masking is supplied on the periphery of the field of view for a short time interval, unconsciously activating wide-angle human “periscopic vision” that possesses low spatial and high temporal resolution. In these studies, the selective attention of the narrow-angle central-vision channel with high spatial resolution was charged with a pseudotarget. We assumed that peripheral vision is capable at that instant of unconsciously perceiving signals hidden by a mask and storing them in memory. It was established that the unconscious low-frequency descriptions of the signals stored in memory influence decision making and control the operator’s involuntary motions under conditions of indeterminacy. Opponent-style implementation of the interaction of central and peripheral vision can serve as a pattern for further refining artificial control systems.

Neuroimaging and functional magnetic resonance imaging have been used to study the change in the activity of the human brain during visual spatial tests of varying complexity. It is shown that, along with an increase in activity in brain structures responsible for the task, there is a decrease in activity in other areas that are not engaged during the visual signal processing. To estimate the equilibrium of activation and deactivation processes in the brain, the number of voxels (the minimum element of a 3D image in the brain) with a significant change in activity relative to the resting state was calculated for each subject. The results of data averaging for all subjects showed that the total volume of activated regions increases with the problem complexity. The volume of brain regions that decreased their activity during the test showed a similar dependence on complexity—the more difficult the task, the greater the number of deactivated voxels. The data obtained suggest the existence of mechanisms for the redistribution of neuronal activity in the human brain to maintain a balance between the activated and deactivated regions, which makes it possible to reduce the energy expenditure of the brain with an increase in cognitive loads.

In behavioral experiments rhesus macaque monkeys were trained to interact with the computer using a tactile display. We used grayscale Gabor patches of low spatial frequency as stimuli. Monkeys' task was to touch the screen with his hand in the area of the target stimulus, followed by automatic food or juice reinforcement. After two successive correct answers, stimulus contrast gradually decreased. Using a two-alternative forced choice method the contrast threshold was measured within which monkeys can detect the appearance of low-frequency images. It was shown that the contrast sensitivity decreased with the decrease of stimulus spatial frequency, while the reaction time increased. The findings extend our knowledge of the primates' activity in the virtual environment and open new possibilities for modeling and studying various human diseases.

The work of an operator in solving two classification problems when working with one image alphabet is studied. From ninety visual stimuli, half of the images contained animate objects, and the other half contained inanimate objects. The first task was to classify the images according to a semantic attribute—whether they contained an animate or inanimate object. This alphabet of stimuli was then subjected to wavelet filtering in a low- and high-spatial-frequency region, regardless of semantic significance. The second task was to classify the stimuli according to a physical attribute—a blurred or unblurred object in the image. Electrophysiological monitoring of the operator’s work—recording of the induced visual potentials from the entire surface of the head—made it possible to detect that, from the beginning of the stimulation until the organization of the motor response, parallel processing of the observed signal occurs according to the different semantic and physical attributes. The responses of the temporal and frontal sections of the brain associated with the semantics of the images are distinguished, even under those conditions in which the subject’s task was to classify the physical properties of an image of an object.

This research is devoted to a study of features of the operation of the neural structures of the human brain responsible for “identification, friend-or-foe” patterns when different instructions are being carried out. Digital-image processing methods are used to synthesize stimuli adequate for the task, consisting of images of optoclones of virtual people. Functional magnetic-resonance tomography (fMRT) is used to investigate the basic patterns of brain activity. The dynamics of blood flow in different phases of stimulation is estimated. The opposition principle of the interaction of the regions of the brain responsible for making decisions is detected. It is shown that, first, there is a complex system that jointly operates the zones of the brain, each of which makes its own specific contribution to the accomplishment of mental processes. Second, each of these zones of the brain can be involved in the implementation of various functions, depending on the instruction and the experimental conditions. Third, various structures of the brain interact on the opposition principle. Changing the instruction substantially affects the distribution over the brain of the BOLD signal, which reflects the functional architecture of a large-scale neural network. These results make a substantial contribution to the development of new algorithms for the operation of neuromorphic recognition systems and their practical application in control systems—for example, in analyzing masked mimetic facial expressions.

The methods of neuroiconics and functional magnetic-resonance tomography are used to investigate the factors that limit the possibilities of visual search. The influence of an image of a human face hidden in the background on the activity of the observer’s brain was recorded during the task of tracking a moving ring. It is established that images are unconsciously perceived under threshold-presentation conditions, and this is reflected in the activation of the fusiform gyrus—a region of the brain that participates in face recognition. Under above-threshold presentation conditions, the parietal and frontal regions of the brain were also activated, but activity in this case decreased in the auditory, motor, and certain other regions of the brain not occupied in signal processing. The resulting data reveal the significance of the background semantics under conditions of visual search and explain how the unconsciously perceived optical characteristics of a background image can affect the operator’s functional state.

Методами цифрового синтеза изображений созданы системы тестов для измерения функционального состояния различных уровней зрительной системы, включая высшие когнитивные процессы. Для оценки реакций мозга на синтезированные тесты использовали психофизические методы измерения и функциональную магнитно-резонансную томографию (фМРТ). Особое внимание уделено клиническим аспектам исследования зрительной коры и ее взаимодействия с другими отделами мозга: с теменной, префронтальной, фузиформной и поясной корой. В рамках комплексного исследования при помощи фМРТ осуществлено определение остроты зрения, контрастной чувствительности, поля зрения, распознавание тестовых изображений разной сложности, в частности лиц, а также проведено исследование механизмов избирательного внимания, принятия решений и подготовки двигательного ответа.

We report here our electrophysiological and psychophysiological studies of the mechanisms by which the visual system recognizes structured images with different levels of ordering. Visual stimuli consisted of textures, i.e., a set of matrixes consisting of Gabor grids. Matrixes differed in terms of the degree of ordering resulting from changes in the probability that grids with the same orientation would appear. The subject's task was to identify the dominant orientation in the stimulus. The relationship between response accuracy, reaction time, and the main characteristics of evoked potentials on the one hand, and the number of identical grids in the matrix on the other was identified. The proportion of correct responses increased and the reaction time decreased as the degree of ordering of stimuli increased. Visual evoked potentials recorded in the occipital areas showed a relationship between the amplitudes of the N2, P2, and P3 waves, with latent periods of 180, 260, and 400 msec, respectively, and matrix parameters. The amplitudes of the P3 component and the positive component recorded in the frontal leads, with a latent period of 250 msec, increased gradually as the task became simpler. The amplitude of the N2 wave also increased with increases in the number of identically oriented elements in the matrix, though this relationship was S-shaped. The magnitude of the P2 component, conversely, was maximal in response to presentation of those matrixes which were most complex to recognize and gradually decreased as the content of identically oriented grids in the matrix increased. These relationships were compared with the statistical characteristics of the stimuli and assessed in terms of the view that the visual system contains two mechanisms, i.e., local and integral image descriptions.

The development of methods of digitally synthesizing and processing images has made it possible to use the methods of iconics to deliberately create test images that selectively activate various structures of the visual system. The methods of processing neurophysiological data, including not only images of the activity of the entire brain but also so-called neuroimaging methods, have made it possible to discriminate the brain structures activated as a result of this selective action. The goal of this study is the spatiotemporal localization (mapping) of the regions of the brain that participate in making decisions concerning the shape of textures. It is established that a subject’s reaction time correlates with the degree of ordering of the textures and with the latency of the late components of the induced potentials in the frontal cortex. The time for a person to make decisions in the task of recognizing a specified class of textures is thereby determined. Mapping of the brain by the method of functional magnetic-resonance tomography showed that the activity of the brain in the process of making decisions involving recognition occurs in the frontal cortex of the human brain.

The aims of this study were to identify the locations of areas in the human cortex responsible for describing fragmented test images of different degrees of ordering and to identify the areas taking decisions regarding stimuli of this type. The locations of higher visual functions were determined by functional magnetic resonance imaging (fMRI) using a scanner fitted with a superconducting magnet and a field strength of 1.5 T. The blood oxygen level-dependent (BOLD) method was based on measurements of the level of hemoglobin oxygenation in the blood supplied to the brain. This level was taken to be proportional to the extent of neuron activation in the corresponding part of the gray matter. Stimuli were matrixes consisting of Gabor elements of different orientations. The measure of matrix ordering was the ratio of the number of Gabor elements with identical orientations to the total number of elements in the image. Brain neurons were activated by simultaneous changes in the orientations of all the elements, leading to substitution of one matrix by another. Substitution of the orientation was perceived by observers as rotation of the elements in the matrix. Stimulation by matrixes with a high level of ordering was found to activate the occipital areas of the cortex, V1 and V2 (BA17–BA18), while presentation of matrixes with random element orientations also activated the parietal-temporal cortex, V3, V4, V5 (BA19), and the parietal area (BA7). Brain zones responsible for taking decisions regarding the level of order or chaos in the organization of the stimuli are located in different but close areas of the prefrontal and frontal cortex of the brain, including BA6, BA9, and BA10. The results are assessed in terms of concepts of the roles and interactions of different areas of the human brain during recognition of fragmented images of different degrees of complexity.

In electrophysiological and psychophysical experiments, we investigated mechanisms of the visual system underlying local and global texture processing. Textures included rectangular matrixes composed of Gabor patches (sine wave grating windowed by a Gaussian envelope). Orientation of each grating varied from 0 to 165 degrees with the step of 15 degrees. Matrixes differed by the amount of Gabor patches with vertical or horizontal orientation. The observers' task was to discriminate the dominant orientation. The advantage of such stimuli involved a possibility to calculate global statistics of the textures, which we considered as the difference between whole amount of vertical and horizontal orientations in the stimulus irrespective of their location. The local statistics was calculated as relative amount of spatially organized nearby gratings (i. e. collinear contours). The subjects' accuracy was low in discriminating less organized textures and gradually improved with the amount of vertically of horizontally oriented Gabor patches, while the reaction time decreased. Visual evoked potentials (VEPs) recorded from occipital lobes revealed different dependencies of their components' magnitude on the amount of equally oriented gratings. Amplitude of the late positive component P3 with latency 400 ms directly depended on the texture discriminability, and N2 wave with latency 180 ms had an S-like dependence. Opposite to that, the magnitude of P2 wave with latency 260 ms was maximal in response to less organized textures and gradually decreased with the amount of equally oriented gratings. The dependencies received were compared with the textures' statistics. Data analysis allowed us to suppose that, in the conditions of our experimental paradigm, two mechanisms were involved in discrimination of the textures--the local and the global processing. We believe that by recording VEPs one can separately investigate activity of these two processes.

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).

Electrophysiological studies were performed to measure the threshold (upper end of range) spatial frequency using visual evoked potentials and comparison with visual acuity neuron 26 healthy subjects. The aim of the present work was to create a method for objective measurement of visual acuity. This was addressed by initial measurements using a universally accepted method of visual stimulation and processing of electroencephalograms, which allows errors due to individual differences in visual system function to be minimized. These experiments yielded a strong correlation between the threshold spatial frequency of the test grid yielding an evoked potential on presentation and visual acuity, in degrees, expressed as the resolving ability of the visual system for this optotype. A logarithmic relationship was found between these values and an equation allowing automated calculation of visual acuity (resolving ability) from electrophysiological data was derived. The results were independent of the subject's responses and therefore provides a maximally objective assessment of visual acuity.