[Receptive fields of cat striate neurons: dynamics of weight and topographical characteristics of the excitatory zone of the neuron receptive fields in the visual cortex].

In 22 acute experiments on anesthetized and immobilized adult cats, the dynamics of 83 on- and/or off-receptive fields (RF) was studied in 47 striate neurons with the method of temporal slices by mapping with single-flash stimuli. The latency of the RF appearance was 88 +/- 5 ms, and its duration was 192 +/- 12 ms. Dynamical wavy changes in the RF sizes and weights repeating up to three times were revealed. The mean duration of one wave was 95 +/- 4 ms. In 99% of cases, the RF discharge center defined for one temporal slice also displaced in an undulatory way relatively the RF's center defined for whole analysis period. The mean duration of this undulatory cycle was 67.3 +/- 3 ms. In 72.5% of cases, the displacement of the discharge center followed different trajectories inscribed in an ellipse. The functional significance of such changes in the RFs of striate neurons is discussed with regard to the dynamics of their detector features and underlying mechanisms.

[Receptive fields of detectors of bar and cross-like figures in classical and combined mapping].

Excitatory and inhibitory zones of receptive fields (RF) of detectors of cross-like figures and bars were studied in a primary visual cortex of cats under conditions of classical and combined mapping. Factor and correlation analysis demonstrated that detectors of bars and crosses differ in those interrelations, which are formed between the basic characteristics of neurons and their RF in each of group. The factor analysis of these connections showed that variables with the greatest weights, combined into the same factor, are different for different detectors. In detectors of bars, positive correlation was found between the background activity and weight characteristics of the RF excitatory and inhibitory zones. In detectors of crosses, inhibition indices showed a positive correlation with sensitivity to this figure, characteristics of excitatory RF zone and the background activity. Enlargement of the additional excitatory zones of cross detectors in combined mapping was significantly greater than in bar detectors. Formation of the RF and orientation sensitivity of bar and cross detectors is discussed, with account made of the finding that the sensitivity to bars is mostly formed by direct excitatory connections, whereas sensitivity to cross-like stimuli is formed by direct and recurrent intracortical inhibitory connections between the nearby neurons.

[Overlapping of optical answers for cross-like figures and oriented bars in the cats primary visual cortex].

For the first time by the optical method the population activity of neurons in cat primary visual cortex was observed simultaneously for detectors participating in analysis of first-order (orientation) and second-order (line intersection) features. The maps for cross-like figures and oriented single bars were compared. The comparative analysis allowed us to estimate the degree of overlapping of the activated regions and parts of cortex that were free from overlap. Overlapping zones provided the evidence for the fact that neuronal detectors for line intersections are located in the same neuronal columns as neurons detecting orientations. Differences were observed between maps for vertically oriented and oblique crosses. Those differences were pronounced not only in topography but also in degree of overlapping of activity zones. This may evidence on different contribution of neurons detecting basic and intermediate orientations.

Recognition of lateralized halftone and outline images of everyday objects in conditions of masking.

Recognition of the shapes of halftone and outline images of everyday objects in conditions of lateralized tachystoscopic presentation and different levels of noise masking (with "raindrops") by humans was studied. Mean group data for 15 subjects demonstrated significantly better recognition of outline images of everyday objects by the left hemisphere of the brain than the right at all levels of masking. Increases in masking produced gradual and significant degradation of recognition as compared with controls (recognition of unmasked figures). Recognition of outline images at all levels of masking was significantly better than recognition of halftone images of the same objects. In men, there were no significant differences between hemispheres either at different levels of masking or for different types of stimuli. The neurophysiological mechanisms and functional significance of these effects are discussed.

Trajectories of shifting dipole sources of visual evoked potentials across the human brain.

Visual evoked potentials in response to images of a set of horizontal and vertical lines or crosses were recorded from the brains of 18 human subjects in 34 leads. Inverse EEG analyses were used for the dynamic location of the dipole current sources of the N1, P1, and N2 waves using a two-dipole spherical model with a 1-msec step. The occipital lobes of all subjects showed significant displacement of the dipoles of evoked potential waves along predominantly arc-shaped trajectories (75.8% of cases). Trajectory durations (average about 25 msec) were characterized by insignificant interindividual variability and were independent of the type of stimulus and the phase of the evoked potential. A characteristic (occurring in 85% of cases) "jump" in the coordinates of the dipole, which constituted a rapid, sharp, and significant medial displacement, was seen between the first and second trajectories of the equivalent current dipoles (at 110-120 msec after stimulus onset). The possible significance of these data for understanding the dynamics and kinetics of processing of local image features in the human visual cortex is discussed.

[Recognition of halftone and contour figures under their lateralization and masking].

Recognition of shape of natural objects was studied during lateralized tachistoscopic presentation and different degree of noise-like ("rain drops") masking in 15 healthy subjects. Two sets of figures were used: halftone and contour ones. In all masking conditions, the mean group data showed a significantly better recognition of contour images by the left hemisphere as compared to the right hemisphere. The probability of correct response decreased with increase in the degree of masking. Contour figures were recognized significantly better than halftone figures. Gender differences in recognition were revealed. Male subjects displayed no hemispheric preference in recognition of both types of stimuli in both masking conditions. Possible neurophysiological mechanisms and functional significance of the findings are discussed.

[Spatial gradient of P300 area of the brain visual evoked potential in the brain-computer interface paradigm].

In 12 adult healthy subjects we studied Brain-Computer-Interface recognition of different intended letters by the square of P300 wave in the averaged VEP. Horizontal and vertical spatial gradient of this square was studied as well as tuning acuteness of visual attention to a significant letter. High acuteness of this tuning was found (width of the tuning curve at its half height was equal to 1.6 grad) independent of the letter position on the letter matrix. Horizontal and vertical gradient of P300 were found to be very similar, but in the half of cases the first one revealed some kind of the "lateral inhibition": decrease of P300 square for the columns neighboring to the meaningful one. Tuning acuteness was found to be reliable and directly interrelated with P300 square. The data are discussed in relation to selectivity of the local visual attention.

[Striate receptive fields mapped with single and bipartite stimuli].

In 22 acute experiments with anesthetized and immobilized adult cats, 364 maps of receptive fields (RF) of 47 striate neurons were obtained by means of single local stimuli flashed at different parts of the visual field, or with additional asynchronous activation of the RF excitatory center with oscillating bar of the optimal orientation. Under bipartite stimulation, considerable and significant decrease in the square and weight of the central excitatory RF zone was revealed in more then 75% of the studied cells. Additional excitatory zones appeared in 54% of cases, or the square and weight of the excitatory zones substantially increased, and inhibitory zones developed in 90% of cases. These effects were correlated with the degree of increase in the background firing during transition from the mode of mapping with single stimulation to that with bipartite stimulation. The mechanism and possible functional role of cooperative excitatory and inhibitory intracortical interactions in organization of receptive fields and detection of features of a visual image are discussed.

[Dipole tracing of visual evoked potentials in human brain].

3D tracing of equivalent current dipoles (ECDs) of averaged human visual evoked potentials (VEP) by their distribution across a 34-electrode array was obtained under short presentation of pattern-onset stimuli (sets of 45 horizontal, vertical bars or crosses). Using a 2-dipole spherical three-layer model, we dynamically (step of 1 ms) localized dipoles in four healthy subjects. Dipole locations were matched to anatomical brain regions visualized in structural MRI. Best-fitting source parameters were superimposed on MR images of each subject to identify the anatomical structures giving rise to the surface patterns. It was found that during 50-300 ms following the onset of the stimuli, the ECDs in all subjects were localized in the occipital cortex and demonstrated reliable systematic shift in localization. Two local (1-2 cm3) zones of the preferable dipole attendance were found at 5-6 cm behind zero line: the first one was localized near the midline of the brain, whereas the other zone was situated in the right hemisphere at a distance of 6-7 cm from the first zone. Their localization and strength of activation were reliably different for crosses and lines and changed during VEP generation. Zones of relatively rare dipole attendance were found also. The data are discussed in relation to localization of initial and endpoint of ECDs trajectories, as well as with sensitivity of the visual cortex to line crossing and branching.

Neurophysiological and simulation studies of striate cortex receptive field maps: the role of intracortical interneuronal interactions.

Acute experiments on 27 adult anesthetized and immobilized cats investigated 101 on and off receptive fields in 67 neurons in visual cortex field 17 by mapping using single local stimuli presented sequentially at different parts of the visual field, as well as in combination with additional stimulation of the center of the receptive field. Both classical and combined mapping identified receptive fields with single receptive zones (63.4% and 29.3% respectively), along with fields consisting of several (2-5) excitatory and/or inhibitory zones (36.6% and 70.7%). We provide the first report of receptive fields with horseshoe, cross, and T shapes. Simulations of horizontal interneuronal interactions in the visual cortex responsible for the multiplicity of excitatory and inhibitory zones of receptive fields were performed. A role for cooperative interactions of neurons in this effect was demonstrated. The possible functional role of receptive fields of different types in extracting the features of visual images is discussed.

[Recognition of halftone, contour, and silhouette figures under conditions of their masking].

Shape recognition of natural objects was studied under conditions of tachistoscopic (12 ms) presentation and different degree of noise-like ("rain drops") masking in 15 healthy subjects. Three set of figures were used: halftone, contour and silhouette. For all sets of figures, mean group data reveal a drop of the correct response probability with increase in masking. Recognition of contours was shown to be significantly better than that of halftone and silhouette figures, whereas halftone figures were recognized better than silhouettes. Possible neurophysiological mechanisms of the revealed phenomena and their functional implication are discussed.

Characteristics of the responses of visual cortex neurons with sensitivity to bars or cross-shaped figures in cats.

The magnitudes and latent periods of spike responses were recorded from 280 individual neurons tuned to the orientation of light bars or cross-shaped figures in the primary visual cortex (field 17) of the cat. In control experimental conditions, half of 195 cells preferred the bar (first group), the remainder preferring crosses (second group); the responses of neurons of the first group to bars and crosses were of similar magnitude, while in the second group, responses to crosses were significantly larger than responses to bars. The latent periods of responses to optimal bars in the first group of neurons were shorter than those in the second group, and became longer on exposure to crosses, while latent periods in the second group were shorter on exposure to crosses. In conditions of local bicuculline blockade of intracortical inhibition, about a quarter of 85 neurons were sensitive only to the bar, regardless of the presence or absence of inhibition. The remaining neurons were sensitive to crosses in at least one of the states and continued to have responses which were smaller in terms of absolute magnitude than the responses of group 1 neurons. The significance of these data for understanding the mechanisms of tuning of striate neurons to signal features and the temporal sequence of their operation is discussed.

[Study of the letter and word recognition by the brain-computer-interface with P300 wave of human visual evoked potential].

In four adult healthy subjects in 18 experiments, we studied Brain-Computer-Interface recognition of different intended words by P300 wave in the VEP. The set of optimal characteristics of visual stimulation which rise reliability of recognition up to 100 %, as well as effective registration locus (Pz) were determined. It was found that the best processing criteria for letter recognition were: P300 square and superposition of all three criteria (P300 amplitude, square and covariation coefficient).

[Trajectories of shifting of dipole sources of visual evoked potentials over the human brain].

Dynamic study of 3D localization of the equivalent current dipole (ECD) sources of visual evoked potentials (EP) in the human brain was performed in 18 healthy subjects using a two-dipole model. Dipole tracing was performed for relatively early EP components (N1, P1, and N2) with 1-ms step. The analysis confirmed localization of these ECDs mainly in the right occipital cortex and revealed their successive shift over this area in the anterior-medial direction and then backwards in all subjects during generation of the EP components. Typically, some successive arch-like trajectories of the shift were revealed (75.8%); their duration was relatively standard (about 25 ms) and did not depend on the stimulus shape and EP phase. Between the 1st and the 2nd trajectories (110-120 ms after the stimulus onset) a jump in ECD coordinates in the medial direction was found in 85% of cases. Possible significance of the findings for the insight into dynamic topography of the visual feature processing in the human brain is discussed.

[The area of the optically activated zones of cat area 17 under stimulation with grids of different orientation].

In acute experiments with 9 anesthetized and immobilized cats, the relative tangential square of the activated cortical columns in area 17 was mapped by the intrinsic optical signal under stimulation with grids of different orientation. We examined the "oblique effect", i.e. the greater representation of neurons tuned to the vertical and horizontal orientations vs. oblique orientations in the primary visual cortex. The square of the activated parts of the cortex was estimated under different threshold criteria (80, 60 and 40% of the maximum). The "oblique effect" was not observed in our study: the areas of activation of the cortical columns did not differ statistically for two basic vs. oblique orientations. Reasons for the difference between the results of electrophysiological and optical mapping are suggested and possible contributions of the experimental protocol (anesthesia) and individual visual experience in different animals' samples to the origin of these differences are discussed.

[Neurophysiological and simulation study of the striate receptive fields maps: the role of intracortical interaction].

In 27 acute experiments with anesthetized and immobilized adult cats 101 maps of receptive field (RF) in 67 striate neurons were studied by means of mapping with single flashed stimuli presented in different parts of the visual field and under conditions of additional activation of the RF excitatory center by the local oscillating or flashing grid. Under conditions of both classical and combined modes of mapping, the RFs of the classical shape with a single excitatory zone (63.4 and 29.3% of cases, respectively) and RFs with multiple (2-5) excitatory and/or inhibitory zones (36.6 and 70.7%, respectively) were found. We were the first to describe, also, some RFs of horseshoe-like, cross-like and T-like shapes. Simulation of non-classical RFs revealed possible contributions of cooperative excitatory and inhibitory intracortical interactions to the effects under study. The functional role of RFs of different types in the feature detection is discussed.

[Responses of neurons in cat visual cortex with sensitivity to bar and cross-like figure].

In the cat primary visual cortex (area 17) the response magnitude and latency were studied in 280 neurons sensitive to bar or cross-like-figure. Under natural conditions half of the studied 195 cells preferred bar (first group) or cross (second group). In the first group responses to both figures were near equal, while in the second one cross evoked much stronger response. Response latencies with the optimal bar in the first group were shorter than in the second group and longer to a cross than to a bar while in the second group they were considerably shorter to a cross than to bar. Under local blockage of GABA-ergic inhibition by microiontophoretic application of bicuculline about one-fourth of 85 neurons generated greater responses and were bar-sensitive irrespective to presence or absence of inhibition. Other neurons were cross-sensitive at least in one of the conditions (with and/or without of inhibition). They responses grew under bicuculline action relatively more than in the first group. Significance of the data obtained for tuning to image features and temporal succession of their detection is discussed.

[Human visual evoked potentials to bars and cross-like figures].

Temporal and amplitude characteristics of averaged visual evoked potentials to presentation of lines, corners and crosses with different orientation recorded in 19 adult healthy subjects were compared in 34 derivations. In all subjects, the latencies of P1, N2, and P3 were shorter while their amplitudes were higher for crosses than for lines. The effect of lengthening of P1 peak latency from occipital to temporal cortex was mostly pronounced for EPs to a bar, whereas as increase in the P1 amplitude was most evident for a cross-like figure. Correlation of these data with: i) greater magnitude and shorter latencies of responses to crosses vs. bars in a half of cat striate neurons, ii) sensitivity of cells in monkey inferior temporal cortex to star-like figures, and iii) relatively better human recognition of figures with comers than with lines, as well as significance of the effects obtained for detection of image features in different areas of the human visual cortex is discussed.

Simulation studies of the role of intracortical inhibition in the formation of sensitivity to cross-shaped figures.

The receptive fields of detector neurons for cross-shaped figures in the visual cortex were modeled in conditions of blockade of intracortical inhibition. The tuning of simulated neurons was compared with and without inhibition in the receptive field. In a simulated detector with convergence from two orientation detectors, acute tuning to the cross widened in the absence of inhibition, becoming invariant to the shape and orientation of the cross. A detector based on the disinhibition mechanism lost cross sensitivity when inhibition was blocked and became a detector for the orientation of a single bar. A model of a receptive field in which the inhibitory zones mask the tuning to a cross-shaped figure and in which blockade of inhibition affects only sensitivity is also proposed. We identified which of the properties of receptive field (configuration, location, zone weightings) allow them to simulate the properties of cat visual cortex field 17 neurons, these being sensitive to the shape and orientation of cross-shaped figures.