Simulation of the role of intracortical inhibition as a source of sensitivity to cross-like figures

Simulation of receptive fields of striate neurons sensitive to cross-like figures under the blockade of intracortical inhibition was performed. It was shown that without the inhibition, a neuron with convergence of signals from two orientation detectors widens its tuning to a cross in such a way that the tuning becomes invariant to the shape and orientation of the cross. Detector of a cross whose orientation is based on the disinhibition mechanisms becomes a bar orientation detector under conditions of inhibition blockade. Another scheme of receptive field is advanced, in which the inhibitory zones mask tuning to cross-like figures, but the blockade of inhibition unveils such sensitivity. We specified the features of the receptive fields (shape, localization, and weight of their zones), which, being applied in simulation, imitate properties of the real striate neurons sensitive to shape and orientation of a cross-like figures.     

Effects of local adaptation of receptive field on sensitivity of the cat visual cortex neurons to cruciform images

The responses to flashing single light bars of different orientation and to cruciform images (CI) were compared in 9 neurons of the cat striate cortex possessing high specific sensitivity to CI, during local adaptation of various receptive field (RF) zones. In most neurons, a two- to threefold reduction in the response to CI with a constantly present bar of optimum or orthogonal orientation, if compared with a response to the figure consisting of two flashing bars, was found. Responses to the CI including an adaptation bar were often increased, if compared with those observed at usual orientation tuning. The role of a cross-orientation inhibition in the formation of a selective sensitivity to CI in the neurons of the visual cortex is discussed.Neirofiziologiya/Neurophysiology, Vol. 27, No. 2, pp. 134–139, March–April, 1995.     

Double orientation tuning of cat visual cortical neurons

Orientation tuning of 148 primary visual cortical neurons was studied in acute experiments on unanesthetized, curarized cats by analysis of their spike responses to flashes in a receptive field of a bar of light of optimal size. Orientation tuning of 88 neurons (59%) was found to be bimodal: Besides the principal preferred orientation there was a second, making an angle with the first. The second tuning maximum in some cases (64%) was exhibited only with a change in stimulus intensity or background brightness. Analysis of orientation tuning by the time-slice method, i.e., on the basis of individual cuts of the spike trace, showed double tuning to be present in 69% of cases only at certain moments after the beginning of stimulation. The results of analysis of the model showed that the double orientation tuning effect may be the result of the specific configuration of the receptive field, the use of a stimulus longer than the receptive field, the presence of a series of alternating excitatory and inhibitory zones in the receptive field, and also of end inhibitory zones on the narrow ends of the field. The unequal change in zones of the receptive fields in time explains the appearance of double orientation tuning in individual fragments of the spike trace. The functional role of double, "cross-wise" tuning in some primary visual cortical neurons and their role in the detection of the features of visual patterns are discussed.     

Sensitivity of striate neurons to Y-like figures: experiment and simulation

Under stimulation of the receptive fields (RF) of neurons in the cat area 17 by flashing Y-like figures of different shape and orientation, the sensitivity to these figures was revealed in 72% of the studied cells, while 62% of units were sensitive to cross-like figures as well. Tuning to Y-like figures was typically selective to their shape and orientation, but in some cases it was invariant to these features. Response magnitudes to single bar, Y-like figure and cross were positively correlated. Simulation showed that the disinhibition might be a sufficient mechanism for effective detection of Y-like figures in a classical receptive field.     

Receptive fields of visual cortical neurons during changes in parameters of photic stimulation in cats

Spatial excitability contours in receptive fields of visual cortical neurons during changes in the physical and physiological parameters of photic stimulation were investigated in acute experiments on immobilized cats under conditions of dark, mesopic, and low photopic adaptation. With the change from dark to low mesopic adaptation the shape and size of the receptive fields detected by testing with flashes of constant intensity are unchanged, but with the transition to low photopic adaptation the receptive field becomes long and very narrow in 72% of cases, and the acuity of its orientational and directional tuning becomes much sharper. Against an unchanged background illumination, loss of brightness of the test light slit leads to narrowing of the measurable receptive field. Excitability contours of the receptive field estimated on the basis of absolute threshold of the cell response and level of intensity necessary to obtain the same number of spikes in the response become much narrower as the threshold criterion rises and during dark adaptation. Reactivity contours of the receptive field in response to stimulation of physiologically equal intensities (equal to the increase in threshold) under conditions of photopic adaptation also are much narrower than reactivity contours under conditions of dark adaptation. Evaluation of receptive fields with allowance for the possible contribution of effects of light scatter on the screen and in the ocular media showed that in most cases their size cannot be explained by these phenomena.Institute of Higher Nervous Activity and Neurophysiology, Academy of Sciences of the USSR, Moscow. Translated from Neirofiziologiya, Vol. 12, No. 2, pp. 115–123, March–April, 1980.     

Role of intracortical inhibition in orientation tuning dynamics in the cat striate cortex neurons

A suggestion about the leading role of GABA-induced intracortical inhibition in the dynamics of orientation tuning (OT) of the cat striate cortical neurons was tested in acute experiments before and during the local blockade of their inhibition by iontophoretic application of bicucculine. In the course of the investigation of these dynamics, with the use of a temporal scanning method, two types of neurons differing in the inhibition blockade-induced OT changes were found. In the neurons of the first type (57%), bicuculline induced the OT dynamics or enhanced it, if it pre-existed before the bicuculline application. In the neurons of the second type (43%), bicuculline strongly reduced or eliminated the dynamic shift of a preferred orientation. These results mean that under normal conditions the inhibition stabilizes and sharpens OT in some cells, while in other cells, in contrast, it causes the OT dynamics. The following mechanisms may underlie the observed effects: an elimination of the inhibition originating from lateral non-isoorientational excitatory inputs of a receptive field; an inhibition of these inputs via the adjacent interneurons activated by a powerful discharge of the examined neuron; a long-term afterhyperpolarization of the neuron, and the dynamics of the excitatory and inhibitory zones of the receptive field.Neirofiziologiya/Neurophysiology, Vol. 27, No. 2, pp. 100–109, March–April, 1995.     

On the mechanisms underlying appearance of responses to movement,directional sensitivity and velocity tuning of the cat's striate cortical neurons

The responses to moving and stationary stimuli of 27 cat's striate cortical units were studied. Two stationary light bars located in different parts of the receptive field were used. The order of presentation and the time-interval between the stimuli varied; so, the presentation of a pair of stationary stimuli was an analogue of a moving stimulus.It was shown that responses occurred in neurons previously unresponsive to stationary stimuli when two stationary stimuli were presented successively in certain order. In the direction-sensitive units an asymmetry of the temporal course of the inhibitory processes was observed. The inhibitory zone located on the side of the preferred direction of movement was characterized by an early inhibitory phase followed by a phase of disinhibition and by a second inhibitory phase. For the inhibitory zone located on the side of the null direction no disinhibitory phase was demonstrated.The significance of the spatial and temporal characteristics of the receptive field for the appearance of responses to movement, the directional sensitivity and the velocity tuning in striate neurons is discussed.     

Simulation model of the tuning mechanism in the visual cortex neuron for the perception of Y-shape images

We performed an imitation simulation of receptive fields (RF) of cat cortical neurons in the primary visual cortex, which were able to detect symmetrical and asymmetrical Y-like figures. We investigated the models of the receptive fields of neurons sensitive to Y-like figures through either the convergence from half-bar detectors or disinhibition mechanism. The model of an of the receptive fields of neurons sensitive to Y-like figures through either the convergence from half-bar detectors or disinhibition mechanism. The model of an-like figure detector on the basis of convergence from the angle and orientation detectors was advanced. Tuning of the simulated receptive fields to Y-like figures was compared with their tuning to cross-like figures. It was shown that the detectors of asymmetric Y-like figures are also detectors of a cross, whereas the detectors of symmetric Y-like figures are more sensitive to Y-like figures than to crosses. The features of the model critical for sensitivity to Y-like figures (the shape, localization, and weight of the RF zones) were specified.     

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.     

Stereopsis and the random element in the organization of the striate cortex.

Receptive field position and orientation disparities are both properties of binocularly discharged striate neurons. Receptive field position desparities have been used as a key element in the neural theory for binocular depth discrimination. Since most striate cells in the cat are binocular, these position disparities require that cells immediately adjacent to one another in the cortex should show a random scatter in their monocular receptive field positions. Superimposed on the progressive topographical representation of the visual field on the striate cortex there is experimental evidence for a localized monocular receptive field position scatter. The suggestion is examined that the binocular position disparities are built up out of the two monocular position scatters. An examination of receptive field orientation disparities and their relation to the random variation in the monocular preferred orientations of immediately adjacent striate neurons also leads to the conclusion that binocular orientation disparities are a consequence of the two monocular scatters. As for receptive field position, the local scatter in preferred orientation is superimposed on a progressive representation of orientation over larger areas of the cortex. The representation in the striate cortex of visual field position and of stimulus orientation is examined in relation to the correlation between the disparities in receptive field position and preferred orientation. The role of orientation disparities in binocular vision is reviewed.     

A model study of the dynamic mechanisms of the double orientation tuning of the visual cortex neurons]

The dynamics of double orientation tuning of neurons in the visual cortex of cat was studied by means of computer simulation. It was possible to test the role of shape, relative localization and weight of the inhibitory and excitatory zones of a receptive field and the dynamics of these characteristics. It was shown that selective and acute double orientation tuning can be achieved only through changes in the weight of zones in the receptive fields with the end-stopping and lateral inhibitory zones whereas only the presence of the end-stopping inhibitory zone in the receptive field is sufficient during changes in zone localization and area.     

Simulation of mechanisms of tuning to incomplete cross-like figures in neurons of visual cortex

We performed a digital simulation of the receptive fields (RF) of cat cortical neurons in the area 17 that are able to detect cross-like figures with partly masked central or peripheral fragments. It was shown that the reciprocal interaction between the RF center and periphery may produce sharp, selective, and pronounced tuning to a cross shape and orientation due to blocking the end-stopping inhibition in the RF by its side-disinhibitory zone. Under conditions of cooperative interaction between the RF center and periphery the sensitivity index (cross/bar response ratio) was typically lower than in the reciprocal model. Features of the model that are critical for sensitivity to cross in cases of small or incomplete figures (the shape, localization, and weight of the RF zones) are specified.     

Vibrissectomy in rats in early ontogeny leads to disordered functional properties of the cortical projection neurons

Vibrissae trimming during the first 20 days of postnatal life caused alternations of the properties of the receptive fields of single neurons in the barrel-field cortex in rats. The following changes were found in the deafferented cortex: (i) an extension of the receptive fields of single neurons as judged from an increase in the number of vibrissae with short-latency excitatory responses to stimulation and (ii) a depression of the inhibitory receptive field tuning mechanisms.     

Selective sensitivity of the cat striate neurons to cruciform and angular figures of various orientations

Selectivity and invariability of tuning were studied in 51 neurons of the primary visual cortex (area 17); cruciform and angular figures (CF and AF, respectively) of different configurations and orientations were presented in their receptive fields. Twenty-three neurons, or 45% of the studied cells, demonstrated selective sensitivity to these figures. Their responses considerably (2.38±0.36 times, on average) increased, as compared with those evoked by presentation of a single bar of preferred orientation. In the examined group, 2 cells demonstrated sensitivity both to the CF and AF. A wide range of detector properties related to the CF and AF analysis was found in the analyzed neuronal population. Detectors of configuration of these figures are described. Selective sensitivity to the angle between branches of these figures was observed in 17 neurons, and responses of 2 neurons among them showed invariability to orientation of these figures. Four cells were selective for orientation and were insensitive to configuration, and 4 other cells showed no specific sensitivity to either of these properties, but were sensitive to the appearance of a CF itself in their receptive field (these cells were regarded as invariant detectors of crossing nodes). Data inconsistent with the hierarchic principle of detection of the above properties are presented. Possible mechanisms and functional significance of selective sensitivity of striate neurons to the CF and AF are discussed.Neirofiziologiya/Neurophysiology, Vol. 27, No. 5/6, pp. 403–412, September–December, 1995.     

Response histogram shapes and tuning curves: The predicted responses of several cortical cell types to drifting gratings stimuli

The responses to visual stimuli of simple cortical cells show linear spatial summation within and between their receptive field subunits. Complex cortical cells do not show this linearity. We analyzed the simulated responses to drifting sinusoidal grating stimuli of simple and of several types of complex cells. The complex cells, whose responses are seen to be half-wave rectified before pooling, have receptive fields consisting of two or more DOG (difference-of-Gaussians) shaped subunits. In both cases of stimulation by contrast-reversal gratings or drifting gratings, the cells' response as a function of spatial frequency is affected by the subunit distances 2 and the stimulation frequency . Furthermore, an increased number of subunits (a larger receptive field) yields a narrower peak tuning curve with decreased modulation depth for many of the spatial frequencies. The average and the peak response tuning curves are compared for the different receptive field types.     

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.     

Pattern-dependent response modulations in motion-sensitive visual interneurons--a model study

Even if a stimulus pattern moves at a constant velocity across the receptive field of motion-sensitive neurons, such as lobula plate tangential cells (LPTCs) of flies, the response amplitude modulates over time. The amplitude of these response modulations is related to local pattern properties of the moving retinal image. On the one hand, pattern-dependent response modulations have previously been interpreted as 'pattern-noise', because they deteriorate the neuron's ability to provide unambiguous velocity information. On the other hand, these modulations might also provide the system with valuable information about the textural properties of the environment. We analyzed the influence of the size and shape of receptive fields by simulations of four versions of LPTC models consisting of arrays of elementary motion detectors of the correlation type (EMDs). These models have previously been suggested to account for many aspects of LPTC response properties. Pattern-dependent response modulations decrease with an increasing number of EMDs included in the receptive field of the LPTC models, since spatial changes within the visual field are smoothed out by the summation of spatially displaced EMD responses. This effect depends on the shape of the receptive field, being the more pronounced--for a given total size--the more elongated the receptive field is along the direction of motion. Large elongated receptive fields improve the quality of velocity signals. However, if motion signals need to be localized the velocity coding is only poor but the signal provides--potentially useful--local pattern information. These modelling results suggest that motion vision by correlation type movement detectors is subject to uncertainty: you cannot obtain both an unambiguous and a localized velocity signal from the output of a single cell. Hence, the size and shape of receptive fields of motion sensitive neurons should be matched to their potential computational task.     

Dynamics of light intensity detection by visual cortical neurons in cats

The dynamics of the intensity function of 32 neurons in area 17 of the visual cortex to photic stimuli of optimal size, shape, and orientation flashing in the center of the receptive field was studied by the time slices method, with a step of 10 or 20 msec, in unanesthetized, curarized cats. All neurons tested showed instability of their intensity function reflected in characteristics of successive fragments of the response: It changed both in preferred intensity and in width of the intensity range within which the neuron generated an above-threshold response. In 72% of cases the preferred intensity for the neuron changed successively during the 4–200 msec after the beginning of stimulation by 4–36 dB from greater toward lesser brightnesses, but later it changed more rapidly (in 20–60 msec), rising again apparently in a jump. In four cases the response optimum was shifted up the intensity scale from its initial value by 10–20 dB. Analysis showed that the observed effects are the simple result of the shape of the relationship between temporal characteristics of the response (latent period, time taken to reach the maximum, and time of ending of the burst) to photic stimulus intensity. The possible functional role of these effects for dynamic time coding of information on brightness of photic stimuli by visual cortical neurons is discussed.     

Binocular visual processing in the owl's telencephalon.

Single neurons recorded from the owl's visual Wulst are surprisingly similar to those found in mammalian striate cortex. The receptive fields of Wulst neurons are elaborated, in an apparently hierarchical fashion, from those of their monocular, concentrically organized inputs to produce binocular interneurons with increasingly sophisticated requirements for stimulus orientation, movement and binocular disparity. Output neurons located in the superficial laminae of the Wulst are the most sophisticated of all, with absolute requirements for a combination of stimuli, which include binocular presentation at a particular horizontal binocular disparity, and with no response unless all of the stimulus conditions are satisfied simultaneously. Such neurons have the properties required for 'global stereopsis', including a receptive field size many times larger than their optimal stimulus, which is more closely matched to the receptive fields of the simpler, disparity-selective interneurons. These marked similarities in functional organization between the avian and mammalian systems exist in spite of a number of structural differences which reflect their separate evoluntionary origins. Discussion therefore includes the possibility that there may exist for nervous systems only a very small number of possible solutions, perhaps a unique one, to the problem of stereopsis.