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.

The time course of disinhibition of visual cortex neurons and sensitivity to cross-shaped figures.

The relationship between the responses of 74 neurons in field 17 of the cat cortex to presentation of cross-shaped figures flashing in their receptive fields and the asynchronicity with which the lines of the figures were presented were investigated. The cross sensitivity of neurons was studied with simultaneous, leading, and delayed activation of the disinhibitory zone of the receptive field in relation to the time at which its major excitatory and end-stopping inhibitory zones were stimulated. Two types of temporal interaction were identified between the receptive field zones determining cross sensitivity. In cells of the first type (14 of 23 cells), the response was maximal in conditions of simultaneous stimulation of the major and disinhibitory zones of the receptive field; neurons of the second type (nine of 23 cells) showed the opposite temporal relationship. Digital simulation showed that cross sensitivity in neurons of the first type was supported by disinhibition of end-stopping inhibition, while in neurons of the second type it depended on a combination of disinhibitory and convergence mechanisms.

Dynamic changes in the tuning of striate neurons to the shapes of cross-shaped figures.

Time slice analysis was used to study the dynamics of tuning to the shapes of cross-shaped figures flashing in the receptive fields of 83 neurons in the primary visual cortex (field 17) of the cat brain. Tuning was assessed in terms of the numbers of spikes in the overall response and its sequential 20-msec fragments. Only 11.7% of neurons produced reproducibly developing spike responses to a given shape (defined as the angle between the lines), i.e., had a preferred cross-shaped figure. In the remaining cases (88.3%), tuning of neurons to the shape of the cross showed dynamic changes. In 7.2% of cases, changes in the preferred shape of the cross occurred monophasically; changes were biphasic in 27.0% of cases, while in the remaining 54.1% of cases, the dynamics in changes in the preferred cross shape were undulatory. The tuning of receptive field zones is assessed as the cause of these effects and their difference from the previously observed dynamics of preferred orientations of single bars and cross-shaped figures; the functional significance of these effects is also discussed.

The disinhibitory zone of the striate neuron receptive field and its sensitivity to cross-like figures.

Acute experiments on immobilized anesthetized cats were used to confirm the suggestion that the sensitivity of many neurons on the primary visual cortex to cross-shaped, angular, and Y-shaped figures may be determined by the presence within their receptive fields of disinhibitory zones, which block end-stopping inhibition. A total of 55 neurons (84 functions, i.e.. on and off responses) were used for studies of sensitivity to crosses, and responses to single bars of different lengths were compared before and after stimulation of an additional lateral zone of the field (the presumptive disinhibitory zone), which was located in terms of responses to crosses. Seventeen of the 55 cells in which increases in the length of a single bar decreased responses, i.e., which demonstrated end-stopping inhibition, showed significant increases in responses (by an average factor of 2.06 +/- 0.16) during simultaneous stimulation of the lateral zone of the receptive field, which we interpreted as a disinhibitory effect on end-stopping inhibition. These data provide the first direct evidence for the role of end-stopping inhibition and its blockade by the disinhibitory zone of the receptive field in determining the sensitivity of some neurons in the primary visual cortex of the cat to cross-shaped figures.