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Neurons of the primary visual cortex (V1) respond to stimuli presented at the classical receptive field (CRF), and the responses are suppressively modulated by stimuli simultaneously presented at the receptive field surround (SRF) [1, 2]. The response modulation by an SRF stimulus depends on the stimulus orientation and spatial frequency; thus, it was suggested that intracortical mechanisms contribute to this phenomenon [2]. We recently reported that this suppressive modulation is not due to a direct inhibition to the cell by the intracortical inhibition [3] (also see the chapter by H. Ozeki et al., this volume).

In neurons of the primary visual cortex (V1), the response to the grating stimulus presented in the classical receptive field (CRF) is suppressively modulated by the grating presented at the receptive field surround (SRF). This surround suppression (SS) depends on the relationship of stimulus parameters, such as orientation and spatial frequency, between the CRF and SRF gratings. The strength of SS depends on the stimulus contrast of both CRF and SRF. To assess the input mechanism underlying this stimulus-context-dependent suppression, we performed two series of experiments on the contribution of intracortical inhibition to SS and the nature of the contrast-gain control mechanism of SS.

To assess the network dynamics in the primary visual cortex, we performed simultaneous recordings of neuronal activity from cell pairs within 1 mm apart in anesthetized and paralyzed cats. Cross-correlograms (CCs) were calculated for the responses to grating stimuli with varying orientations and sizes for the 30 cell pairs to date. There were three patterns of cell pairs in respect of tuning property to stimulus sizes. First, both cells exhibited strong suppression of firings with stimuli larger than their classical receptive field (CRF). Second, only one cell exhibited suppression to the large stimuli while the other did not. In these two patterns, the contribution of the peak CCs became smaller when the grating stimulus was larger than the CRFs. And third, both cells did not significantly reduce activity to the grating patch larger than their CRFs. In this case, the peak contribution in CCs did not vary with an increase of the stimulus size. We concluded that the strength of firing interaction is positively correlated with the change in firing rate of cell pairs because of the change of stimulus sizes. We also analyzed CCs in respect of orientation preference of cell pairs. Although the significant peak CCs were most often observed for the common and optimal orientation of the cell pairs, there were some pairs that showed strong peaks for the nonpreferred orientation (more than 60° apart from the optimal) of stimulus, which evoked responses with low firing levels. This result suggests that the strength of firing correlation of cell pairs does not simply depend on the firing rate when the stimulus orientation is varied.

The response of neurons in the primary visual cortex (V1) is modulated by the stimulus presented at the surround of the classical receptive field (CRF), and the predominant effect of this surround stimulation is suppressive [1–5]. The strength of the surround suppression is dependent on the relationship of stimulus parameters, such as orientation and spatial frequency, between grating stimuli at the inside and outside of the CRF [1, 2]. Although properties of the surround suppression have been well characterized, input mechanisms underlying the stimulus-context-dependent response modulation and the size-tuning properties of V1 neurons have not been clarified yet. There is evidence that the surround suppression in VI is both sub-cortical [5] and intracortical [2, 4] in origin.