抄録
It is known that orientation tuning of neurons in the primary visual cortex (V1) becomes sharper as the size of stimulus increases beyond the classical receptive field (CRF) (Orban et al., 1979; Chen et al., 2005; Xing et al., 2005). We investigated relationships between the strength of the orientation selectivity and that of the surround suppression in 74 V1 neurons recorded from anesthetized and paralyzed cats. The orientation selectivity became stronger as an increment of surround suppression due to an enlargement of stimulus that covered the CRF and the receptive field surround (SRF). However, the preferred orientation of the neurons was independent of stimulus sizes and did not vary according to a change in stimulus size. Also, both the surround suppression and the orientation selectivity of responses to the stimulus larger than the CRF were significantly stronger in layers II/III than in layers V/VI. We compared our results with predictions of a simple iceberg model where a large stimulus that covered the CRF and SRF equally and linearly suppressed responses to all stimulus orientations. The model, however, overestimated the sharpening of the orientation tuning by large stimulus. These results suggest that surround suppression in V1 exhibits similar to but less-selective orientation tuning than the response to CRF stimulation. Such an effect of surround suppression can be a reason why the stronger orientation selectivity is observed for large stimulus. [J Physiol Sci. 2006;56 Suppl:S176]
