Overview

The extraclassical surround (ECS) generally suppresses the firing rate of visual neurons in the primary visual cortex (V1), especially when the surround stimulus has the same orientation (iso-oriented). However, it has been shown that the ECS can actually enhance the firing rate when the stimulus has a low contrast. In (Ichida et al. 2007), the authors test a prediction from a model they have published (Schwabe et al. 2006): that the far ECS, and not just the immediate ECS, can enhance the response. They find that the far ECS can indeed enhance the response, but only when the immediate ECS does not contain the iso-oriented stimulus.

Methods

The authors define the classical receptive field (CRF, called the minimum response field in the paper) as the region that can be driven using a small high contrast 0.1° grating. The size of the CRF depends on the contrast of the stimulus.

It is larger for low contrast stimuli than high contrast stimuli. The authors define the immediate ECS as the area beyond the high contrast CRF where a low contrast stimulus would increase the response.

Instead of using the CRF, they used what they call the high-contrast and low-contrast summation RF. They first found the CRF (minimum response field) by using a 0.1° grating. They then used this to center a high-contrast grating patch. They then varied the size of the patch and found the size that optimally simulated the cell. They called this the high contrast summation RF (SRF_high) or simply the RF center. They used the same protocol with a low contrast grating to find the low-contrast summation RF (SRF_low).

They called the region between SRF_high and SRF_low the near surround. The far surround’s outer diameter was set to 14°. The inner diameter varied but was no smaller than the SRF_low.

References