Abstract
The ability to detect a target situated in the visual periphery is impaired if a stimulus with similar physical characteristics is located surrounding the target. In particular, the contrast detection threshold of a grating located in the periphery is increased if a surrounding grating of the same frequency and orientation is present. This inhibition between center and surround has been termed surround suppression. We measured the spatial frequency bandwidth of surround suppression in the periphery for different spatial frequencies (0.5, 1.1, 3 and 5 cpd) of a sinusoidal grating (target) surrounded by a grating with different spatial frequencies (surround). Using a Bayesian adaptive staircase we have measured contrast detection thresholds in an 8AFC detection task where the target (grating with 2.3 deg-Butterworth window) could appear in one of eight possible positions at 4 deg eccentricity. The target appeared on top of a surround grating (grating with 9 deg-Butterworth window) with the same or orthogonal orientation. In each session we fixed the spatial frequency of the target and changed the spatial frequency and the orientation of the surround. When the surround is orthogonal to the target, the thresholds are similar to those obtained without surround and independent of the surrounding spatial frequency. However, when the target and surround had the same spatial frequency and orientation, a strong suppression was obtained. When the spatial frequency of the surround differed from that of the center, the suppression rapidly reduced, with detection thresholds decreasing as a Gaussian function of the spatial frequency difference. The bandwidth in octaves of the Gaussian function fitted to the detection thresholds was around 4 for a target frequency of 0.5 cpd, and decreased with the increasing target spatial frequency to around 1 at 5 cpd. We suggest that this result could be a consequence of the decreasing bandwidth of the simple cells present in the striate cortex.
Supported by Medical Research Council New Investigator Award 80154 and Royal Society Society University Research Fellowship UF041260 to JCAR.