These results are often interpreted as evidence of long-range interactions in V1 and/or other low-level visual areas. Such interactions are usually assumed to be either a multiplicative (contrast gain) or an additive form of input modulation implemented via lateral cortical connections. It has been shown that such low-level input modulation could provide a basis for various image segmentation tasks, for example, contour integration or boundary detection (Li,
1998; Stemmler, Usher, & Niebur,
1995; Yen & Finkel,
1998). In support of this hypothesis, several neurophysiological studies have reported that lateral facilitation was a common effect in striate cortex. Notably, Kapadia, Ito, Gilbert, and Westheimer (
1995) compared single-cell recordings in monkey V1 with the performance of human observers and found qualitative agreement between the two. In their study, 42% of complex cells responded more strongly to a bar presented in the suprathreshold (classical) receptive field if another (collinear) bar was presented in the surround. Other studies (e.g., Chen, Kasamatsu, Polat, & Norcia,
2001; Kapadia, Westheimer, & Gilbert,
2000; Nothdurft, Gallant, & Van Essen,
1999; Polat, Mizobe, Pettet, Kasamatsu, & Norcia,
1998; Sengpiel, Baddeley, Freeman, Harrad, & Blakemore,
1998; Toth, Rao, Kim, Somers, & Sur,
1996) have reported similar effects.