Surround suppression and facilitation in the fovea: very long-range spatial interactions in contrast perception.

Surround modulation of perceived contrast has been almost exclusively studied in short-range conditions, i.e., in situations where a tiny gap, at most, separates center from surround. Existing long-range studies suggest that suppression extends to 12-cycle distance, whereas facilitation of perceived contrast is suggested to arise from visual field regions enclosing the center. In V1 neurons, however, long-range surround modulation involves both suppression and facilitation. Thus, we investigated short- and long-range surround modulation by measuring the perceived contrast of a center in the presence of a surround either near (0.3 cycles, 0.1 degree) or far (19.8 cycles, 6.6 degrees) from the center. This study demonstrates that in addition to the well-known suppression, surround modulation involves remarkably long-range facilitation of perceived contrast. At low center contrasts, the long-range facilitation was stronger than the long-range suppression, whereas at high center contrast we found mainly long-range suppression. Because the current models of perceived contrast could not account for our data, we considered our results in the context of models developed for surround modulation in V1 neurons. However, neither mechanistic nor phenomenological models proved satisfactory. Moreover, with the current knowledge, it seems that straightforward pooling of V1 neurons' responses cannot account for surround modulation of perceived contrast.