Foveal contour interaction on the edge: Response to 'Letter-to-the-Editor' by Drs. Coates and Levi

Recently, we reported that, when considered as a function of the edge-to-edge target-toflanker separation in min arc, the spatial extent of foveal contour interaction is the same for high and low contrast acuity targets. This result resolved an apparent discrepancy in the literature, which suggested that foveal contour interaction was absent or reduced for low contrast targets. In commenting on our results, Drs. Coates and Levi suggest a two-mechanism model for foveal crowding that depends on the center-to-center separation between the acuity target and flanking stimuli, and is based in part on a reanalysis of data from our recent work and a number of other studies. In our reply, we show that the spatial extent of foveal contour interaction for both high and low contrast targets is essentially unchanged by the width of the flanking targets when the target-to-flanker separation is depicted in terms of edge-to-edge separation, but varies systematically when depicted in terms of center-to-center separation. We therefore conclude that for foveal contour interaction in the range of a few min arc, edge-to-edge target-to-flanker separation is the more appropriate metric.