Contrast dependence of spatial summation revealed by classification image analysis

Detection of low-contrast luminance-defined stimuli can involve spatial summation over a large portion of the visual field. However prior psychophysical results suggest that the summation region may shrink substantially in the presence of high-contrast masking gratings or noise (Legge & Foley, 1980; Kersten, 1984). This may be related to recent findings that the extent of spatial summation in V1 neurons depends upon contrast (Sceniak et al., 1999; Kapadia et al., 1999). Here we use a classification image technique to directly test whether the psychophysical receptive field for a simple stimulus (a vertical edge in noise) is dependent upon contrast.