Contrast dependence of spatial summation revealed by classification image analysis

Abstract

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). 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. Classification images for yes/no edge detection and 2IFC contrast discrimination were estimated at signal (edge) contrasts ranging from 0.6%–50% and noise contrasts ranging from 4%–50%. Estimated receptive fields were found to be well approximated by elongated 2D Gaussian derivative or Gabor filters, and were much longer than V1 receptive fields. Both signal contrast and external noise contrast were found to strongly affect the extent of summation: a good predictor for receptive field size and shape was found to be signal-to-noise ratio (SNR). As SNR was increased from as low as 0.07 to as high as 2.0, receptive fields were found to shrink dramatically, by a factor of more than 5 in width and by roughly 60% in length. The dependence of summation on SNR may be due to ‘late’ noise sources and/or to nonlinear summation mechanisms.