The mechanisms for detecting compressively sampled gratings

Contrast thresholds for sine-wave gratings are raised when the gratings are compressively sampled into a set of narrow bright bars on a dark background, even though this method of sampling preserves the mean luminance and contrast of the grating. Burr et al. [(1985). Vision Research, 25, 717-727] suggested threshold elevation was due to localized luminance adaptation to the sample bars, whose average peak luminance necessarily increased when fewer bars per cycle were present. Previously, we reported results using decrement-bar compressively sampled gratings (CSGs), which consist of dark sample bars on a bright background, which favoured the local luminance adaptation hypothesis (Kingdom & Rainville, 1995). Here we report experiments that suggest that this hypothesis is untenable. Using increment-bar CSGs (bright sample bars on a dark background) we found that raising background luminance while holding sample bar luminance constant reduced thresholds by as much as a factor of ten. This suggests that it is the contrast of the bars, rather than their luminance, which determines thresholds. Further experiments showed that CSG detection was facilitated by unsampled grating pedestals, and thresholds were elevated when the fundamental was physically cancelled. This implied that CSGs were detected by the same mechanisms as the unsampled gratings from which they are derived. Finally, we provide evidence for the involvement of a dynamic gain control component for increment-bar CSG detection.