Abstract
Masking from 2D white noise is typically attributed to an increase in variance in the detecting mechanism. Direct evidence for this comes from the double pass technique, in which noise masks increase the consistency of observers’ responses (albeit by less than expected). But noise masks must also activate non-target mechanisms, and these will have a suppressive effect on the detecting mechanism similar to that of cross-oriented grating masks. A consequence of this gain control suppression is that masks should attenuate the perceived contrast of targets. We demonstrate this using a 2IFC matching paradigm for 1c/deg horizontal log-Gabor targets embedded in either white noise or an oblique mask of three times the target frequency (3F mask). What was previously unknown is how the two contributions to masking – variance and suppression – interact with each other. We assess this here by jittering the contrast of a zero-mean pedestal on a trial-by-trial basis (e.g. Cohn, 1976, J Opt Soc Am, 66: 1426-1428), producing a noise stimulus that is entirely within-mechanism. We measured masking functions using a 2IFC procedure for this jitter mask with and without cross-orientation suppression from a high-contrast 3F mask. Arguably, the effects of these different masks might be expected to sum. However, the standard gain control model predicts that when one source is more potent than the other, it will dominate, accounting for all of the masking. At low jitter variances, the 3F mask raised thresholds fourfold for all three observers. At higher jitter variances the masking functions converged, as predicted by the model. However, since masking by suppression and masking by variance produce identical forms of masking function, it is not possible to use (noise) masking functions to assess the equivalent internal noise unless the relative contributions of each source of masking are known. This might be difficult to achieve.
Meeting abstract presented at VSS 2012