Contiguity requirement of metacontrast masking depends on frame of reference

Classical metacontrast masking requires test and masking stimuli to be spatially contiguous. This dependence suggests that the masking interaction occurs in a retinotopic cortical representation such as area V1. However, many other characteristics of metacontrast, such as the influence of attention, suggest a higher level for the interaction. To resolve this apparent contradiction, we examined whether contiguity is required in retinal coordinates or in an object centered frame of reference. We measured the detectability (2AFC) of a 0.35° black test square presented 2° below fixation in the presence of 2 masking squares of the same size above and below the test square (28 ms presentation times, asynchrony 0, 28, 56, or 84 ms). Spatial selectivity was assessed by presenting the masking squares at various horizontal positions while the test square position remained fixed. The background was either a blank white screen or a 5° white square ("panel") that moved from left to right across the screen at 14°/sec. If retinal position is critical, the maximum of masking should always occur at the position of the test square. If the background is the critical frame of reference, the maximum of masking under the moving panel condition should be displaced to the right. Strong masking was found for asynchronies of 28 and 56 ms. Eight out of 11 subjects showed sufficiently narrow spatial tuning of masking to compare the peaks of masking with and without the moving panel. For 56 ms asynchrony 5 subjects showed a displacement to the right, 3 showed no change. For 28 ms asynchrony, none of the subjects showed displacement. The displacement of the spatial masking functions indicates that metacontrast masking depends on the frame of reference in which test and mask are represented, suggesting that the interaction occurs at a higher level than the retinotopic visual cortex.