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Perceptual bistability with counterphase gratings

Suprathreshold counterphase modulated gratings induce a bistable percept of drift or flicker. It is argued that these perceptual alternations might provide a new means for the investigation of directional selective mechanisms. The prevalence of either of the 2 perceptions was examined as a function of the spatio-temporal characteristics of the stimulus and compared with (1) the spatio-temporal contrast sensitivity surface for counterphase modulated gratings and (2) the motion counterphase sensitivity ratio. The authors and another observer served as Ss in suprathreshold experiments varying in spatial and temporal frequency combinations and in a 2-alternative forced-choice experiment designed to assess the threshold sensitivity for all stimuli used in the suprathreshold experiment. Data show that drift perception elicited by suprathreshold counterphase gratings attained a maximum for 8 cycles/deg, 12 Hz stimuli and decreased for any other experimental condition. For spatial frequencies below 1 cycle/deg, or temporal frequencies below 2 Hz, only flicker perception was reported. These phenomenal experiences did not show any systematic dependence on Ss' involuntary eye movements. Comparison with the threshold measurements did not support their explanation in terms of the transient-sustained dichotomy, nor did it allow for a straightforward equivalence between the spatio-temporal characteristics of direction-selective mechanisms at threshold and at suprathreshold levels. (34 ref) (PsycINFO Database Record (c) 2005 APA, all rights reserved)



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