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Peripheral influences on motion integration in foveal vision are modulated by central local ambiguity and center-surround congruence.

PURPOSE
To study how central visual motion integration and segmentation processes are influenced by the congruence or incongruence of peripheral contextual moving surrounds and to determine their clinical relevance.


METHODS
Nine subjects participated in experiments 1 and 2 (12-second blocks containing 2-second static fixation and 10-second surface plaid movement) and 15 in experiment 3 (72-second blocks, with 12-second fixation, and 60-second motion). Observers reported whether they perceived nontransparent (corresponding to visual integration of motion cues into one surface) or transparent (segmentation of two surfaces from motion cues) plaid motion within a 5 degrees central circular region. Surround stimuli were 20 degrees transparent or nontransparent moving plaids.


RESULTS
Contextual effects required the presence of both local and global ambiguity. If central local motion became unambiguous, then surrounds became ineffective. Under local and global ambiguity, transparent surrounds invariably induced central congruence while also strongly suppressing incongruent percepts. Nontransparent surrounds produced similar but less consistent congruent bias, especially for longer viewing times. In the latter case, however, suppression of incongruent central interpretations became barely detectable compared to the observed significant facilitation of congruent percepts.


CONCLUSIONS
Local ambiguity is critical in contextual modulation, and the peripheral enhancement or suppression of central motion integration depends both on transparency bias and center-surround congruence. The importance of local ambiguity in contextual modulation is clinically relevant, because it implies that contextual effects will be stronger in disorders with impaired central vision, such as macular degeneration. Moreover, the increased efficacy of global context under conditions of increased local ambiguity may be useful in future rehabilitation approaches.