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Duje Tadin, Joseph S. Lappin, Randolph Blake; Relative timing of center and surround signals in motion revealed by temporal reverse correlation. Journal of Vision 2005;5(8):493. doi: 10.1167/5.8.493.
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Center-surround interactions are a hallmark of neural motion processing. Different neural circuitry underlies center and surround responses, thus changes in their relative latencies (e.g., by varying contrast) may affect their interactions. We approached this question using temporal reverse correlation.
random-dot surround (r=6deg) moved up-down randomly (20deg/s) for 500ms. A new surround direction was chosen every 5ms (200Hz). Observers perceived this as fast jitter. Surround contrast, in different conditions, ranged from 92% to 1.7%. In the center of the surround was a small hole (r=0.7deg) where, at a randomly chosen time, high-contrast center motion (20deg/s) was briefly presented. The observer's task was only to report the direction of center motion. Performance was kept near 70% by adjusting center duration (duration thresholds were low, effectively restricting center motion to ∼3 5ms frames). 3 observers completed 10000 trials each. Temporal patterns of surround motions were classified for correct and incorrect trials, yielding curves depicting the effect of the surround on the center motion for each 5ms frame.
For all surround contrasts, surround motion impulses matching the direction of the center motion increased the probability of making a mistake, indicating surround suppression. This suppressive influence of the surround spanned about 50ms and its timing depended on surround contrast: decreasing surround contrast from 92% to 7%, shifted the peak of the surround influence 11ms backwards (decrease to 1.7% contrast resulted in a 36ms shift). That is, a low-contrast surround signal must be initiated well before the center signal in order to arrive on time to suppress center motion - a finding consistent with longer latency of low-contrast stimuli. Repeating the same study with low-contrast center motion yielded analogous results but with surround facilitation, indicating that the “sign” of center-surround interactions depends on center contrast.
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