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Uri Polat, Dov Sagi; Modulation of the decision criterion by collinear lateral facilitation. Journal of Vision 2005;5(8):485. doi: https://doi.org/10.1167/5.8.485.
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© ARVO (1962-2015); The Authors (2016-present)
The increased contrast sensitivity for targets placed between collinear flankers is modeled as target's excitation mediated by lateral interactions. The interactions are thought to underlie filling in along contours. Here we probe the filling in process by measuring false-alarm (FA) rates in such a detection task, taking advantage of results showing observers inability to adjust FA to a particular, identifiable, stimulus in experimental conditions where the different stimuli are mixed (Sagi & Gorea, VSS 2004). We measured d' and zFA for a Gabor target (9 cpd) using YES-NO paradigm and compared FA rates across target-flankers separations of 3,4,6,9 and 12λ (wavelength). The target contrast was fixed. Two sets of experiments were used; in the first, each separation was measured in a fixed block (Fix) of 50 trials for each separation; in the second experiment all separations were randomly mixed in a single block of trials (Mix). At least 400 trials per datum point were measured with 7 observers. In the Mix condition the criteria were found to vary according to the separation, the probability of reporting target present (FA and Hit) was very high at the shorter separations (3,4λ) and decreased with increasing separation. With larger separations (9,12λ) there was a bias toward No response (Miss and Correct Rejection). On the average, across the different separations, the Yes and No reports were balanced, showing no significant response bias. d' behaved as expected from previous 2AFC measurements. The results show that subjects can not optimize their performance in the Mix condition despite having exact knowledge of the stimulus parameters (separation), rather they seem to equate the global FA and Miss rates by reducing the FA at large separations to compensate for the high FA at the small separations. The high FA rate with small target-mask separations in the Mix condition, suggests that the target location is being “filled in” by lateral excitation from the flankers.
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