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Christina M Anderson, Ione Fine, Karen R Dobkins; Contrast, coherence and directional tuning. Journal of Vision 2003;3(9):285. doi: 10.1167/3.9.285.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: Previous psychophysical and neurophysiological studies report broad directional tuning of motion mechanisms, with bandwidths (width at half height) ranging from 70 –120 (e.g., Ball & Sekuler, 1980; Albright, 1984). In motion area MT, this tuning is also largely invariant with coherence (Britten & Newsome, 1998). We measured directional tuning psychophysically by having subjects detect global motion across a wide range of motion coherence and contrast levels. Methods: A 2-IFC procedure was used; the ‘noise’ interval contained randomly moving dots and the ‘signal’ interval contained noise plus two superimposed fields of coherently moving dots. We tested 7 angular differences between the directions of the two dot fields (delta=0 , 22.5 , 45 , 67.5 , 90 , 135 , and 180 ). We obtained two types of thresholds for 3 subjects: (1) coherence thresholds, where the proportion of dots moving coherently in the ‘signal’ interval was varied. Coherence thresholds were obtained for several fixed contrasts (rms, 3.2 – 30.2%), and (2) contrast thresholds, where the contrast of the dots was varied. Contrast thresholds were obtained for several fixed coherence levels (20 – 100%). Thresholds for a given contrast and coherence did not depend on whether coherence was fixed and contrast varied, or vice versa. Directional tuning bandwidths were calculated by fitting Gaussian functions to threshold ratios (threshold for delta=0 / thresholds for the other deltas) plotted as a function of delta. Results and Conclusions: We found that directional tuning had a bandwidth of 60–70 that was nearly invariant across a wide range of luminance contrasts and coherence levels, including contrasts and coherences for which MT responses saturate. However, we found broader tuning, of ∼110–120 , at the intersection of very low contrasts (< 4%) and high coherences (> 45%).
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