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Joseph S. Lappin, Jeffrey B. Nyquist, Duje Tadin; Spatial interactions in fast and slow motion mechanisms. Journal of Vision 2006;6(6):1085. doi: 10.1167/6.6.1085.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: Visual motion mechanisms exhibit center-surround antagonism. Both psychophysical (Tadin et al., 2003, 2005) and physiological (Pack et al., 2005) experiments have found surround antagonism at high contrasts and spatial summation at low contrasts. These and other correspondences suggest that psychophysically observed surround suppression is a perceptual correlate of center-surround antagonism of MT neurons. MT neurons, however, prefer high velocities and are unresponsive below ∼0.5deg/s. Therefore, suppression may vary with velocity. To find out, we quantified spatial characteristics for speeds from 0.06–30deg/s.
Method & Results: Duration thresholds for direction discrimination (L/R) were measured for high-contrast Gabor patches of varied sizes, using a high-speed (200Hz) monitor. Duration thresholds decreased rapidly with increasing velocity, with lower thresholds for smaller stimuli. Importantly, surround suppression increased substantially with velocity, and was nonexistent at very slow speed.
Slow-speed acuity and spatial summation: Spatial displacement thresholds were substantially better at low speeds in the preceding experiment. Functional specializations of fast and slow motion mechanisms were further studied in additional experiments. Spatial acuities for detecting slow relative motion exhibited spatial summation, improving with both size and contrast. Thresholds for high speeds, however, exhibited spatial antagonism at high contrast.
Summary: Visual motion mechanisms have different functional characteristics at different speeds. The change in spatial antagonism with changing velocity seems to reflect changing functional demands associated with maximizing spatial vs. temporal resolution. These results are consistent with the apparent role of area MT in spatial antagonism.
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