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Timothy Barnes, Ennio Mingolla; An augmented Barlow–Levick model detects onsets and offsets of motion. Journal of Vision 2011;11(11):762. doi: https://doi.org/10.1167/11.11.762.
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© ARVO (1962-2015); The Authors (2016-present)
Freezing is an effective defense mechanism for some prey because many animals rely on visual motion to distinguish objects from their surroundings. Kinetic boundaries, defined as discontinuities in the optic flow field, are constantly produced by motion parallax during egomotion as well as by kinetic occlusion during object motion, even under conditions of perfect static camouflage. Perceiving a kinetic boundary, the foundation of motion–based figure–ground segregation, requires the detection of an unexpected onset or offset of otherwise constantly moving contrast patches. Many cells in primate V1 are directionally selective for moving contrasts, and recent reports suggest that this selectivity arises through inhibition of contrast signals moving in the cell's null direction, as in the rabbit retina. The Barlow–Levick motion detection circuit is extended in this work to also detect motion onsets and offsets by reapplying its mechanism to the output of directionally selective cells. The selectivity of this circuit, measured as its peak response to motion onset/offset compared to its peak response to constant motion, is analyzed as a function of stimulus speed. Because of the temporal ordering of excitation and inhibition that model onset and offset cells receive, offset cell speed tuning is biased towards higher speeds than onset cell tuning, similarly to MT cell speed tuning when exposed to speed ramps (Schlack et al., 2007, J. Neurosci., 27(41):11009–11018). In the context of a population of neurons with different preferred speeds, this asymmetry addresses a behavioral paradox – why subjects in a simple reaction time task respond more slowly to motion offsets than onsets for low speeds (Kreegipuu & Allik, 2007, Psych. Res., 71(6):703–708), even though neuronal firing rates react more quickly to the offset of a preferred stimulus than to its onset (Bair et al., 2002, J. Neurosci., 22(8):3189–3205).
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