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Andrew T. Smith, Velia Cardin; Combination of optic flow fields and stereoscopic depth fields in the encoding of self-motion. Journal of Vision 2011;11(11):740. doi: 10.1167/11.11.740.
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© ARVO (1962-2015); The Authors (2016-present)
The principal visual cue to self-motion (egomotion) is optic flow, which is specified in terms of local 2D velocities in the retinal image without reference to depth cues. However, in general, points near the centre of expansion of natural flow fields are distant while those in the periphery are closer, creating gradients of horizontal binocular disparity. To assess whether the brain combines disparity gradients with optic flow when encoding egomotion, stereoscopic gradients were applied to expanding dot patterns. The gradients were radially symmetrical, forming a cone with the apex at the centre of expansion. The depth cues were either consistent with self-motion (concave disparity cone, as in a tunnel) or inconsistent (convex cone with central dots near, peripheral dots far). The stimuli covered the same range of disparities (±52 minarc relative to the fixation plane) and had a diameter of 23 deg. They were presented via binocular LCD goggles during 3T MRI scanning. To maintain attention, participants performed a demanding counting task at fixation. The BOLD activity generated was compared in a wide range of pre-defined visual regions in 13 participants with good stereo-acuity. Three regions reliably responded more strongly to consistent than inconsistent depth when added to expanding flow. These were (i) hV6, in the parieto-occipital sulcus, (ii) a small region in the precuneus that we refer to as PcM and (iii) vestibular-visual area PIVC, all of which can be identified by comparing responses to egomotion-compatible and—incompatible flow (Cardin & Smith, Cerebral Cortex 2010, 20, 1964). All other visual areas examined (V1-V3B, V7, MT, MST, VIP, CSv) responded well to the stimuli but were indifferent to their depth-flow relationship. The results confirm the involvement of V6, PcM and PIVC in encoding egomotion and suggest that depth and flow cues may be integrated in an egomotion-dependent way in these regions.
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