Manipulation of 3D object perception with mid-level cortical micro-stimulation

Our LONG-TERM GOAL is to test patterned micro-stimulation of intermediate and higher-level visual cortex as a prosthetic strategy for restoring 3D object vision. We think that compositional (parts-based) coding of shape in the ventral visual pathway can be hijacked as an unusually efficient way to generate unlimited shape percepts with a practical number of stimulation sites. Moreover, targeting intermediate/higher-level neurons that explicitly encode 3D shape fragments may be the only way to evoke 3D shape percepts, which normally depend on subtle shading, specularity, texture flow, and stereoscopic cues that would be difficult or impossible to duplicate with the phosphenes produced by micro-stimulation of retina or V1. We will combine array recording, stimulation, and behavior in macaque monkeys, to test the HYPOTHESIS that micro-stimulation-generated activity in recently demonstrated V4 clusters tuned for specific 3D geometric fragments causes perception of those fragments. The causal role of specific part signals in ventral pathway has never been tested. We will use linear probes to measure local V4 multiunit activity at 32 sites during passive fixation. We will analyze tuning for part geometry and spatial position at these V4 sites with our previously published methods. We will then start a 4-alternative 3D shape matching task, in which, for some trials, correct perception of the sample stimulus can partly depend on stimulation of a V4 site that adds signal strength for the geometric part and spatial location that the V4 site encodes. Visibility of the sample stimulus at this location will be variably obscured by dynamic pixel noise. Preliminary behavioral data with no stimulation show that match choice accuracy is a roughly sigmoidal function of signal to noise ratio of the obscured object part, near chance up to ~20% signal and at maximum above ~80% signal. Our EXPECTED RESULT is that stimulation of the V4 site during sample presentation will bias behavioral choices toward stimuli with the encoded geometry at the encoded spatial location. The sensitivity of this approach in dorsal pathway motion studies argues that we will be able to detect even weak causal V4 influences. We also expect to demonstrate that micro-stimulation adds specifically 3D perceptual evidence, by showing the same choice bias when 3D information is needed to choose between stimuli with identical 2D shape. If we do not observe micro-stimulation effects in V4, we will perform experiments in inferotemporal cortex, where multi-part 2D and 3D tuning have been observed, and where micro-stimulation is known to produce observable effects, though not yet for complex, specific shape information. The SIGNIFICANCE of the expected results would be (a) The first causal test of perception produced by compositional, 3D parts-based shape coding, which would have a major impact in resolving the nature of object representation in the brain, and (b) Justification for further exploration of 3D parts-based prosthetic strategies in intermediate/higher-level visual cortex, which could impact clinical approaches to blindness.