Neurophysiological mechanisms underlying object recognition in humans
Murray, Micah Middelmann
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Humans readily identify objects despite sub-optimal conditions affecting either the quality or quantity of the visual features defining that object. Qualitative changes may derive from alterations in an object's contrast, feature cues, orientation, size, or position. Quantitative changes can be due to partial occlusion, fragmentation, overlaid patterns, or absent luminance boundaries. In addition to detecting features within a visual scene, the visual system must also determine which of these features correspond to one object vs. another. This process has been referred to as the binding problem and figure-ground segregation. The primary goal of this thesis was to elucidate the neurophysiological mechanisms underlying object recognition in humans. Our premise was that the ability to recognize a visual scene as well as objects within the scene is a product of neural integration processes operating on both global and local levels. That is, while visual space is perceived as a seamless whole, its cerebral representation is initially anatomically divided. Normal visual perception therefore relies on recombination of separated anatomical representations via neural response interactions. Study I combined electrophysiological (VEP) and psychophysical techniques to investigate inter- and intrahemispheric neural response interactions at the root of the visual binding problem and figure-ground segregation. Neural response interactions to stimuli presented simultaneously in separate visual quadrants began at ∼70--80ms post-stimulus onset, lagging visual cortical response onset by ∼25ms. These latencies place an upper limit on the onset of activity in neuronal ensembles responsive to both spatial locations, thus providing a time course for the conjoining of separate visual representations. Study 2 combined VEP and hemodynamic (fMRI) techniques to elucidate brain mechanisms underlying illusory contour (IC) processing, which we reasoned also relied on neural response interactions. With IC stimuli, object borders are perceived in regions of uniform luminance, representing an experimental means to investigate object recognition during conditions of quantitative variation. In particular, we sought to determine the cortical level where IC processing occurs first. The collective VEP and fMRI results indicate that IC processing begins at ∼80--90ms, lags the cortical response onset by ∼40ms, and is localized in higher-tier visual areas.