The effect of high-level image structure in early human visual cortex
Joo, Sung Jun
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A commonly held view about neurons in early visual cortex is that they serve as localized filters. Here, however, we demonstrate that the responses of neurons in early visual cortex are sensitive to perceptual grouping based on global image structures. First we measured neural responses to an oriented Gabor ("target") embedded in various orientation patterns using multiple methodologies--psychophysics, functional magnetic resonance imaging (fMRI), and electroencephalography (EEG). Specifically, we varied whether a central target deviated from its context by changing distant orientations while leaving the immediately neighboring flankers unchanged. The results of psychophysical contrast adaptation and fMRI experiments show that a target that deviates from its context results in more neural activity compared to a target that is grouped into an alternating pattern. For example, the neural response to a vertically oriented target was greater when it deviated from the orientation of flankers (HHVHH) compared to when it was grouped into an alternating pattern (VHVHV). We then found that this pattern-sensitive response manifests in the earliest sensory component of the event-related potential to the target. In a forced-choice classification task of "noise" stimuli, perceptions are biased to "see" an orientation that deviates from its context. These results show that neurons in early visual cortex are sensitive to large-scale global patterns in images in a way that is more sophisticated than localized feature detection. Finally, we show using event-related potential (ERP) that the well-known orientation-specific surround suppression effect is dependent on the surface structure in the image, demonstrating an important role of high-level, global processes in determining when contextual effects occur in early visual cortex. Overall, our results demonstrate an important role of high-level, global processes in determining when contextual effects occur in early visual cortex.
- Psychology