When a suprathreshold visual stimulus is flashed for 60-300 ms and masked, though it is no longer visibly degraded, the perceived shape is vulnerable to distortion effects, especially when a 2nd shape is present. Specifically, when preceded by a flashed line, a briefly flashed circle appears to be an ellipse elongated perpendicular to the line. Given an appropriate stimulus onset asynchrony, this distortion is perceived when the 2 stimuli (approximately 4 degrees) are presented as far as 12 degrees apart but is not due to perception of apparent motion between the 2 stimuli. Additional pairs of shapes defined by taper and overall curvature also revealed similar nonlocal shape distortion effects. The test shapes always appeared to be more dissimilar to the priming shapes, a distortion termed a shape-contrast effect. Its properties are consistent with the response characteristics of the shape-tuned neurons in the inferotemporal cortex and may reveal the underlying dimensions of early shape encoding.
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