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Erik Runeson, Huseyin Boyaci, Judith McLaughlin, Lee Osterhout, Scott Murray; Two representations of object size in early human visual cortex. Journal of Vision 2009;9(8):37. doi: 10.1167/9.8.37.
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Three-dimensional context has a dramatic effect on how we perceive object size - the exact same size projected on the retina can appear small if the object is perceived to be close and large if the object is perceived to be far away. These size illusions raise the question of how 3D context influences neural representations of object size. Here we show, using averaged scalp-recorded electrical potentials (ERPs), that there are two successive representations of object size in early visual cortex: an early representation that reflects the size projected on the retina and a later representation that incorporates 3D context and reflects perceived size. In an initial experiment, we presented circular 2D checkerboards that varied in size on a uniform gray background. We found two latency-defined ERP components centered approximately at the occipital pole that were modulated by stimulus size. In a second experiment, we presented sphere-shaped checkerboards that varied in size at near and far apparent distances in a 3D rendered scene, making the perceived size of the stimulus dependent on its location. The magnitudes of the early and late components were again modulated by absolute stimulus size, replicating the finding from experiment 1. However, the components differed in how they interacted with the location of the stimulus. The early component was not affected by perceived size - the magnitude was the same for stimuli at near and far distances. In contrast, the magnitude of the late component was modulated by perceived size. The stimulus in the far location produced a systematically higher-magnitude response than the near location, reflecting behaviorally quantified perceived size differences between the two locations. These results suggest that the close relationship between the perception of object size and neural activity in early visual cortex results from feedback of 3D contextual information from higher visual areas.
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