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Marian Berryhill, Geoffrey Aguirre, Ingrid Olson; Superior occipital regions track perceived viewing distance in two dimensional images. Journal of Vision 2008;8(6):81. doi: 10.1167/8.6.81.
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© ARVO (1962-2015); The Authors (2016-present)
Perceived depth is apparent in two-dimensional images due to the presence of monocular depth cues. Consequently, viewing distance, the distance between the observer and the observed, is implicitly included in visual images. The present study used a continuous carry-over fMRI design (Aguirre, 2007) to identify focal adaptation and distributed cortical patterns reflecting implied viewing distance. Stimuli were photographs of 40 objects positioned at 4 viewing distances within a naturalistic background, and the background alone. Stimuli were presented continuously, every 1500 msecs. The sequence of implied viewing distances was specified by a fourth-order counterbalanced m-sequence, whereas the particular object positioned at that distance was randomized per trial. Subjects performed an object categorization judgment orthogonal to the manipulation of viewing distance. Presentation of any stimulus compared to the empty background evoked broad activations throughout dorsal and ventral visual areas. Adaptation analysis examined the response modulation as a function of the change in viewing distance across trials. Only the bilateral superior occipital gyrus (corresponding to V3d) demonstrated recovery from adaptation proportional to the change in viewing distance, apart from absolute viewing distance. In a separate analysis, the distributed pattern of cortical response to stimuli at each viewing distance, independent of stimulus features, was submitted to linear support vector machine (SVM) classification. Area V1 voxels provided accurate classification, as expected given the greater angular size of stimuli at the near viewing distance. Importantly, the optimal classification site for viewing distance was not the superior occipital region identified by within voxel adaptation. This suggests that the V3d site can represent viewing distance by a within voxel population code. Studies of a patient with bilateral parieto-occipital lesions also suggest that this region is critical for accurate distance perception. Viewing distance is represented automatically in superior occipital regions similar to those that process binocular disparity (stereopsis).
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