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Leonard Matin, Wenxun Li, Todd Hudson, Joy Hirsch; Perceptions of elevation and orientation: From the stimulus basis to the cerebral cortex. Journal of Vision 2001;1(3):195. doi: 10.1167/1.3.195.
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By changing the orientation of individual pitched or rolled lines in the visual field, large changes of perceived elevation of visual targets are induced during monocular viewing with steady gaze. Systematic changes in the orientation of bilaterally symmetric line pairs at bilaterally symmetric locations produce these large systematic changes; changes in the orientation of parallel line pairs have no influence on perceived elevation although they do influence perceived orientation in the frontoparallel plane. Our computational model predicts these results as well as results from a fully-structured well-illuminated pitched visual field. Since elevation of the eye in the head and/or the head in space has essentially no influence on these perceptions, we conclude that the induced changes in perceived elevation with steady gaze result from our having extracted and manipulated the visual signal normally fed back to maintain visual constancy of elevation in the face of head and/or eye movements. fMRI measurements with monocular steady gaze (left eye) record signals for the symmetric line pairs in conjunction with none for the parallel line pairs in a segment of Brodmann 40 of the inferior parietal lobule of the right hemisphere and area 7 in the left hemisphere in all five subjects, suggesting that these areas are involved in processing perceived elevation. The result is duplicated with all subjects so far run with the right eye alone. A segment of area 19 (occipital gyrus) in the left hemisphere is the only area activated in all subjects by the parallel line stimuli in conjunction with no activation by the symmetric line pairs, suggesting that area 19 is involved in processing perception of frontal plane orientation.
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