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Jonathan Zadra, Dennis Proffitt; Hemispheric differences in the perception of hills. Journal of Vision 2007;7(9):424. doi: 10.1167/7.9.424.
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Previous research has demonstrated a distinction between conscious perception and visually guided action in people's responses to hills. Verbal reports (reflecting conscious perception) of the slant of a hill are greatly overestimated, while motoric responses (visually guided action) are far more accurate. Similarly, in the Ebbinghaus Illusion (also known as the Titchener circles illusion), conscious verbal reports of the size of the inner circles indicate that people perceive the circles to be different sizes, however recorded motoric responses to the circles indicate accurate visually guided actions, as the grip aperture is not affected by the illusion (Aglioti, S., DeSouza, J.F., & Goodale, M.A., 1995). Recently, Gonzalez, Ganel, and Goodale (2006) reported that this distinction is not true when participants grasp with their left hand; the left hand is affected by the illusion, indicating that there may be a left hemisphere specialization for visual control of action.
The possibility of a hemispheric specialization for slant perception and visually guided action on slants was investigated in the current study. Participants viewed various slants in a virtual environment. Visual information was restricted to either the left or right visual hemifield on a given trial, and motoric responses were recorded with both the left and right hand. Preliminary results indicate that the response hand is not a significant predictor of motoric slant estimates, however verbal responses are overestimated to a greater extent when the hill is presented to the right visual hemifield than to the left.
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