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Martijn Barendregt, Serge Dumoulin, Bas Rokers; The neural basis of stereomotion scotomas. Journal of Vision 2016;16(12):438. doi: https://doi.org/10.1167/16.12.438.
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© ARVO (1962-2015); The Authors (2016-present)
Many observers experience difficulty discriminating the direction of binocular motion through depth in part of their visual field. These regions of stereomotion blindness have traditionally been termed stereomotion scotomas. While we have previously shown that these scotomas are not the result of any monocular deficit, the underlying cause remains unknown. Here we investigate two potential neural impairments that could underlie this deficit: failures in processing changes in binocular disparity (CD) and interocular velocity differences (IOVD). We assessed the sensitivity to stereomotion across the visual field in observers with a stereomotion scotoma. Subsequently, we measured the sensitivity across the visual field to each of the two neural mechanisms, CD and IOVD, individually. Lastly, we measured BOLD responses using ultra-high field MRI (7T) elicited by viewing stereomotion stimuli presented in different locations in the observer's visual field. We find that at the spatial scale at which these visual impairments occur, the processing of binocular disparity is unimpaired, but the changes in binocular disparity contribute little to the perception of motion through depth.Sensitivity to IOVD, on the other hand, varies systematically with locations of stereomotion scotomas in the visual field. Further, we find that several early visual areas show a difference in the BOLD response to stereomotion stimuli when comparing responses within and outside the stereomotion scotoma. Our results show that stereomotion scotomas are the result of a failure to combine velocity signals from the two eyes. Further, we find that the inability to discriminate direction of motion through depth is reflected in the responses in early visual cortex. Since this visual deficit is the result of a cortical impairment and basic visual functions are intact, we conclude that the inability to discriminate the direction of motion in depth, should be considered a novel type of visual agnosia, namely a motion agnosia.
Meeting abstract presented at VSS 2016
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