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Pascal Mamassian; Locating the functional vertical midline with a motion probe. Journal of Vision 2010;10(7):1379. doi: 10.1167/10.7.1379.
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© ARVO (1962-2015); The Authors (2016-present)
The vertical midline splits the visual fields into two halves that are represented in contralateral hemispheres. While space is retinotopically encoded across most visual areas within each hemisphere, the vertical division between hemifields necessarily disrupts this topological organization. We are interested here in measuring the functional consequences of the vertical split. In particular, we investigate how crossing the vertical midline impairs motion sensitivity.
Observers were engaged in a motion speed change detection that occurred midway along the trajectory of a rotating dot. Two dots diametrically opposed on a virtual circle travelled each a quarter of the circle. Only one of the dots changed speed for a brief duration and observers had to report which dot presented the speed change (the one above or below fixation). The speed change could occur just before or just after the dot crossed the vertical midline. Viewing was monocular.
Observers were significantly worse in detecting the speed change when it occurred after crossing the midline than before. In addition, the range over which motion sensitivity was impaired increased with the speed of the stimulus.
On the theoretical side, the loss of motion sensitivity after the vertical midline possibly reflects an impairment to predict the future location of the moving dot, or an impairment to communicate this prediction across hemispheres. On the practical side, this phenomenon is useful to estimate the location of the functional midline and to determine the extent to which the area around the vertical midline is represented in both hemispheres.
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