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Linda Lanyon, Jason Barton; Why the contralesional hemifield is scanned by patients with hemianopia but not with hemineglect: computational modeling of mechanisms of neural compensation. Journal of Vision 2010;10(7):279. doi: 10.1167/10.7.279.
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© ARVO (1962-2015); The Authors (2016-present)
Hemianopia patients have a contralesional visual hemifield deficit yet, during visual search, direct eye movements toward and explore their blind side. In fact, during line bisection tasks, eye movements are guided preferentially to the contralesional blind hemifield and there is a line bisection bias toward this hemifield. In contrast, scan paths from hemineglect patients typically ignore the contralesional hemifield during both line bisection and visual search, and these subjects show an ipsilesional bisection bias. What strategies do hemianopia patients have or develop that compensate for the lack of visual information in their blind hemifield and why is such a compensatory process not accessible in visual neglect? We used a neurophysiology-based computational model to examine possible neural compensatory processes implemented in hemianopia and why these are ineffective in hemineglect following parietal lesions. We propose two different compensation mechanisms that could be employed during hemianopic adaptation to facilitate scanning eye movements towards objects they cannot see in their blind fields. First, a spatial compensatory bias can facilitate search scanning in a complex scene and allows locations in the blind field to attract attention and be fixated. Second, a strategy based on Gestalt grouping, which we implement through extrastriate lateral interactions, permits accurate placement of fixations when viewing the portion of a continuous object that falls into the blind field, such as a horizontal line. We show that, while these compensatory mechanisms facilitate attentional scanning in the blind hemifield in hemianopia, these same mechanisms are ineffective in hemineglect following parietal lesion. We conclude that this type of neurobiologically realistic computational modeling can suggest plausible neural mechanisms of compensation in hemianopia, which can be tested empirically, and which may have some use in guiding rehabilitation strategies.
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