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Anish Mitra, Jaya Viswanathan, Mathias Abegg, Jason Barton; Line bisection in simulated homonymous hemianopia. Journal of Vision 2010;10(7):535. doi: 10.1167/10.7.535.
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Homonymous hemianopia is a frequent visual field defect after injury of the postchiasmatic visual pathway. It has long been known that hemianopic patients make systematic errors during line bisection, placing bisection markers not in the centre of the line but biased instead toward their visual field defect. The cause for this contralesional bisection error is unknown. Various hypotheses attribute the error to the visual field defect, long-term strategic adaptation to the defect, or, more recently suggested, a consequence of extrastriate brain injury. To determine if bisection error can occur without the contribution of the latter two factors, we studied line bisection in healthy subjects with simulated homonymous hemianopia using a gaze-contingent display paradigm, with different line lengths and the presence or absence of line-end markers. We found that simulated homonymous hemianopia induced a line bisection error towards the simulated hemianopia, that this was associated with a significant bias of fixations toward the blind field, and that the effect was present with all line lengths but accentuated when line-end markers were present. In a second experiment we showed that the eccentric fixation alone, without a simulated hemianopia, is sufficient to produce a similar bisection error, with or without line-end markers. Our results indicate that a homonymous visual field defect alone is sufficient to induce a line bisection error and previously described alterations in fixation distribution, and does not require long-term adaptation or extrastriate pathology.
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