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Sara Ajina, Christopher Kennard, Geraint Rees, Holly Bridge; Abnormal contrast saturation in V5/MT+ following damage to V1. Journal of Vision 2014;14(10):288. doi: https://doi.org/10.1167/14.10.288.
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© ARVO (1962-2015); The Authors (2016-present)
Residual vision, or blindsight, following damage to V1 appears to be particularly salient when high contrast moving stimuli are presented in the blind field. In healthy cortex V1 shows a steady increase in response with increasing luminance contrast. In comparison, V5/MT+ shows an early saturation effect with contrast, and even very low contrast levels elicit significant activation above baseline. Here we use fMRI to determine the pattern of response to contrast in V5/MT+ when V1 is damaged. Methods: 10 patients with adult-acquired homonymous hemianopia and chronic unilateral V1 damage and 9 age-matched controls were recruited. fMRI responses to drifting achromatic Gabor stimuli (5 or 8 degree diameter) were measured while participants performed a fixation task. Five contrast levels (1%, 5%, 10%, 50%, 100%) were presented to each hemifield (within the scotoma when in the blind field) in 3 scan runs (300s each). In separate 2-AFC psychophysical testing participants indicated whether a stimulus appeared in the first or second time-interval. Results: In the control group, V1 showed a linear increase in BOLD signal change with increasing contrast (R2=0.97). Response in V5/MT+ was best described by a logarithmic curve (R2=0.92), suggesting an early saturation effect. In patients, response in V5/MT+ in the damaged hemisphere showed a positive linear relationship with increasing contrast (R2=0.89), a pattern that correlated significantly with healthy V1 response in the sighted hemifield of patients, and control V1 response (r=0.30, p=0.03; r=0.92, p=0.02). There was no correlation with V5/MT+ response in patients' sighted hemifield, or to V5/MT+ in controls (r=0.17, p=0.24; r=0.78, p=0.12). Conclusions: V1 appears to be required for typical early-saturation responses to contrast in V5/MT+. When V1 is damaged, detection of stimuli within a scotoma improves as contrast increases. fMRI signal change in V5/MT+ in the damaged hemisphere also increases with contrast, according to a linear relationship.
Meeting abstract presented at VSS 2014
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