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Abstract:
An important question is whether the adult visual cortex is able to reorganize in subjects with visual field defects (scotomas) as a result of retinal or cortical lesions. Functional magnetic resonance imaging (fMRI) methods provide a useful tool to study the population receptive field (pRF) properties and assess the capacity of the human visual cortex to reorganize following injury. However, these methods are prone to biases near the boundaries of the scotoma. Retinotopic changes resembling reorganization have been observed in the early visual cortex of normal subjects when the visual stimulus is masked to simulate retinal or cortical scotomas. It is not known how the receptive fields of higher visual areas, like hV5/MT+, are affected by partial stimulus deprivation. Here, we measured responses in human area V5/MT+ in five healthy subjects under two stimulation conditions. FMRI measurements were obtained under the presentation of a moving bar stimulus spanning the entire visual field while the subjects were fixating. In a second session the stimulus was masked in the left upper quadrant of the visual field to simulate a quadrantanopic scotoma (“artificial scotoma” or AS) occurring often as a result of partial V1 or optic radiation lesions. PRF estimates were obtained using a recent method of pRF topography estimation (Lee et al., A new method for estimating population receptive field topography in visual cortex, NeuroImage, 2013) which is consistent with other pRF methods. Responses obtained under the AS condition were compared with simulations obtained from a linear AS model (or LAS model). The LAS model provides an estimation of the pRF changes expected to occur as a result of the truncated stimulus assuming that the pRF linearly integrates the AS. We found that pRFs in hV5/MT+ are nonlinearly affected by the truncated stimulus presented: pRF centers shifted towards the border of the AS, pRF size decreased and pRF amplitude increased near the AS border. In addition, using the full bar stimulus to estimate the pRF topography (when in fact the stimulus presented included the AS) produced erroneous pRF estimates inside the region of the artificial scotoma. These biases are not the result of a trivial methodological artifact but appear to originate partly from asymmetric BOLD responses occurring when the stimulus moves from seeing to non-seeing locations of the visual field. Distinguishing between pRF changes that occur as the result of true reorganization versus different test-stimulus presentation conditions is an important task that needs to be undertaken when studying visual cortex organization in patients with visual field deficits.
