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Abstract

Macular degeneration (MD) is a common cause of human visual impairment. Typically MD deprives the foveal part of the primary visual cortex from retinal input. It has been reported that visual areas undergo extensive plastic reorganization in response to such deprivation (Baker et al., J. Neurosci. 2005), but this question remains not conclusively settled.
We used 4.7 Tesla functional magnetic resonance imaging (fMRI) to study the visual cortex of an adult macaque monkey with binocular central retinal lesions due to a form of juvenile MD. FMRI experiments were performed under light remifentanyl induced anesthesia. Standard moving horizontal/vertical bar stimuli were presented to the subject and the population receptive field (RF) method (Dumoulin and Wandell, Neuroimage 2008) was used to measure retinotopic maps and population receptive field sizes in early visual areas. RF size was plotted as a function of eccentricity in early visual areas. As expected, population based RFs increase in size as a function of eccentricity within each visual area, and as we move from lower to higher visual areas at a fixed eccentricity. In general, there is good agreement between maps obtained by fMRI and previous results obtained by anatomical and physiological methods.
The pattern of activity elicited in the MD monkey was compared to the pattern of activity elicited in two control monkeys. The primary visual cortex of the MD animal shows an extensive area devoid of BOLD (blood oxygen level dependent) activity that includes the fovea and roughly corresponds to the expected size of the retinal lesion projection zone (LPZ). Visually driven activity starts beyond the border of the calcarine sulcus, at approximately 9 degrees eccentricity a distance of ~ 36 mm from foveal V1 in this animal. RF size maps derived from non-deafferented cortex abutting the retinal LPZ were comparable to RF size maps derived from the corresponding area in control subjects. Further investigation using fMRI and standard electrophysiology methods is in progress.