Abstract
In achiasmia (absence of optic chiasm), the nasal retina is abnormally projected to the ipsilateral hemisphere such that the LGN and early visual areas in one brain hemisphere receive retinal inputs of the full visual field of the ipsilateral eye. This rarely diagnosed condition presents a unique opportunity to study common developmental rules of the visual cortex, which maintains a topological map of retinal inputs. A 23-year-old male with a congenital nystagmus and achiasmia (confirmed with MRI) was evaluated using BOLD fMRI. The participant monocularly viewed visual stimuli, which included a rotating wedge and an expanding ring for retinotopy, an expanding-contracting ring and a stationary ring to map MT/MST, and rapidly presented faces, scenes, objects, and scrambled images to map LOC. Resting-state data was also acquired while the participant closed his eyes but stayed awake. The obtained retinotopy was of excellent quality. Within the retinotopically defined visual areas V1– V3, the participant’s retinotopic representation of the ipsilateral visual field is a mirror image of its contralateral field representation. The two are superimposed on the ipsilateral hemisphere of each eye such that two points symmetrically located across the vertical meridian are mapped to the same point on the cortex, consistent with a recent study (Hoffmann et al., 2011 SfN). With monocular viewing, activations of V1-V3 are limited to the ipsilateral hemisphere relative to the viewing eye. In contrast, activations of MT and LOC are bilateral, as in normally sighted individuals. Moreover, even though evoked activities in V1-V3 are unilateral to the eye of origin, the resting-state activities in these areas showed the normal high correlation (r=0.6-0.9) between the hemispheres. These findings suggest that while the early visual areas develop according to the abnormal topological organization of the retinal projections, later stages of visual processing have "fixed" the topological abnormality.
Meeting abstract presented at VSS 2012