Triple visual hemifield maps in optic chiasm hypoplasia

In humans, each hemisphere comprises an overlay of two visuotopic maps of the contralateral visual field, one from each eye. Is the capacity of the visual cortex limited to these two maps or are plastic mechanisms available to host more maps? Using an integrative approach of submillimeter fMRI, diffusion-weighted imaging and population receptive field mapping, we found three hemiretinal inputs to converge onto the left hemisphere in a rare individual with chiasma hypoplasia. This generates extremely atypical responses in striate and extrastriate cortices, specifically an overlay of three hemifield representations. Unexpectedly, the effects of this large abnormality on visual function in daily life are not easily detected. We conclude that developmental plasticity including the re-wiring of local intra- and cortico-cortical connections is pivotal to support the coexistence and functioning of three hemifield maps within one hemisphere.