Abstract
Achiasma is a congenital condition in which the optic chiasm does not develop. Consequently, all retinal efferents from each eye project only to the respective ipsilateral cerebral hemisphere. In the early visual cortex (EVC), the representation for the left and right visual hemifield are "folded" onto each other such that each small patch in EVC has two population receptive fields (pRFs) situated symmetrically across the vertical meridian. Our earlier work has shown that within each small patch of EVC, there reside two sub-populations of neurons, each serving one of the two pRFs. The physical proximity of these sub-populations in cortex may suggest that they have local neural connections, but extensive psychophysical testing has failed to detect any inter-field interactions. Here we used EEG to detect neural interaction between the two sub-populations in EVC. Using frequency-tagged stimuli with different frequencies in the two visual hemifields, we found steady-state visual evoked potentials (SSVEP) at intermodulation frequencies, demonstrating the presence of inter-field neural interactions. However, source localization suggested that the neural loci of these intermodulations were not in EVC (V1-V3). Next, we placed stimuli in either the upper or lower visual field in order to test whether the sources of intermodulation followed the expected pattern of dorsal or ventral EVC, as computed using a cortical model based on the subject's own anatomy. Intermodulation sources did not follow the expected pattern, which again suggests that the locus of inter-field intermodulation is outside of EVC. These findings, along with prior work, support the hypothesis that in achiasma, early visual cortex consists of sub-populations separately representing the two visual hemifields that are physically overlapped but do not interact.
Meeting abstract presented at VSS 2014