In collaboration with Jong Yoon, Richard Maddock, and Cameron Carter, we have studied visual processing in schizophrenia (see Yoon et al., 2013 for a review). Patients with schizophrenia have diminished surround suppression, a specific form of contextual modulation of visual perception in which the presence of a surround stimulus decreases perceived contrast of the central surrounded region. Reduced surround suppression allows patients with schizophrenia to perform visual perception tasks in the presence of a surround as well as or even better than healthy controls (Yoon et al., 2009). We have found that the magnitude of surround suppression correlates with the concentration of the inhibitory neurotransmitter GABA in early visual cortex, as measured with magnetic resonance spectroscopy (Yoon et al., 2010). In addition, patients with schizophrenia have reduced levels of GABA in visual cortex relative to control subjects. We have also found that patients with schizophrenia show broader tuning of a perceptual measure of tuning for stimulus orientation, a type of selectivity that has been associated with GABAergic transmission in early visual cortex (Rokem et al., 2011).
We have also employed magnetic resonance spectroscopy to quantify visual cortical GABA levels in individuals with amblyopia (adults with visual impairment that is caused by abnormal visual experience in childhood) (Mukerji et al., 2022). We found that higher visual cortical levels of GABA are associated with more severe amblyopia. Also, lower GABA levels in visual cortex were associated with weaker perceptual suppression of the non-amblyopic eye by the amblyopic eye and stronger suppression of the amblyopic eye by the non-amblyopic eye. These results suggest the possibility of therapeutic interventions to restore vision in the amblyopic eye through enhancement of signaling of GABA in visual cortex of individuals with amblyopia.