Abstract
People with psychosis experience abnormal visual percepts (e.g., hallucinations), and have shown reduced sensitivity to visual contrast. Although the neural basis of psychotic disorders such as schizophrenia remains unknown, a prominent theory suggests impaired GABAergic inhibitory functioning. We investigated the role of inhibition in the visual system of people with psychosis using ultra-short echo time (8 ms) STEAM MR spectroscopy at 7 tesla, and a visual contrast discrimination task performed outside the scanner. Data were acquired from 41 participants with psychosis, 25 unaffected biological relatives of participants with psychosis, and 33 healthy controls as part of the Psychosis Human Connectome Project. Participants with psychosis showed impaired discrimination across all pedestal contrasts, but we saw no difference in mid-occipital GABA levels between groups. Across all participants, we found that higher GABA levels were correlated with reduced visual contrast discrimination (higher thresholds). In particular, we found that GABA levels correlated with thresholds for low (< 2%) but not higher (≥ 5%) contrast stimuli. Our findings suggest that higher resting GABA levels in visual cortex may inhibit perception of visual stimuli based on luminance contrast in both people with and without psychosis. Previous observations of lower occipital GABA levels among people with psychosis might be explained by differences in either transverse relaxation time (T2) and / or macromolecular content between groups, as we sought to control for these effects in our MRS results.