UC Berkeley

Anatomical, physiological, and psychophysical approaches have revealed a great deal about the structure and function of the visual system. Non-invasive techniques based on nuclear magnetic resonance supplement these approaches by allowing the possibility of recording from the awake, behaving human brain. Here, we present two experiments using different non-invasive techniques to answer questions about the neural basis of human vision. In Chapter 2, we examine the relationship between γ-aminobutyric acid (GABA), perceptual suppression, and amblyopia (“lazy eye”). In amblyopia, abnormal visual experience during development leads to an enduring loss of visual acuity in adulthood. Physiological studies in animal models suggest that intracortical GABAergic inhibition may mediate visual deficits in amblyopia. To better understand the relationship between visual cortical GABA and perceptual suppression in persons with amblyopia (PWA), we employed magnetic resonance spectroscopy (MRS) to quantify GABA levels in both PWA and normally-sighted persons (NSP). In the same individuals, we obtained psychophysical measures of perceptual suppression for a variety of ocular configurations. In PWA, we found a robust negative correlation between the depth of amblyopia (the difference in visual acuity between the amblyopic and non-amblyopic eyes) and GABA concentration that was specific to visual cortex and was not observed in a sensorimotor cortical control region. Moreover, lower levels of visual cortical GABA were associated with weaker perceptual suppression of the fellow eye by the amblyopic eye and stronger suppression of the amblyopic eye by the fellow eye. Taken together, our findings provide evidence that intracortical GABAergic inhibition is an important component of the pathology of human amblyopia and suggest possible therapeutic interventions to restore vision in the amblyopic eye through enhancement of visual cortical GABAergic signaling in PWA. In Chapter 3, we use functional magnetic resonance imaging (fMRI) to identify cortical and subcortical components of the visual system. In particular, we localized many topographically-organized visual cortical areas as well as the lateral geniculate nucleus (LGN) and the magnocellular and parvocellular subdivisions thereof. We report the extension of this M/P localization procedure to fMRI data acquired with multiband acceleration. We also examined thalamocortical coupling between the LGN subdivisions and visual cortical areas but found no effects of stimulus type or spatial attention.