Influence of visual cortical GABA concentration on perceptual suppression and binocular summation in amblyopia

摘要

Amblyopia is characterized by reduced visual acuity due to abnormal early visual experience and is often accompanied by suppression of signals from the amblyopic eye. Recent studies indicate a role for intracortical GABAergic inhibition in amblyopic suppression as well as a link between occipital GABA levels and fMRI visual cortical response amplitudes in healthy individuals. To better understand the relationships among behavioral suppression, GABA, and visual responses, we measured several types of perceptual suppression, visual cortical GABA levels with magnetic resonance spectroscopy (MRS), and fMRI response amplitudes in amblyopes and healthy controls. Specifically, we obtained the magnitude of surround suppression, overlay suppression, and interocular suppression for each participant using well-established psychophysical approaches. Additionally, we recorded fMRI activity in retinotopically-defined early visual cortical areas for four stimulus conditions: binocular, dichoptic (different stimuli in both eyes), and monocular stimulation of either the amblyopicon-dominant or non-amblyopic/dominant eye. Finally, MRS was performed without visual stimulation to measure resting GABA concentration in bilateral occipital cortex. As expected, we found increased surround suppression in amblyopes when the target was presented to the amblyopic eye and the surround to the non-amblyopic eye, relative to the reverse stimulus configuration and to interocular surround suppression in controls. We quantified binocular summation of fMRI responses by computing the difference between binocular versus monocular and between binocular versus dichoptic response amplitudes. Amblyopes exhibited less fMRI binocular summation than controls for both metrics, and these measures of binocular summation were inversely correlated with visual cortical GABA concentration. Surprisingly, especially given our findings of increased perceptual suppression in amblyopia, visual cortical GABA levels were significantly lower in amblyopes than controls. Ongoing analyses are focused on correlating behavioral metrics of interocular interactions and perceptual suppression obtained from psychophysical data with visual cortical GABA levels and with fMRI response amplitudes in visual cortex.