Occipital GABA levels in older adults and their relationship to visual perceptual suppression

Kabilan Pitchaimuthu,Allison M. McKendrick,Bao N. Nguyen,Gary F. Egan,Sinyeob Ahn,Qi-zhu Wu,Olivia Carter

doi:10.1038/s41598-017-14577-5

Abstract

Several studies have attributed certain visual perceptual alterations in older adults to a likely decrease in GABA (Gamma Aminobutyric Acid) concentration in visual cortex, an assumption based on findings in aged non-human primates. However, to our knowledge, there is no direct evidence for an age-related decrease in GABA concentration in human visual cortex. Here, we estimated visual cortical GABA levels and Glx (combined estimate of glutamate and glutamine) levels using magnetic resonance spectroscopy. We also measured performance for two visual tasks that are hypothesised to be mediated, at least in part, by GABAergic inhibition: spatial suppression of motion and binocular rivalry. Our results show increased visual cortical GABA levels, and reduced Glx levels, in older adults. Perceptual performance differed between younger and older groups for both tasks. When subjects of all ages were combined, visual cortical GABA levels but not Glx levels correlated with perceptual performance. No relationship was found between perception and GABA levels in dorsolateral prefrontal cortex. Perceptual measures and GABA were not correlated when either age group was considered separately. Our results challenge current assumptions regarding neurobiological changes that occur within the aging human visual cortex and their association with certain age-related changes in visual perception.

Highlights

GABAergic inhibition is central to the output and activity of visual neural circuitry[1,2,3,4]
Using Magnetic resonance spectroscopy (MRS), we demonstrate elevated levels of GABA+/water in the visual cortex of older adults
We show associations between visual cortical GABA+/water levels and performance on two visual tasks that are hypothesised to, at least in part, be mediated by GABAergic inhibition

Summary

Introduction

GABAergic inhibition is central to the output and activity of visual neural circuitry[1,2,3,4]. At a perceptual level in humans, a simple model of reduced GABA levels in visual cortex does not readily explain observations of unaltered levels of orientation discrimination[14,15], increased centre surround contrast suppression[16,17,18,19], and prolonged binocular rivalry percept durations[20,21] in older adults compared to younger adults. Some of these findings are more consistent with increased intracortical inhibition[16,17,18,22]. We discuss our findings in the context of models of visual inhibition of relevance to the specific perceptual tasks studied, and in the context of possible changes to cellular GABA and GABAergic neurotransmission that cannot be directly measured by MRS

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