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Yeo Bi Choi, Adam Steel, Thomas Botch, Caroline Robertson; Measuring Transitory Modulation of GABA Levels in the Visual Cortex. Journal of Vision 2020;20(11):598. doi: https://doi.org/10.1167/jov.20.11.598.
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Binocular rivalry is thought to rely on the balance of excitatory and inhibitory neurotransmission in the visual cortex. Using Magnetic Resonance Spectroscopy (MRS), we have previously linked rivalry dynamics to excitatory (glutamate-glutamine; glx) and inhibitory (gamma-aminobutyric-acid; GABA) levels in the visual cortex in a trait-dependent manner (i.e. individuals with higher visual cortex GABA levels show greater perceptual suppression during rivalry; Robertson et al., 2016). Here, we sought to understand this link in a state-dependent manner (i.e. the tie between rivalry and transient GABA level changes) in vivo using functional magnetic resonance spectroscopy (f-MRS).
We acquired visual cortex GABA and Glx concentrations using a MEGA-PRESS sequence at 3T while participants (n=8) viewed alternating blocks (24s each, 6s ISI, 256 spectra total) of rivalry stimuli (orthogonal gratings, one presented to each eye) or fused stimuli (plaids, both presented to each eye). Participants were asked to indicate which stimulus they were seeing using a button press. Individual spectral acquisitions were aligned and averaged by condition (rivalry/harmony).
Contrary to our hypothesis that GABA levels would be higher during rivalry as compared with fusion blocks, we found no significant difference across conditions. None of the between condition differences in the measurement of GABA (p=0.1144), Glx (p=0.3037) and the ratio of the two (p=0.7295) were significantly different from zero (Figure 1).
In sum, we examined the underlying neurometabolic concentration of binocular rivalry fluctuations at a higher temporal resolution using a block design. Current results show no relation between transient GABA levels in the visual cortex and behavioral measure of rivalry. Future studies are needed to better understand the temporal dynamics of functionally-driven changes in neurotransmitter levels using MRS.
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