Relating Eye Dominance to Neurochemistry in the Human Visual Cortex Using Ultra High Field 7-Tesla MR Spectroscopy

Abstract

Although our view of the world looks singular, it is combined from each eye’s separate retinal image. If the balanced input between eyes is disrupted during early childhood, visual acuity and stereoscopic depth perception are impaired. This is because one eye dominates over the other, causing a neurological condition called ‘amblyopia’ [1]. In the normal, healthy visual system, the balance between eyes can be determined using various methods to provide a measure of ‘eye dominance’. Eye dominance is the preference for using image from one eye over another [2], suggesting that the visual system applies different weights upon their input. Hence, eye dominance is relevant for understanding the mechanisms underlying binocular vision. As an investigative strategy to understand the binocular visual system in health in disease, we want to characterize eye dominance in the normal visual system. This information can then be used to serve as a baseline to compare to extreme eye dominance in ‘amblyopia’. Specifically, we ask to which degree variations in eye dominance are related to visual cortex concentrations of major excitatory neurotransmitter and metabolite glutamate (‘Glu’) and inhibitory neurotransmitter γ-aminobutyric acid (‘GABA’). Their relationship is formalised as the ‘Glu/GABA’ ratio. 13 participants took part in a 1-h psychophysical experiment to quantify eye dominance and a separate 1.5-h 7-Tesla MRI brain scan to measure hemodynamic and neurochemical responses during visual stimulation. The degree of eye dominance was predicted by the inter-ocular difference in V1 Glu/GABA balance. Stronger eye dominance correlated with an increase in inhibition during dominant relative to non-dominant eye viewing (r = −0.647, p = 0.023). In contrast the hemodynamic response, measured with functional magnetic resonance imaging, did not correlate with eye dominance. Our findings suggest that normally occurring eye dominance is associated with the balance of neurochemicals in the early visual cortex.