Measuring binocular interactions and adaptation in the early human visual system

Within the visual system, humans have three chromatic pathways: luminance, L-M (red/green) and S (blue/yellow). These pathways begin as early as the retina, through the LGN and to V1. The primary visual cortex is responsible for a number of functions (in this thesis, we particularly focus on binocular combination and adaptation) which may differ in their process based on the stimulus chromaticity. Research into both of these areas typically relies on the use of achromatic luminance stimuli, even though the same findings may not be generalisable to the chromatic pathways.

In Chapter 2, EEG is used to measure SSVEP suppression in response to binocular stimuli of the different chromatic pathways. The results show that significant levels of binocular combination must be occurring in neurons that carry half-wave rectified signals, as well as neurons that are sensitive to both achromatic and chromatic stimuli. In Chapter 3, psychophysics, SSVEPs and pupillometry are used to measure adaptation both during and after the adaptation period. It was found that all chromatic pathways show no change in SSVEP responses during the adaptation period, but show an increase in response after adaptation. This differs drastically from the psychophysical findings in which perceptual contrast matches decrease after adaptation. These findings are used to propose a gain control model of adaptation in which neurons shift their preferred sensitivity to match the environment. Finally, Chapter 4 investigates phase-dependent binocular combination. It was found that the chromatic pathways show a reduction in fMRI BOLD response when eye inputs are in anti-phase, but there is no difference in the in- and anti-phase conditions in the luminance pathway.

Overall, the work in this thesis provides novel findings and highlights the differences in V1 responses between chromatic pathways, particularly for binocular combination and adaptation.