Binocular combination of light in canonical and non-canonical pathways

Humans possess two forward-facing eyes allowing combination of individual images to form a cyclopean representation of the world. Image quality is determined by the amount of light hitting the retina, regulated by the pupils. The anatomical pathways behind binocular combination and that regulate pupil size are well understood. Many studies have investigated how the brain combines information across the eyes: for spatial patterns, visual areas implement ‘ocularity invariance’, which equalises monocular and binocular responses. However, not much is known about the computational processes that regulate binocular combination of the pupil response. This was the main aim of this thesis: to investigate and understand the rules of binocular combination in the cortical pathways (the visual cortex) and the subcortical pathways that govern the pupil diameter.

In Chapter 2, a novel combination of EEG, pupillometry, psychophysics, and computational modelling was used to investigate if this invariance is implemented for luminance flickering stimuli. The results show violations of ocularity invariance in the cortex, with strong facilitation of binocular signals, and in the subcortical pathways governing pupil diameter, where non-linear combination was observed. In Chapter 3, these findings were expanded using fMRI to investigate where, within the visual cortex, this facilitation happens. Facilitation was observed mainly in V1 but not as strong as for EEG (likely due to the different nature of EEG and fMRI recordings). Finally, in Chapter 4, silent substitution and pupillometry were used to investigate binocular combination while targeting the intrinsically-photoreceptive retinal ganglion cells, the L-M and the S-(L+M) pathways. The results showed non-linear combination in all pathways but with differences in interocular suppression levels.

The studies undertaken in this thesis contribute to the wider understanding of binocular combination of light signals and provide methodological developments (simultaneous EEG and pupillometry) that can benefit researchers interested in binocular combination disorders (e.g. amblyopia).