Characterisation of theta oscillations in the primate primary visual cortex

Abstract

Oscillatory neural activities are a prominent aspect of brain function. Oscillations of different frequencies and in different brain areas have been proposed to play important roles in brain function and behaviour. In the visual system, there is an emerging view that theta oscillations (3 - 8 Hz), modulate visual processing, possibly resulting in rhythmic sampling of visual information. This sampling can manifest in the rhythm of behavioural performance. Recent investigations point to activities in higher-level cortical areas as the neural correlates of rhythmic sampling. However, several lines of evidence suggest that the correlates may already be found in sensory areas, including the primary visual cortex (V1). The aim of this thesis is to probe for theta oscillations in V1, characterise their stimulus dependencies, and identify the link to behaviour. In the first chapter, I introduced the current state of knowledge on rhythmic visual processing, embedded in what we know about brain oscillations and visual processing pathways. In the second chapter, I describe how stimulus-induced, neuronal theta oscillations in V1, measured as multi-unit activity (MUA), were affected by low level stimulus properties. More specifically, I found that stimulus size, contrast, and orientation were critical for the emergence of theta oscillations. I also showed evidence of a correlation between V1 neural theta and theta rhythmic visual detection fluctuations. In the third chapter, I further describe theta oscillation in V1 with three main findings. First, theta in the pre-stimulus activity influenced stimulus-induced theta. Second, there was weak evidence that stimulus repetitions decreased theta power. Third, theta also existed at the level of local field potential (LFP), a measure that is tightly correlated with non-invasive EEG measures. In the fourth chapter, I discuss my findings in the context of existing knowledge. Taken together, this thesis demonstrates that neuronal theta oscillations in primate V1 depend on the fundamental visual stimulus properties and the possible relevance of these theta oscillations on rhythmic sampling of visual information.