Temporal orienting in the human brain: neural mechanisms of control and modulation

The main aim of the experiments reported in this thesis was to explore the neural mechanisms underlying the temporal orienting of attention. In Chapter 3, I explored the possible dissociation between exogenous and endogenous temporal orienting by comparing reaction times to targets appearing after rhythmic or symbolic cues. Behavioural results provided evidence for the existence of dissociable exogenous and endogenous types of temporal orienting of attention. The experiment in Chapter 4 combined spatiotemporal expectations using rhythmic moving cues to test the modulatory effect of exogenous temporal orienting in the brain. Specifically, I used EEG to test the effect of temporal orienting on perceptual and motor stages of target analysis, as well as on anticipatory oscillatory brain activity. The time-frequency analysis revealed that rhythmic cues can entrain slow brains oscillations, providing a putative mechanism for enhancing the perceptual processing of expected events. Spatiotemporal expectations also modulated the amplitude of visual responses and the timing and amount of preparatory motor activity. In Chapter 5, I used a novel task to explore the neural modulatory effects of spatial and temporal expectations acting in isolation or in coordination. For the first time, the analysis of early visual responses demonstrated that temporal expectations alone, independently of spatial orienting, can enhance early visual perceptual processes. The time-frequency analysis in this experiment showed a desynchronisation of alpha oscillations focused over central-parietal electrodes induced by rhythmic cues that were independent of spatial expectations. When rhythmic cues carried spatiotemporal information, the alpha desynchronisation also spread over contralateral occipital electrodes. In Chapter 6, fMRI was used to test the possible neural dissociation between motor and temporal orienting. The results confirmed the large overlap between these two processes, but also indicated independent behavioural and neural effects of temporal orienting. Temporal orienting activated the left IPS across motor conditions, further implicating the left IPS in temporal orienting. Based on the results of these experiments, directions for future studies are discussed.