A multimodal investigation of visuospatial attention asymmetries in humans

2017-02-24T02:02:13Z (GMT) by Newman, Daniel P.
The degree to which each cerebral hemisphere manages different information processing roles is one of the most enduring questions of brain science. Understanding hemispheric specialisation in the healthy brain aids the prediction of functional outcomes for patients with acquired brain injury, such as unilateral spatial neglect, and may also provide insights into a number of prominent disorders in which hemispheric lateralisation appears to be atypical, such as attention deficit hyperactivity disorder (ADHD). Behavioural measures have shown that healthy subjects tend to exhibit a subtle bias (or ‘asymmetry’) of visual attention favouring left space, termed ‘pseudoneglect’, where visual stimuli in the left hemifield are processed more quickly or accurately than stimuli on the right, and spatial attention is oriented more efficiently towards the left than right. This phenomenon is thought to reflect right hemisphere lateralisation of the networks governing spatial attention. Although a range of indirect evidence also suggests that visuospatial biases can be modulated by non-spatial factors, such as alertness, the underlying physiology for this interaction has remained unclear. This thesis aimed to further understand the biological drivers of visuospatial attentional bias in neurological healthy subjects, examining the impact of state factors such as alertness and trait factors such as genetics on visuospatial bias. The first experimental chapter of this thesis examined the neurophysiological bases of the rightward shift in attention bias that is observed with declining alertness over time-on-task. Posterior α-band EEG - a marker of cortical excitability linked to spatial attention orienting – was recorded from healthy participants who detected transient, unilateral visual targets while also monitoring stimuli at fixation. Asymmetry indices were derived for both lateral target reaction times and hemispheric differences in α-activity before lateral target onsets. Pre-target α became more prominent over the right, relative to left, hemisphere as the task progressed over 48-min, and this change was correlated with a significant rightward shift in spatial bias. Contrary to past studies of posterior α-asymmetry and orienting, here participants did not receive pre-target cues. Thus we show that asymmetries in the hemispheric distribution of anticipatory α are not only apparent during externally-cued attention orienting, but are also sensitive to decreasing alertness over time. These data were the first to link rightward attention drift over time with change in hemispheric activation asymmetry, providing important implications for our understanding of interacting spatial attention and non-spatial alertness networks. The second experimental chapter extends on this research by showing that contrary to the suggestions of other recently published work, the direction and magnitude of attention bias exhibited by a participant early in an attention task is unlikely to represent a stable trait that predicts the direction of their subsequent shift in spatial bias over time-on-task. This chapter concluded that although evidence supports both the contention that trait-like individual differences in spatial bias exist within the healthy population, no clear evidence is yet available for participant/observer subtypes in the direction of time-on-task shift in spatial biases. The third experimental chapter then provides direct evidence for the influence of alertness on spatial attention. This chapter asked if exposure to blue-enriched light (which is known to increase alertness) could activate right-hemisphere attention networks, thus enhancing attention to left space. Our results showed that exposure to higher, relative to lower, intensity blue-enriched light enhanced response-times for stimuli in the left, but not right, visual hemifield. This processing benefit was mediated by a specific effect of light intensity on right-hemisphere parieto-occipital α-power. These behavioural and neurophysiological effects were sustained over task duration (~36 minutes). These data provide evidence for a direct modulatory influence of alertness on the physiological substrates of spatial attention, using a non-invasive, non-pharmacological manipulation of alertness which lasts post light exposure. The final experimental chapter aimed to assess genetic influence on spatial attention bias. Two putatively functional tandem repeat polymorphisms of the dopamine transporter gene (DAT1;SLC6A3) which are argued to influence the level of available synaptic dopamine and confer risk to disorders of inattention, were selected a priori and tested for an association with spatial attention bias. As expected, results suggested that participants without the 10-repeat allele of the DAT1 tandem repeat polymorphism possess an enhanced attentional ability to suppress task-irrelevant stimuli in the left hemifield. The findings of this thesis converge to show that both trait and state factors influence visuospatial attention bias. The exciting finding that right hemisphere activation and visuospatial attention bias can be directly manipulated by exposure to blue enriched light gives rise to the tantalising possibility of light-based interventions for right hemisphere disorders of spatial attention such as unilateral spatial neglect. Future studies will benefit from testing the generalisation of this effect of blue enriched light on spatial attention across different contexts and in different samples.

Awards: Vice-Chancellor's Commendation for Doctoral Thesis in Excellence in 2016.