The Contribution of the Vestibular System to Temporal Attention

The vestibular end-organ is phylogenetically old and placed deep inside the inner ear, detecting head movement and gravitational pull. The multiplicity of anatomical projections of the vestibular system, reaching cortical and subcortical brain regions, has proven to influence a wide range of higher-level functional relationships, including working memory (WM) and attention. The literature has established an association between the vestibular system and spatial attention, but little or no research has investigated the vestibular system's contribution to temporal attention in either normative or clinical groups. To this end, this thesis explores whether the periodicity of vestibular information can be used by visual and auditory temporal attention and thereon be utilised therapeutically. Chapter 2 examined whether visual and auditory attentional responses were enhanced if the presentation of visual and auditory stimuli coincided with the rhythmic presentation of vestibular pulses induced by Galvanic Vestibular Stimulation (GVS). Unexpectedly, synchronous vestibular signals were shown to inhibit rather than enhance visual response, more so when the vestibular waveform was sinusoidal (AC) than a square wave (DC) pulse. However, enhancement was observed when visual stimuli were replaced with auditory stimuli, although it did not matter if the Onset of the auditory stimuli was synchronised with that of the background vestibular stimuli. Chapter 3 moved on to show that the auditory enhancement was boosted when using an AC bilateral bipolar or AC bilateral bipolar with positive Offset instead of a DC bilateral bipolar signal. A group of participants with Specific Learning Difficulties (SpLDs), such as attention deficit hyperactivity disorder (ADHD), and dyslexia were recruited to investigate the therapeutic importance of any such improvement; however, the results were inconclusive. This chapter also showed that the visual inhibition seen in Chapter 2 was the product of the viewing conditions, which did not favour multisensory facilitation. Finally, Chapter 4 utilised a Rapid Serial Visual Presentation (RSVP) paradigm to assess the time course and characteristics of the visual interference effect observed in Chapters 2 and 3. The experiments presented in this chapter found modest evidence of facilitation during GVS but did not cast light on the time course of the interference effect shown in Chapters 2 and 3. Together these thesis findings show that visual and auditory discrimination processes are sensitive to the temporal and morphological characteristics of the vestibular signal. However, this sensitivity differs across the visual and auditory senses and is constrained by task and the multisensory integration principles seen in other non-vestibular types of cross-modal interaction. The findings give further reason to understand how the brain exploits the high temporal fidelity of the vestibular system but do not yet support allied investigations into neuro-rehabilitative potential.