The cortical mechanisms underlying human spatial navigation: from afferent processing to motor efference

Abstract

The research described in this thesis examined the cortical mechanisms underlying human spatial navigation, both from afferent and efferent perspectives, using healthy individuals and patients with focal cortical lesions. Specifically, the role of the posterior parietal cortex in the neural integration of vestibular signals for self-location perception was examined in a series of behavioural experiments (Chapter 2) assessing position, motion, and timing perception during angular rotations in the dark. Hippocampal cells have been associated with spatial navigation involving allocentric (map-based) co-ordinates. A combination of landmark-based orientation and vestibular path integration paradigm was used to evaluate the role of the right hippocampus in vestibular working memory related to allo-, and ego-centric navigation (Chapter 3). A new hypothesis relating to theta oscillation synchronisation for binding spatial (and non-spatial) conjunctions is proposed. Vestibular-guided self-motion relies upon visual calibration, for sighted individuals. Blind individuals, however, are said to possess a "supersense" as a result of cross-modal plasticity. Blind versus sighted performance was compared using a vestibular-guided auditory localisation task in footballers (Chapter 4), and both in turn compared to sighted non-footballers. This enables a fair assessment of the relative contribution of practice versus innate "supersense" for spatial performance. Gait is a necessary component of spatial navigation in humans, and the ability to adapt locomotor behaviour to a change in the environment is essential for everyday activity. Using the "broken escalator" paradigm, the role of the primary motor cortex and premotor regions for gait adaptation was assessed in healthy participants, using non-invasive transcranial direct current stimulation (tDCS; Chapter 5). This technique was then applied in combination with physical therapy to patients with Parkinson‟s disease (Chapter 6) and patients with small vessel disease (Chapter 7) – a common cause of gait dysfunction in the elderly –to improve gait and balance function. The complementary study in a single patient with Parkinson‟s disease combining tDCS with tango dancing examined whether non-invasive brain stimulation could preferentially augment the effects of this form of physical therapy in a clinically-relevant manner (Appendix).