Late-Holocene Mass Movements in High Arctic East Lake, Melville Island (Western Canadian Arctic Archipelago)

East Lake, located at Cape Bounty (Melville Island, Canadian High Arctic), was mapped using a high-resolution swath bathymetric sonar and a 12 kHz sub-bottom profiler, allowing for the first time the imaging of widespread occurrence of mass movement deposits (MMDs) in a Canadian High Arctic Lake. Mass movements occurred mostly on steep slopes located away from deltaic sedimentation. The marine to lacustrine transition in the sediment favours the generation of mass movements where the underlying massive mud appears to act as a gliding surface for the overlying varved deposits. Based on acoustic stratigraphy, we have identified at least two distinct events that triggered failures in the lake during the last 2000 years. The synchronicity of multiple failures and their widespread distribution suggest a seismic origin that could be related to the nearby Gustaf-Lougheed Arch seismic zone. Further sedimentological investigations on the MMDs are however required to confirm their age and origin.

ROLE OF THE FRONTAL EYE FIELD, SUPERIOR COLLICULUS, AND BASAL GANGLIA IN FLEXIBLE SACCADE BEHAVIOR

A distributed network of cortical and subcortical brain regions mediates the control of voluntary behavior, but it is unclear how this complex system may flexibly shift between different behavioral events. This thesis describes the neurophysiological changes in several key nuclei across the brain during flexible behavior, using saccadic eye movements in rhesus macaque monkeys. We examined five nuclei critical for saccade initiation and modulation: the frontal eye field (FEF) in the cerebral cortex, the subthalamic nucleus (STN), caudate nucleus (CD), and substantia nigra pars reticulata (SNr) in the basal ganglia (BG), and the superior colliculus (SC) in the midbrain. The first study tested whether a ‘threshold’ theory of how neuronal activity cues saccade initiation is consistent with the flexible control of behavior. The theory suggests there is a fixed level of FEF and SC neuronal activation at which saccades are initiated. Our results provide strong evidence against a fixed saccade threshold in either structure during flexible behavior, and indicate that threshold variability might depend on the level of inhibitory signals applied to the FEF or SC. The next two studies investigated the BG network as a likely candidate to modulate a saccade initiation mechanism, based on strong inhibitory output signals from the BG to the FEF and SC. We investigated the STN and CD (BG input), and the SNr (BG oculomotor output) to examine changes across the BG network. This revealed robust task-contingent shifts in BG signaling (Chapter 3), which uniquely impacted saccade initiation according to behavioral condition (Chapters 3 and 4). The thesis concludes with a published short review of the mechanistic effects of BG deep brain stimulation (Chapter 5), and a general discussion including proof of concept saccade behavioral changes in an MPTP-induced Parkinsonian model (Chapter 6). The studies presented here demonstrate that the conditions for saccade initiation by the FEF and SC vary according to behavioral condition, while simultaneously, large-scale task dependent shifts occur in BG signaling consistent with the observed modulation of FEF and SC activity. Taken together, these describe a mechanistic framework by which the cortico-BG loop may contribute to the flexible control of behavior.