Assessing the contribution of a subcortical circuit for express visuomotor responses

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Vision is the main sense humans use to identify objects in the external environment. Many everyday actions require a transformation of visual information into the appropriate patterns of muscle activity. Such visual-to-motor transformations can occur remarkably fast: express saccades or the earliest phase of neck or limb muscle recruitment can occur within 100 ms of visual target onset. My research program seeks to understand how the brain generates such rapid responses. We hypothesize that express responses, be they generated at the eye, head, or limb, are initiated by a common tecto-reticulo-spinal pathway originating in the superior colliculus (SC). To test this, we propose complementary behavioural and neurophysiological experiments in humans and non-human primates (NHPs). In Project 1, we will measure eye movements and the activity of neck and upper limb muscles while humans perform visually-guided reaches in a novel emerging target task developed during the past Discovery Grant. In this task, subjects try to catch a moving target that emerges from behind a barrier. Express responses are produced on upper limb muscles in virtually every subject in this task, allowing us to assess subject-by-subject and trial-by-trial correlations of express responses across the eyes, head, and limb. We will also test whether startling acoustic stimuli, which engender short-latency responses via the reticulospinal system, influence the timing and/or magnitude of express responses. Finally, since the reticulospinal system projects bilaterally, we will examine whether express responses in the upper limb are expressed bilaterally in a modified task where subjects choose which arm to use to catch the emerging target. The work in Project 1 can only indirectly test underlying neural mechanisms. In Project 2, we will examine the neurophysiology of express responses in NHPs performing the emerging target task, recording eye, neck, and limb responses as in humans. We will then record from the SC to assess signal timing after target emergence and relative to express responses at the eyes, head and limb. Finally, we will causally test the prediction that inactivation of SC should commonly modify express responses of the eyes, head, and limb. It has long been appreciated that a hierarchy of neural circuits contributes to voluntary visually-guided actions, with spinal cord and brainstem circuits acting in concert with cortical circuits to ensure contextually-appropriate actions. By leveraging a novel behavioural task that we developed to reliably evoke express responses, we can now test the contribution of a largely unappreciated role for a phylogenetically-conserved tecto-reticulo-spinal circuit in express responses. This work will ultimately contribute to a better understanding of how cortical and subcortical circuits work together in a dynamic and uncertain world to generate contextually-appropriate actions when time is of the essence.