The role of the FEFsem in smooth pursuit: Insights from varying retinal input
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Primates are exceptionally adept at tracking moving objects with their eyes, providing high-acuity vision throughout the duration of tracking. This behavior, called smooth pursuit, relies on incoming visual signals being successfully transformed into eye movement commands. Though a seemingly simple process, in practice this requires the processing of retinal signals, integrating them with extraretinal signals like memory, prior experience, prediction, motivation, and attention, among others, and weighting these various factors appropriately. One area known to be vital to the execution of volitional smooth pursuit is the smooth eye movement subregion of the Frontal Eye Field (FEFsem). I sought to advance our understanding of the role of the FEFsem in smooth pursuit by varying the retinal stimuli during pursuit, and estimating the relative contributions of retinal and extraretinal signals to neuronal activity. In chapter 1, I review what is known about smooth pursuit behavior with an emphasis on the way different signals impact the behavior. I also review the signal transformations at different nodes in the pursuit pathway, focusing on those areas known to be most closely linked to the FEFsem. In chapter 2, I manipulate the retinal input during pursuit by temporarily extinguishing the target. This affords me the opportunity to assess the relative contributions of retinal and extraretinal signals to both the behavioral and neuronal response. I found that both the smooth pursuit system as a whole, as well as individual FEFsem neurons exhibit changing reliance on retinal input throughout pursuit. The degree to which the weights of these components change and the type of response observed allows me to discuss their possible functions. In chapter 3, I use a large-field, textured background moving concurrently with a small target spot to investigate the behavioral and FEFsem neuronal response to a combined volitional and reflexive pursuit task. I found that the addition of the large-field motion increased the gain of pursuit eye movements, and decreased the neuronal activity level in the FEFsem, on average, consistent with the hypothesis that a different cortico-ponto-cerebellar pathway underlies reflexive ocular following compared to volitional pursuit. Additionally, I found that the response of individual FEFsem neurons to the addition of the background motion was positively correlated with the response to the target blink, suggesting that these neurons may respond to “disruptions” in pursuit similarly, irrespective of whether the disruption consists of the addition or subtraction of retinal input. Finally, in chapter 4, I summarize the findings, discuss their relevance to current models of pursuit, and suggest future directions to address remaining questions as to the mechanisms underlying volitional smooth pursuit eye movements.
- Neuroscience