2025 Theses Doctoral
Contributions of the superior colliculus to covert decision termination
Many decisions require us to actively interrogate the world using our senses. Based on what we perceive, we may commit to an immediate course of action or plan for future actions.
The neuroscience of perceptual decision-making examines how the brain gathers sensory information and uses it to guide behavior. A widely accepted model represents this decision process as the gradual accumulation of sensory evidence until a threshold or criterion is reached. For decisions about the direction of random-dot motion (RDM) stimuli, extensive research has described the mechanisms of evidence accumulation in association areas such as the lateral intraparietal area (LIP). Recent studies have also made progress in identifying the biological basis of the decision threshold. For overt decisions marked by an immediate saccadic response, neurons in the superior colliculus (SC) are thought to set this threshold by initiating eye movements when LIP activity is sufficiently elevated. Additionally, reversible inactivation of SC neurons has been shown to prolong evidence accumulation in LIP, suggesting that SC neurons play a causal role in terminating the decision process. However, it remains unknown whether SC neurons also contribute to covert decision termination, where commitment results in a planned, rather than an immediate, eye movement.
To address this gap, I recorded neural activity from two rhesus macaques as they performed a RDM discrimination task, where I varied the length of the stimulus and delay periods from trial to trial. This design required the animals to make a covert commitment to a choice on each trial, which they later reported with a saccade following the delay period. Using high channel-count electrodes, I recorded simultaneously from large populations of neurons in SC, LIP, and the dorsal pulvinar of the thalamus (dPul) unilaterally. In a majority of trials, SC neurons exhibited non-saccadic bursts — bursts of activity not associated with eye movements or specific trial events. The timing of these non-saccadic bursts suggested they might signal the moment of decision termination. Testing this hypothesis, I found that non-saccadic bursts effectively divided each trial into an early deliberation phase, where sensory evidence informed the decision, and a later commitment phase, where further sensory evidence was ignored. Additionally, the timing of non-saccadic bursts in the SC corresponded with the end of evidence accumulation in LIP.
Finally, I identified a population of neurons in dPul that may relay this termination signal from SC to LIP. This study advances our understanding of perceptual decision-making by broadening the function of the SC in decision termination. Beyond its established role in terminating overt decisions with an immediate saccadic report, these findings suggest that the SC also contributes to decision termination following covert commitments, where the saccade occurs after a delay. The results have important implications for systems neuroscience by offering a clear example of serial computations occurring across dedicated cortical and subcortical areas to guide flexible behavior. While the path from sensation to deliberation to commitment is far from fully understood, these results lay the groundwork for future research — both to explore the role of additional nodes in the decision-making network and to investigate the local circuits that give rise to distinct functions within each node.
In Chapter 1, I provide historical context on the neurobiology of perceptual decision-making, introduce the bounded evidence accumulation model, and discuss its application to both overt and covert decisions. I also motivate the present study by introducing the superior colliculus and its known role in the decision-making network. In Chapter 2, I investigate physiological markers of decision commitment in the SC. I describe non-saccadic bursts, which are physiological events that occur on single trials, and demonstrate how these events relate to covert decision termination. In Chapter 3, I examine how non-saccadic bursts in the SC might be involved in terminating the process of evidence accumulation in LIP and show how neurons in dPul might mediate this effect. Chapter 4 presents closing considerations and outlines directions for future research.
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Pensack_columbia_0054D_19007.pdf application/pdf 3.28 MB Download File
More About This Work
- Academic Units
- Neurobiology and Behavior
- Thesis Advisors
- Shadlen, Michael Neil
- Degree
- Ph.D., Columbia University
- Published Here
- January 29, 2025