Watanabe, K., Lauwereyns, J., & Hikosaka, O. (2003). Neural correlates of rewarded and unrewarded eye movements in the primate caudate nucleus. Journal of Neuroscience, 23(31), 10052-10057.
Goal: To investigate how neurons in the caudate nucleus differentially respond to rewarded and unrewarded saccadic eye movements
Methods: Two adult monkeys played the biased saccade task (BST) while undergoing extracellular neuronal recordings of GABAergic medium-spiny projection neurons in the caudate nucleus. In the BST, the monkeys had to saccade toward a red spot either left or right of fixation on the screen. Auditory feedback was provided and erroneous/incomplete trials were repeated. During blocks of stimuli, one saccade direction was rewarded but saccades to both directions were enforced.
Results:
- They found a subset of neurons in the caudate nucleus increased their activity around the saccade timing. Most of these neurons showed an interaction effect between saccade direction and reward expectation. Of this further specified subset of neurons, most responded selectively to contraversive saccades.
- Average saccade latency was significantly shorter for rewarded compared to unrewarded saccade trials.
- The authors differentiate between rewarded-saccade neurons and unrewarded-saccade neurons by their sensitivity to change in spiking activity for each condition. Increased activity in the rewarded-saccade neurons was related to earlier saccade responses to rewarded contralateral stimuli. Increased activity in the unrewarded-saccade neurons was related to earlier saccade responses to unrewarded contralateral stimuli.
Discussion: This study showed that reward expectation modulates neural activity in the caudate nucleus, and this activity is related to task performance (i.e., saccade latency). The authors suggest that the functionality of the unrewarded-saccade neurons may lie in a motivational-conflict interaction state where the caudate nucleus as a whole is differentially responding to short term and longer term motivational goals by adjusting motivational weights for behavioral responses.
Thoughts:
- I have no interest in conducting primate work myself. Yikes.
- Is a drop of water really a motivator? If so, how humane is that? Are there other motivating rewards that could be used? Perhaps the strength and valence of the motivator would influence neuronal activity in the caudate nucleus.
- If we think of cognitive control in the fashion that Botvinick and Braver conceptualized in their 2015 review, it is conceivable that the caudate nucleus would show activity for any goal directed behavior. In that light, this paper doesn’t seem especially illuminating. However, it is foundational.
- This paper highlights to me that systems of functional units can be broken down into even more fine-grained communities than region (i.e., caudate nucleus).
- I am interested in the long-range down-stream connections of these GABAergic projection neurons. I will look for literature that has done more mapping of frontostriatal circuitry. Perhaps someone has recorded from neurons across multiple locations in order to elucidate functional interactions.