The head and tail of caudate nucleus code flexible and stable reward value of visual objects
Being able to estimate the reward value of visual objects is a crucial factor in guiding ones behavioral choices. What makes this task even more challenging is that the reward value is often not fixed but can vary quickly, making it necessary for there to be flexibility to take into account the immediate reward history in addition to the stable reward value that has been learned over the longer term. The underlying neural mechanisms have remained a topic of speculation. Existence of two parallel reward-value processing mechanisms, one processing flexibly short-term reward value and another holding the longer-term stable reward value of objects has been hypothesized, however, empirical support for this hypothesis has been lacking.
In their recent study, Drs. Kim and Hikosaka (2013) used combined single-neuron recording and temporal inactivation methods in non-human primates to investigate the roles of distinct caudate nucleus areas in determination of flexible and stable reward values of visual stimuli. They observed that, during behavioral tasks wherein monkeys looked more at objects with high than low value, neuronal firing recorded from head of the caudate nucleus coded reward value flexibly and neurons in the tail of the caudate nucleus coded for the longer-term stable reward value. Temporary inactivation of these two caudate nucleus subregions corroborated the findings obtained in the single-neuron recordings.
These very important and exciting results suggest that there indeed are two parallel neural systems coding for reward value of objects, one enables flexible coding of reward value when there is short-term volatility in value, and another mechanism enables holding and appreciating the stable reward values of objects. In addition to shedding light on the neural basis of reward-value processing under different types of task conditions, these findings offer hypotheses and insights into the neural basis of specific deficits that have been documented in various basal ganglia disorders, as also briefly discussed by the authors in their paper.
Reference: Kim HF, Hikosaka O. Distinct basal ganglia circuits controlling behaviors guided by flexible and stable values. Neuron (2013) e-publication ahead of print. http://dx.doi.org/10.1016/j.neuron.2013.06.044