Tonically Active Neurons in the Monkey Dorsal Striatum Signal Outcome Feedback during Trial-and-error Search Behavior

Hitoshi Inokawa, Naoyuki Matsumoto, Minoru Kimura and Hiroshi Yamada

Neuroscience (2020)

Doi: 10.1016/j.neuroscience.2020.08.007

Brief summary: Inokawa and colleagues found that the tonically active neurons (TANs) in the striatum of monkeys responded to the reward feedback cues specifically at the first time during exploration. The specialized activity pattern (pause-rebound) disappeared during the repeat outcomes. The observations indicated that TANs might play an important role in modulating reward learning under uncertain situations. 

Previous studies had shown that the TANs in striatum could encode motivationally significant stimuli (both conditioned cues and primary rewards) during learning the stimulus-reward associations. Most of these studies recorded the TANs’ activities with a deterministic relationship of the stimulus-reward (S-R) contingency. What about their activities under the uncertain situations? In this study, the authors adopted a multi-step choice task, during which the S-R contingency continuously changed among each sub-block of trials (3-5 trials). Within each trial, three figures were displayed on the screen. One of them will be randomly selected as the rewarded stimulus. Monkey could only search the reward by trial-and-error constantly for each sub-block. After the first choice of reward stimulus (termed as search trial), monkey was required to repeat the choice during the following 1~2 trials to get more rewards (termed as repeat trials). Unexpectedly, they found that most TANs only responded to the search trials, but not to the repeat trials.

This piece of finding is intriguing. It suggests the possible roles of TANs in ever-changing conditions, such as series reversal learning, set-shifting, and the value-updating conditions, etc.