The Journal of Neuroscience, August 1, 2001, 21(15):5730-5739
Reward Unpredictability inside and outside of a Task
Context as a Determinant of the Responses of Tonically Active Neurons
in the Monkey Striatum
Sabrina
Ravel1,
Pierangelo
Sardo2,
Eric
Legallet1, and
Paul
Apicella1
1 Laboratoire de Neurobiologie Cellulaire et
Fonctionnelle, Centre National de la Recherche Scientifique,
13402 Marseille Cedex 20, France, and 2 Istituto di
Fisiologia Umana, Università di Palermo, 90134 Palermo, Italy
Tonically active neurons (TANs) in the monkey striatum are involved
in detecting motivationally relevant stimuli. We recently provided
evidence that the timing of conditioned stimuli strongly influences the
responsiveness of TANs, the source of which is likely to be the
monkey's previous experience with particular temporal regularities in
sequential task events. To extend these findings, we investigated the
relationship of TAN responses to a primary liquid reward, the
timing of which is more or less predictable to the monkey either
outside of a task or during instrumental task performance. Reward
predictability was indexed by the timing characteristics of the mouth
movements. The responsiveness of TANs to reward increased with the
range and variability of time periods before reward, notably when the
liquid was delivered outside of a task. A change in the temporal order
of events in a task context produced an increase of response to reward,
suggesting an influence of the predicted nature of the event in
addition to its time of occurrence. By contrast, we observed no
substantial changes in neuronal activity at the expected time of reward
when this event failed to occur, suggesting that these neurons do not appear to carry information about an error in reward prediction. These
results demonstrate that TANs constitute a neuronal system that is
involved in detecting unpredicted reward events, irrespective of the
specific behavioral situation in which such events occur. The responses
influenced by stimulus prediction may constitute a neuronal basis for
the notion that striatal processing is crucial for habit learning.
Key words:
striatum; basal ganglia; TANs; prediction; reward; learning
Copyright © 2001 Society for Neuroscience 0270-6474/01/21155730-10$05.00/0