Impact of expected value on neural activity in rat substantia nigra pars reticulata

Abstract

SUMMARY The substantia nigra pars reticulata (SNr) is thought to serve as the output of the basal ganglia, whereby associative information from striatum influences behavior via disinhibition of downstream motor areas to motivate behavior. Unfortunately, few studies have examined activity in SNr in rats making decisions based on the value of predicted reward similar to those conducted in primates. To fill this void, we recorded from single neurons in SNr while rats performed a choice task in which different odor cues indicated what reward was available on the left or on the right. The value of reward associated with a left or rightward movement was manipulated by varying the size of and delay to reward in separate blocks of trials. Rats were faster or slower depending on whether the expected reward value was high or low, respectively. The number of neurons that increased firing during performance of the task outnumbered those that decreased firing. Both increases and decreases were modulated by expected value and response direction. Neurons that fired more or less strongly for larger reward tended to fire more or less strongly for immediate reward, reflecting their common motivational output. Finally, value selectivity was present prior to presentation of cues indicating the nature of the upcoming behavioral response for both increasing- and decreasing-type neurons, reflecting the internal bias or preparatory set of the rat. These results emphasize the importance of increasing-type neurons on behavioral output when animals are making decisions based on predicted reward value.

Neurophysiology of Reward-Guided Behavior: Correlates Related to Predictions, Value, Motivation, Errors, Attention, and Action

“…This trend toward common encoding likely reflects the conversion of expected outcome information into appropriate motor signals at the level of NAc (i.e., limbic-motor interface). Consistent with this hypothesis, when looking even further downstream, activity in the substantia Nigra pars reticulata (SNr) does appear to reflect a common evaluation of goals, which likely better reflects its role as a motor output structure rather than a reporter of economic value (Bryden et al 2011b). Even within SNr, significant correlations between delay and size were relatively weak, suggesting that we might have to move even deeper into the motor system to find a common output signal for similarly valued outcomes.…”

Ventral Striatum Lesions Enhance Stimulus and Response Encoding in Dorsal Striatum

“…On each trial, nose poke into the odor port after house light illumination resulted in delivery of an odor cue to a hemicylinder located behind this opening (24, 25). One of three different odors (2-Octanol, Pentyl Acetate, or Carvone) was delivered to the port on each trial.…”

“…Procedures were the same as described previously (24, 26). Electrodes were advanced daily (40–80μm).…”

Ventral Striatum Encodes Past and Predicted Value Independent of Motor Contingencies

“…Procedures were the same as described previously (Bryden et al , 2011). Wires were screened for activity daily; if no activity was detected, the rat was removed, and the electrode assembly was advanced 40 or 80 µm.…”