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Society for Neuroscience 2014, Date: 2014/11/15 - 2014/11/19, Location: Washington DC

Publication date: 2014-11-01

Author:

Arsenault, John
Vanduffel, Wim

Keywords:

VENTRAL TEGMENTAL AREA, MONKEY, STIMULATION

Abstract:

To examine the causal role of VTA in positive reinforcement we electrically microstimulated this region (VTA-EM) during an operant conditioning paradigm. Within this paradigm, monkeys performed a cue preference test measuring their preferences between two simultaneously presented visual cues in a free choice task. Selection of both visual cues was followed by juice rewards (0.07 ml) during 50% of the trials. Critically, juice reward probabilities were equalized across cue positions (left or right) and cue identity (cue A or cue B), and hence, were completely independent of the monkey’s choice. Monkeys first underwent a 400-trail block of the free choice task in which no VTA-EM was delivered to determine the animal’s baseline cue preference. For consistency across sessions, the preferred and nonpreferred cues during the baseline test were deemed cue A and B, respectively. After the baseline preference test was completed, a cue B-VTA-EM block began in which 50% of all cue B selections were followed by VTA-EM. VTA-EM consisted of a 200 ms train of bipolar stimulation pulses (200Hz; 650 μA - 1mA; 2 VTA electrodes). After the cue B-VTA-EM block, we began pairing VTA-EM with cue A selections (using the paradigm explained above) and stopped pairing cue B selection with VTA-EM (cue A-VTA-EM block). In an effort to better understand the effect of juice and VTA-EM reinforcement on trial-by-trial cue selection behavior, we utilized a Kalman filter learning model (http://www.cs.bris.ac.uk/home/rafal/rltoolbox/, Dayan & Abbot, 2001). For each monkey (n=2), two separate learning models were generated. One model used juice administration as the reward input while the other model used VTA-EM. We found that the models utilizing VTA-EM as the reward input provided a better fit for the cue selection behavior. Thus, VTA-EM reinforcement better explains trialby- trial cue selection behavior. Further examination demonstrated that VTA-based models exhibited a lower learning rate and less noisy selections (higher inverse ‘temperature’). Thus, the Kalman filter learning models confirmed in both animals that VTA-EM reinforcement better accounted for trial-by-trial cue selection behavior while indicating that VTA-EM reinforcement was integrated over longer time periods and used to exploit the high value cue more often relative to equiprobable juice reinforcement.