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  Ramping up to an explanation of accumbens dopamine signals

Lloyd, K., & Dayan, P. (2016). Ramping up to an explanation of accumbens dopamine signals. Poster presented at Computational and Systems Neuroscience Meeting (COSYNE 2016), Salt Lake City, UT, USA.

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Lloyd, K, Author              
Dayan, P1, Author              
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 Abstract: A cornerstone of current theorizing about dopamine’s computational role is the idea that the phasic activity of dopamine neurons represents a temporal difference (TD) prediction error. While substantial evidence supports this mapping, recent reports of ramp-like increases in accumbens dopamine concentration when animals are about to act, or are about to reach rewards, pose an important challenge to this TD hypothesis. This is because, under a TD account, the implied activity underlying such ramps is persistently predictable by preceding stimuli and as such, should be largely predicted away. Nevertheless, we suggest that dopamine ramps are largely reconcilable with standard theory, and offer three, non-mutually exclusive accounts of these phenomena. Firstly, at a computational level, we propose, along with Berke, that ramping may arise as a form of state prediction. In average-reward analyses, average reward rate has been suggested to (i) be a comparison point for the TD error, (ii) control instrumental vigour, and (iii) be represented by tonic dopamine levels. We determine that a suitable counterpart in the discounted case is a state-dependent quantity which, carried by dopamine, would manifest as a ’quasi-tonic’ signal with similar properties to those observed experimentally. Secondly, at an algorithmic level, we suggest that ramping observed just before execution of an instrumental action for reward may be caused by uncertainty about when the action will occur. TD errors occasioned by resolution of such uncertainty, such as may occur just prior to action execution, may explain these signals. Thirdly, at an implementational level, we observe that ramping may arise if dopamine has a direct influence on the timecourse of choice, such as setting the gain of an accumulative decision-making process. Even assuming purely noise-driven fluctuations in dopamine levels, resulting correlated dynamics entail an average dopamine signal which appears to ramp up to the time of decision.

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 Dates: 2016-02
 Publication Status: Published in print
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Title: Computational and Systems Neuroscience Meeting (COSYNE 2016)
Place of Event: Salt Lake City, UT, USA
Start-/End Date: 2016-02-25 - 2016-02-28

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Title: Computational and Systems Neuroscience Meeting (COSYNE 2016)
Source Genre: Proceedings
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Pages: - Volume / Issue: - Sequence Number: II-33 Start / End Page: 126 Identifier: -