Dopamine and reward

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Chapter 10 endnote 18, from How Emotions are Made: The Secret Life of the Brain by Lisa Feldman Barrett.
Some context is:

Many people believe that dopamine is the neurochemical that is linked to positivity and reward... [...] Many people believe that dopamine is the neurochemical that is linked to positivity and reward.

Many scientists still believe that the neurotransmitter dopamine is for reward. Studies of dopamine and reward are a virtual cottage industry in neuroscience. For example, in a standard study of reward learning, a rat is played a sound, then approaches and presses a lever, and then enters a room and eats a sugar pellet. After doing this, the rat will increase the frequency of pushing the bar after hearing the cue, and scientists say that eating the sugar pellet has rewarded the rat to press the lever. Dopamine increases in these experiments, and so dopamine is thought to produce the reward signal.

This is probably a mistaken interpretation, however. Dopamine is indeed necessary for this sequence of events — it is necessary for the rat to walk to the bar and use its muscles to press down. It is necessary for the effortful action.[1] Dopamine is also necessary for the rat to learn the association (i.e., to encode and consolidate the prediction error).[2] If bar-pressing were a habit (i.e., over learned), the rat would not need dopamine.[3][4] It could approach the bar, receive the reward, with no need of dopamine whatsoever. So dopamine allowed the rat to perform an action that gave it information — information with which it could determine that consuming the sugar pellet was yummy and useful for quelling its hunger. Reward learning can occur in animals that are dopamine depleted or when dopamine is inhibited.[5]

Opioids may be necessary for reward, however.[6]


Notes on the Notes

  1. Guitart-Masip, Marc, Emrah Duzel, Ray Dolan, and Peter Dayan. 2014. "Action versus valence in decision-making." Trends in Cognitive Sciences 18 (4): 194-202.
  2. Schultz, Wolfram. 2016. "Dopamine reward prediction-error signalling: a two-component response." Nature Reviews Neuroscience.
  3. Choi, Balsam, and Horvitz. 2015. Extended Habit Training Reduces Dopamine Mediation of Appetitive Response Expression
  4. Salamone, John D. and Mercè Correa. 2012. "The Mysterious Motivational Functions of Mesolimbic Dopamine." Neuron, 76(8): 470–485.
  5. See page 3 of Fields, Howard L., and Elyssa B. Margolis. 2015. "Understanding opioid reward." Trends in Neurosciences 38 (4): 217-225.
  6. Fields, Howard L., and Elyssa B. Margolis. 2015. "Understanding opioid reward." Trends in Neurosciences 38 (4): 217-225.