Friday, March 15, 2013

Is it 'Important' or is it 'valuable'?

We've recently discussed dopamine as a reward prediction signal. But that is really just the start of the complicated dopamine story.

Dopamine's role in reward and punishment (by the hiking artist)
Some research groups have also found that dopamine neurons respond to aversive stimuli, like an air puff to the face or an electric shock. This finding seems to be be completely incompatible with the idea that dopamine is a signal for reward.

Luckily some scientists took the time to try to resolve this discrepancy. Bromberg-Martin, Matsumoto, and Hikosaka (2010) have written an excellent review paper explaining that some dopamine neurons do code for value (reward), but other dopamine neurons code for salience (importance).

Differential Dopamine Coding (Bromberg-Martin et al., 2010 Fig 4)

When researchers are recording from a value coding dopamine neuron, it looks like the neuron responds to reward and actually reduces its response to the air puff. This makes sense as a 'dopamine = good' signal.

However, when a researcher is recording from a salience coding dopamine neuron, it looks like the neuron is responding equally to the good thing (reward) and the bad thing (air puff). This is confusing if you think 'dopamine = good', but makes sense if you think 'dopamine = important'. When the cue comes on (a light or a tone that signifies a reward is coming next or an air puff is coming next), these dopamine neurons fire if that cue means something.

Instead of just being confused about why sometimes dopamine would code for value and sometimes it would code for salience, Hikosaka's group showed that these two types of neurons are actually separate populations, and even seem separated in space.
(Bromberg-Martin et al., 2010 Fig 7B)
The value dopamine neurons are more ventral in the (monkey) brain, while the salience dopamine neurons are more dorsal-lateral. Importantly these two populations of neurons go to slightly different parts of the striatum and receive signals from different parts of the brain. The review paper suggests that the salience coding neurons receive their input from the central nucleus of the amygdala, while the value coding neurons receive their input from the lateral habenula-RMTg pathway.

The important thing here is that dopamine does not do just one thing to the brain. It doesn't just tell the rest of the brain 'yay, you won!' or 'you want that' etc... It says different things depending on different specific conditions. 

Dopamine doesn't 'mean' anything, the cell it comes from and the cell it goes to are what determine what it does. It certainly can't be classified as the 'love molecule'

 © TheCellularScale
Bromberg-Martin ES, Matsumoto M, & Hikosaka O (2010). Dopamine in motivational control: rewarding, aversive, and alerting. Neuron, 68 (5), 815-34 PMID: 21144997

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