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If we are to view pleasure and pain as being essentially synonymous with the more mechanistic concept of reward and punishment (i.e. as a part of learning and motivation system) then do we understand how this actually occurs at a neurobiological level? At least in "lower" animals, pain seems to simply cause an aversion response. Pain motivates the animal to stop whatever it was doing (possibly stopping the pain), and to avoid in the future repeating similar responses to those that preceded (and therefore were possibly causal toward) the incident that caused pain.

This sort of thing is not that hard to replicate in computational models, so it is clearly nothing magical. Obviously, the subjective "experience" of pleasure and pain gets more into philosophy of mind and that's not what I am asking about. I am only concerned about the mechanism which can explain outwardly observable behavior -- especially in simpler animals -- such as the basic stimulus-response-reward/punishment learning that a mouse or bird can do.

Do we understand the mechanism by which a pain signal causes the brain to suppress recently followed decision paths, and a pleasure signal causes the brain to reinforce them? Or is this still completely mysterious at this time?

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migrated from Feb 23 '12 at 16:30

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What's the point of calling it “pleasure” if you are not really interested in pleasure itself (i.e. the subjective experience)? What leads you to believe that conscious experience has no impact on outwardly observable behavior or that mice don't have any? It seems to me that there can be no reasonable answer with such conceptual confusion. – Gala Feb 24 '12 at 12:32
Call it "reward / punishment mechanism" if that makes more sense to you. I'm from the "brain is a biological computer" camp, so I don't see consciousness or qualia as relevant, since they aren't definable in objective terms anyway (in my opinion). Regardless, if it helps, forget I used the terms "pleasure" and just look at it as "do we understand the mechanism by which detection of goal satisfaction by the nerves causes the brain to cause recently used stimulus-response connections to be reinforced such that they are more likely to be reused in the future?" – rob Feb 24 '12 at 17:32
Mildly related question I am also going to tag this question as behaviorism to avoid future qualia confusion. – Artem Kaznatcheev Feb 25 '12 at 2:51
@rob I think that you have to pick a frame of reference for your question before someone is going to be able to answer it effectively. I think its a great question, but I think you are trying to frame it in both a "top down" (which is what I saw in it over in biology, and thought it would be great here) and bottom up (looking at it from the perspective of the individual circuits). Both analysis methods are equally valid, but trying to reconcile one with the other has plagued researchers. Much of what you might call pain is – Chuck Sherrington Feb 26 '12 at 9:07
modulated in the spinal cord, and gated at the brainstem, so from a biological perspective, it's hard to quantify in terms of decision paths or framing it as a behaviorist input/output type model. However, looking at it from the top down, you can gain a lot of powerful insight into what is going on without having to pick apart the minute details. So, it's just a question of which aspect you want to tackle first to make it more manageable. – Chuck Sherrington Feb 26 '12 at 9:14
up vote 14 down vote accepted

I think part of the answer to your question is going to include the dopamine "reward" pathway in the basal ganglia. In particular, a leading theory of dopaminergic function is the predictive reward error or reinforcement learning hypothesis. In this theory, dopamine neurons signal expectations about the outcome of particular stimuli.

Some key experiments are outlined in Schultz, 1998 (PDF). First, an unexpected reward elicits a short activity increase in dopamine neurons. Second, if a conditioned stimulus (CS) precedes the reward, the dopamine neuron fires in response to the CS but not the reward. That is, the dopamine signal is now a predictive reward signal. Third, if the CS is presented and then the reward does not occur, the response to the CS occurs but the dopamine neuron depresses activity immediately after the reward should have happened.

So the theory goes that dopamine neurons are not simply saying whether a reward has occurred, but rather as a learning signal that adjusts predictions about rewards. If an unpredicted reward occurs, the neuron signals a positive error ("Oops, that was a reward I should know about!"). If a predicted reward occurs, the neuron signals no error at the time of the reward ("shrug, I knew that was going to happen"). If a predicted reward does not occur, the neuron signals a negative error ("Aw shucks, guess that bell doesn't mean food after all.").

A somewhat updated revision of the theory is outlined in a review by Redgrave and Gurney, 2006 (PDF). Importantly, they note that dopamine neurons suppress their activity in response to noxious stimuli (the pain aspect of your question). They also raise some tough questions about how exactly this dopamine signal interacts with other parts of the brain to mediate the learning suggested in the theory.

As with basically all neurobiology theory, a full understanding of how this signal that we detect in individual neurons translates into a coherent behavioral response program is a long ways away. But the basal ganglia, in general, are sometimes thought of as an action selection system (or "decision path" selection to use your words), so the ability of dopamine neurons to signal expectations is relevant.

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Where you say "they note that dopamine neurons suppress their activity in response to noxious stimuli"...this is getting at what I am looking for. Obviously it would be nice to see the other side, that 'dopamine neurons increase their activity in response to pleasurable stimuli', although I suppose this might be implied. – rob Feb 28 '12 at 1:33
And I don't really know what you mean by "dopamine neurons"...are these just things that are saying "this feels good!" or are they actually things that can contain logic for future behavior, which is the link that I am looking for? Unfortunately I can't get at the article without paying so all I have to go on is your paraphrase... – rob Feb 28 '12 at 1:36
@rob try – Jeff Feb 28 '12 at 3:18
@rob they actually contain 'logic', as you say. if they simply responded to hedonic stimuli, they would not respond to a CS. And indeed, this logic codes for 'future behavior': studies show that firing dopamine neurons do not simply reflect the current value of the stimulus, but the discounted sum of all future values. Wikipedia TD-learning for more info. – Jeff Feb 28 '12 at 3:23
fyi, this is a pretty good answer, I'm going to dig through those articles. I'm come back and accept this answer soon, unless of course someone else posts one that gets even closer to what I'm trying to understand. :) – rob Feb 28 '12 at 19:08

Neurobiology is not my field of expertise, but this paper seems relevant:

Kent C. Berridge, Chao-Yi Ho, Jocelyn M. Richard, Alexandra G. DiFeliceantonio (2010) The tempted brain eats: Pleasure and desire circuits in obesity and eating disorders. Brain Research, 1350, 43-64.

What we eat, when and how much, all are influenced by brain reward mechanisms that generate “liking” and “wanting” for foods. As a corollary, dysfunction in reward circuits might contribute to the recent rise of obesity and eating disorders. Here we assess brain mechanisms known to generate “liking” and “wanting” for foods and evaluate their interaction with regulatory mechanisms of hunger and satiety, relevant to clinical issues. “Liking” mechanisms include hedonic circuits that connect together cubic-millimeter hotspots in forebrain limbic structures such as nucleus accumbens and ventral pallidum (where opioid/endocannabinoid/orexin signals can amplify sensory pleasure). “Wanting” mechanisms include larger opioid networks in nucleus accumbens, striatum, and amygdala that extend beyond the hedonic hotspots, as well as mesolimbic dopamine systems, and corticolimbic glutamate signals that interact with those systems. We focus on ways in which these brain reward circuits might participate in obesity or in eating disorders.

You may also be interested in these two book chapters:

Smith, Kyle, Stephen V. Mahler, Susana Pecina, and Kent C. Berridge. “Hedonic Hotspots: Generating Sensory Pleasure in the Brain.” In Pleasures of the Brain, edited by Morten L. Kringelbach and Kent C. Berridge, 27–49. New York: Oxford University Press, 2009.

A vital question concerning sensory pleasure is how brain mechanisms cause stimuli to become pleasurable and liked. Pleasure is not an intrinsic feature of any stimulus, but instead refl ects an affective evaluation added to the stimulus by the brain. That is, as Frijda expresses it (Frijda, this volume; Frijda, 2006), a pleasure gloss or hedonic value must be actively ‘painted’ on sweet or other sensations to make them pleasant. Brain mechanisms of pleasure, whatever they are, must take a mere sensory signal and transform it into a hedonic and ‘liked’ reward.

Finding the brain mechanisms responsible for painting a pleasure gloss is a major challenge for affective neuroscience (Barrett and Wager, 2006; Berridge, 2003b; Damasio, 1999; Davidson, this volume; Davidson and Irwin, 1999; Kringelbach, 2005; Kringelbach, this volume; LeDoux, 1996; Panksepp, 1991; Peciña et al., 2006). Fortunately, progress on fi nding hedonic generators in the brain is being made. In this chapter we focus specifi cally on the neuroanatomical hedonic hotspots in the brain where neurochemical signals actually contribute causally to the generation of pleasure.

Aldridge, J. Wayne, and Kent C. Berridge. “Neural Coding of Pleasure: ‘Rose-tinted Glasses’ of the Ventral Pallidum.” In Pleasures of the Brain, edited by Morten L. Kringelbach and Kent C. Berridge, 62–72. New York: Oxford University Press, 2009.

Pleasure is not a sensation. What is it then? Nico Frijda's answer in the "pleasure questions" section of this book (which he suggested a number of years ago) epitomizes an emerging consensus among many psychologists and neuroscientists (Frijda, Chapter 6, this book). He notes that pleasure "is a 'pleasantness gloss' added to whatever is pleasant". [...]

Here we ask: how is a "pleasure gloss" encoded in brain activity? Where in the brain is this glossing operation performed and how does it work? Is it possible for neuroscientists to recognize the signature patterns of neural activity that represent a pleasure gloss? These are difficult questions that are only beginning to be addressed. The "pleasure gloss" metaphor, applied to the transformation of neural signals for a stimulus, is like a varnish that is applied on top of a dull object to transform it into a shiny one. Adding hedonic tone to the signal passed on to downstream structures, the neural gloss effectively gives the entire brain a "rose-tinted" hedonic perception of the stimulus as pleasant.

In the context of neural firing signals, our idea is that a particular pattern of neuronal spikes or action potentials in crucial neurons may apply a glaze of pleasure on what might otherwise be an ordinary sensation or action signal.

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Interesting. I have to say the way they describe this "pleasure gloss" seems so subjective, since it never seems tied back to anything concrete, like a tendency to repeat the behavior in the future. – rob Feb 27 '12 at 18:17

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