J Neurophysiol. 1994 Aug;72(2):1024-7.
Importance of unpredictability for reward responses in primate dopamine neurons.
Mirenowicz J, Schultz W.
We used single neuron recording techniques in two behaving monkeys to investigate the conditions in which dopamine neurons respond to primary rewarding or potentially rewarding stimuli. Animals received drops of liquid either outside behavioral tasks or as rewards during learning or established performance of an auditory reaction time task. Three quarters of dopamine neurons showed a short-latency, phasic response to liquid that was delivered outside the task without being predicted by phasic stimuli. The same neurons responded to liquid reward during learning but not when task performance was established, at which time the neuronal response occurred to the conditioned, reward-predicting, movement-triggering stimulus. These data suggest that the responses of dopamine neurons to rewarding or potentially rewarding liquid are due to the temporally unpredicted stimulus occurrence. A known, reward-predicting, tonic context does not prevent dopamine neurons from responding to the rewarding liquid. The responses during learning apparently occur because reward is not yet reliably predicted by a conditioned phasic stimulus. Because the unpredicted occurrence of reward is of central importance for learning, these responses allow dopamine neurons to play an important role in reward-driven learning.
J Neurophysiol. 1998 Jul;80(1):1-27.
Predictive reward signal of dopamine neurons.
Schultz W.
Most dopamine neurons show phasic activations after primary liquid and food rewards and conditioned, reward-predicting visual and auditory stimuli. They show biphasic, activation-depression responses after stimuli that resemble reward-predicting stimuli or are novel or particularly salient. However, only few phasic activations follow aversive stimuli. Thus dopamine neurons label environmental stimuli with appetitive value, predict and detect rewards and signal alerting and motivating events. By failing to discriminate between different rewards, dopamine neurons appear to emit an alerting message about the surprising presence or absence of rewards. All responses to rewards and reward-predicting stimuli depend on event predictability. Dopamine neurons are activated by rewarding events that are better than predicted, remain uninfluenced by events that are as good as predicted, and are depressed by events that are worse than predicted. By signaling rewards according to a prediction error, dopamine responses have the formal characteristics of a teaching signal postulated by reinforcement learning theories. Dopamine responses transfer during learning from primary rewards to reward-predicting stimuli.
There's even a cool slide to back it up with.
Scientists recorded the activity of dopamine neurons in the VTA of a rat (or presumably some other unfortunate little critter) and graphed them around the stimulus-reward interval, both before they learned the stimulus gave them a reward, during the process of learning, and then afterwards.
from the Neuron review on this
So what does this mean?
Let's say you live on rice and beans for a long, but indefinite period of time. You work 9 to 5 at Burger Parthenon and don't go out much. Now your friend Steve, from out of town, comes in and invites you to party with him. Steve is loaded, a trust fund kid. Maybe you don't trust him at first because of the social standing, or you don't like his cologne or whatever. But regardless, out of boredom, you go out with him, hit the bars, scope out girls (or guys, if that's what you're into). The novelty and feeling of freedom are liberating and make you feel awesome. That night you pass out exhausted, and return to your doldrum day job again. Steve calls you a few weeks later, the same things happen, you go out on the town and feel great, maybe even better than last time.
(A dopamine response is induced via pleasurable behaviour - wine women, and song - in response to a stimulus - Steve calling you)
Since Steve works on an oil rig far away, though, his visits are spotty at best. Steve calls you a few months later, the next time it happens, and then a week after that. Work at Burger Perthnon is hard. You eventually start to anticipate when he'll call - as soon as you see Steve's name flash on your phone you know you'll be having a good time again. You start to relax and feel better knowing there's plans for the weekend - even though they haven't been made yet because you haven't answered his call!
(The dopamine neuron response migrates from the reward = party - to the stimulus - Steve)
Eventually Steve moves next door to you. Seems like a good idea - you can hit the town on a regular basis. So you do. But once it's a planned thing and the Friday call comes in ... bar-hopping with Steve stops being so exciting. The magic is lost.
(The dopamine neurons adjust their response to a continuous reward in response to the stimulus).
The same thing happens in lots of scenarios, with any reward and stimulus.
It seems to me this system is in place for 2 reasons:
1.
Discourage repetitive activity that results in no improvement to encourage energy be spent elsewhere - scratching of itches, sexual activity, defecation etc are normally pleasurable, but you don't see people fixate on them, usually.
2.
Encourage novel risk-taking behaviours. If you go exploring and find a specimen of your favourite fruit tree in a hidden grove, bearing a delicious juicy fruit just for you, you'd feel more satisfied than going to a fruit orchard and picking a fruit from there. Both experiences are more rewarding than going to the supermarket - as long as you don't live on a farm, that is.
The theory that chills during urination are tied to heat loss makes more sense to me than urination being intrinsically rewarding. It is indeed critical to our survival - without waste removal one would surely perish - but in general, defecating more doesn't increase your prospects at advancing the species.
For most people I have to assume it is not going to be a random result whether their bladder is going to empty or not - and even then, I know of nobody toilet-trained who gets really happy when they have to piss, because of the upcoming reward!
