smart mice. Recall that Tsien jazzed up a gene’s expression to improve memory and learning. In contrast, Pecifia and Berridge quieted a gene that controls the amount of dopamine floating around in between neurons. With this gene silenced, dopamine levels increased. Compared with normal mice, these dopamine-plus mice consumed twice as much food and water, and learned much faster where food was located within a maze. But when it came to measuring licking as liking, the dopamine-plus mice were no different from normal mice. Dopamine is therefore essential for the wanting system, but not the liking system. This conclusion has been supported by many other studies, of mice and men, in the context of eating and drug addiction — two topics that I will shortly revisit.
To understand what rodents like, Pecifia and Berridge injected an opioid drug — similar to opium from poppy plants — into two brain regions associated with reward — the nucleus accumbens and the ventral pallidum. Not only have studies of rodents, monkeys, and humans revealed that these areas are associated with reward — recall the brain stimulation studies — but they contain sub-regions known as hedonic hotspots — zones tuned to particular kinds of stimulation, designed to jazz up the liking element of pleasure. Following injection, individuals licked four times more often in response to sugar as the non- injected individuals, but did not show a difference in wanting. The opioid injections also caused a decrease in the aversiveness of bitter quinine, as evidenced by a decrease in mouth gaping. Turning on these hedonic hotspot ramped up the pleasure of sweets, and diminished the displeasure from bitters. Together, the Pecina and Berridge studies highlight the independence of wanting and liking, and the ways in which the brain — or a clever experimenter playing with it — regulates the elements of pleasure.
How does the brain figure out what’s hot and what’s not, delicious or disgusting? It’s one thing to desire a particular experience, and another to derive pleasure from the experience. But the world is not set up with labels that indicate which objects and events are pleasurable and which distasteful. Every object and event has particular properties that, depending on the animal’s sensory ability, can be seen, heard, tasted, smelled, or touched. For all organisms, there are receptors within each of the sensory modalities that are biased to prefer some things over others. This is why no human baby has to be taught to dislike bitter things and like sweet things. From the very first encounter, sugary solutions trigger tongue protrusions and licking, whereas bitter solutions trigger a gaping mouth. We have evolved, as have other animals, sensory systems that are tuned to prefer some things and dislike others, right from the start. These initial biases guide the learning process, facilitating acquisition of new knowledge in some cases, making it almost impossible in others, and setting a course to self-destruction in yet others. Try teaching a young child that the taste of sweet chocolate is disgusting while the taste of bitter endives is delicious. Try teaching cocaine addicts to turn off the magnetic pull of white powder, or convincing alcoholics that
the clinking sound of ice in a glass isn’t meaningful. Try teaching rogue soldiers involved in genocide
Hauser Chapter 2. Runaway desire 60
HOUSE_OVERSIGHT_012806