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How does it work inside the brain?
The firmest evidence comes from EEG records (electro-encephalograms, the traces of electric waves in a person's brain detected by electrodes stuck to his or her skull). If a person is shown a series of pictures, every new picture provokes electric activity. If the same picture appears twice in the series, the second time it appears the person will get a sensation "seen it before!" that they do not get from most items in the series. By testing with many people and many pictures including many repeats, one can filter out just the part of the electrical response that comes with the recognition "seen it before!" Similar results have been gained with words shown instead of pictures, and with words spoken instead of being shown*.
Seeing for a second time a picture you saw a few minutes earlier is something like seeing the second sock of a pair. Subjectively, from socks we get a faint spasm of pleasure and objectively, from a repetition we get a few microvolts of positive electrical potential. Something happens in the brain that triggers the electrical ripples and the feeling of pleasure, but we don't know what that something is. (More.)
On the chemical side, agents such as morphine produce pleasure and agents such as dopamine seem to intensify many effects that would otherwise be milder. One can guess that the sensation "seen it before!" begins as an electrical effect --- some kind of interaction between the hummings of two swarms of busy neurons --- that perhaps releases a particular chemical in a particular sensitive location**. More probably, the interaction initiates a cascade of events that eventually leads to the chemical in the location; nothing in the brain is as simple as "X and Y interact and produce Z."
As to where the critical locations might be, more is known about fear than about joy (for study purposes, it is easier to produce small, controlled, repeatable tremors of fear than tremors of joy). Both pleasure and fear are basic feelings that we have in common with other animals. The fear mechanism exists not in the cortex (the brain-part particularly developed in humans) but in the sub-cortical parts that we have in common with other animals; the amygdala and hippocampus seem to be particularly involved. But even with fear, we do not know the causal chain in detail. We know a number of chemicals that affect one's fear-response and a number of locations where injecting one or another has its effect; that is, we know a number of ways to affect one's fear-response --- but we still do not know whether the thing we ultimately affect is electrical or chemical or a mix of both. And what we know about pleasure is even more incomplete and scrappy --- a smattering of disconnected facts, none very close to the heart of the matter.
On the other hand, knowledge of the brain increases every month. If there were a favorable turn of events, the situation I have described could change in a short time with dramatic results.
* For a different kind of experiment with musical notes, see the page on Mireille Besson.
** Joshua Fost suggests serotonin. His paper published in 1999 contains a section entitled Serotonin in which the first paragraph is very direct. The crucial neural system (1) must be receptive to a wide range of thoughts or perceptions, (2) must be activated specifically by the interaction of two matching items, and (3) must affect the brain-parts that create a sense of pleasure (paraphrase by me; interested readers should see the page on previous writers and Fost's paper itself).