What triggers hunger?

Dave Munger in his blog Cognitive Daily reports research that memory of what we've eaten actually accounts for a significant portion of our hunger:

Rather than being solely a physical sensation, being "full" is largely a matter of recalling that we've eaten a meal appropriate for the occasion.

However, this is what a Reader's Digest book titled, "Making the Most of Your Brain" has to say on the same subject:

In regulating the appetite the brain can match calorie input to energy output almost perfectly. In practice, the region of the brain that controls food intake is frequently overridden by conscious desires for food created by social and other pressures. But left alone, the brain provokes hunger only when glucose levels fall below the optimum level. In this situation, a person's weight would vary less than 5 percent during their entire adult life."

Very interesting!

It appears that hunger is a composite signal based on several inputs, the most important of which is mealtime.

We are conditioned right from birth to regulate our eating pattern. Infants learn quickly to sleep through the major portion of the night, because their mothers discourage feeding during this period. Perhaps, infants that resisted such learning, or mothers that were inclined to be indulgent, were all eaten up by predators thousands of years ago, thus producing the current
race of "infants-trainable-for-feeding-time" humans.

Anyway, mealtimes for human beings, and for at least some other animals, form a stable and regular pattern. It probably conditions us to be hungry at such times, modified by memory of what we have eaten at (and since) the last meal.

Signals from sugar levels or feeling of fullness probably reinforce mealtime response. Over time, our response about all these gets conditioned accordingly:

1. Refractory period between meals, when the hunger-stimulus is suppressed.
2. How much constitutes a full meal
3. Which food items are appropriate at which meal

To the extent this description of hunger response is correct, it may explain the results reported in studies with amnesiacs. (Please see Dave Munger's post about the details. )

The results reported by Dave, however, crucially depend upon the assumption that the amygdala is not related in any way to hunger or satiety?

But amygdala is related closely to sense of smell, which turn is intimately connected with hunger and that complicates matters a bit.

The current issue of Reader's Digest (Indian Edition, June 2005) has an interesting article, "Follow your Nose", on the sense of smell.

It cites research by a Chicago neurologist, Alan R. Hirsch, and claims "... you stop eating a meal because your sense of smell and taste is satisfied, even if you don't feel full." It reports an ongoing study in which 92 participants lost an average of 15.25 kilos each in six months by stimulating their sense of smell! (More info is here.)

Another related question: "How do amnesiacs respond to sex?" Do they have no memory of it after a few minutes? Does that mean they can respond to sexual stimuli all over again, several times in a row?