conditioned hunger
Signals that consistently precede food intake can become conditioned stimuli for a set of bodily responses that prepares the body for food and digestion.
These reflexive responses include the secretion of digestive juices into the stomach and the secretion of certain hormones into the blood stream, and they induce a state of hunger.
An example of conditioned hunger is the "appetizer effect."
Any signal that consistently precedes a meal, such as a clock indicating that it is time for dinner, can cause people to feel hungrier than before the signal.
The lateral hypothalamus (LH) is involved in the initiation of eating.
The nigrostriatal pathway, which includes the substantia nigra, the lateral hypothalamus, and the basal ganglia have been shown to be involved in hunger motivation.
via: http://en.wikipedia.org/wiki/Classical_conditioning