Developmental psychology.pdf

Principles of Motivation 295
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Figure 11.8
Influences on Hunger. The hunger
drive is susceptible to environmental
factors, as well as to conditions of
the body. The role of external
influences is evident even in well-fed
barnyard animals, which begin eating
again when accompanied by
ravenous peers. Among human
beings the time of day, the packaging
of the product, and even the thought
of television can be influential.
indications of changes in air pressure within the balloon. Contractions of the stomach,
presumed to be the basis of hunger, were thereby recorded, and these contractions were
found to coincide with the gnawing feelings, as reported by the subject. On this basis,
stomach contractions appeared to be the critical issue in the experience of hunger
(Cannon, 1934).
In later studies it was found that the stomach could be removed without destroying
the hunger drive. Also, the nerves between the stomach and the brain could
be severed with no significant diminution in the hunger drive (Bash, 1939). Moreover,
some people who had not eaten for considerable periods did not even report experiences
described as hunger pangs. All of these investigations indicated that stomach contractions
are not .a necessary or sufficient condition for hunger. In fact, both stomach
contractions and hunger pangs ceased with the administration of dextrose, a substance
that raises the blood-sugar level.
The next step was to examine the relationship between blood-sugar level and
hunger. In one instance investigators used blood from a starved dog, which was injected
into a normal dog. The stomach of the injected animal showed the kind of contractions
sometimes found in hunger, but injection of blood from a well-fed animal stopped these
contractions (Luckhardt & Carlson, 1915). Such experiments support the idea that
blood-sugar level is closely related to hunger, but again the answer is not easy. This
result also depends upon where in the circulatory system the injection is given, for it
must be readily monitored by the brain or the liver in order to influence the hunger
response.
Hunger and the Brain For these reasons investigators also have considered the role
of the brain in hunger, especially the hypothalamus. Electrical stimulation to a certain
portion of this region, called the lateral hypothalamus, activates and sustains eating in
animals, even those presumably satiated. When this area is damaged or removed,
experimental animals stop eating before their normal needs have been satisfied. They
cease eating despite the availability of food, a condition called aphagia, meaning without
eating. Apparently they do not know when to start eating (Miller, Bailey, & Stevenson,
1950; Anand & Brobeck, 1951; Keesey, Corbett, Hirvonen, & Kaufman, 1984).
Electrical stimulation of another area of the hypothalamus, called the ventromedial
nucleus, promotes the opposite behavior. There is a cessation of eating, even
among animals previously placed on a food-deprivation schedule, and injury to this
brain region causes hyperphagia, which means overeating. In human beings damage
to the ventromedial nucleus sometimes results in obesity, and when this brain area is