36 Motivation
Factors Governing Initiation of Feeding Behavior
Homeostasis
Richter (1927) observed that feeding behavior occurred in
regular bouts that could be specified on the basis of their fre-
quency, size, and temporal patterning. He suggested that find-
ing the determinants of this regularity should be the goal of
psychology and further indicated that homeostasis, the main-
tenance of a constant internal environment, could be one of
these determinants. These observations have been supported
by further research showing that animals frequently act as
though defending a baseline level of intake, leading to the de-
velopment of a depletion/repletion model of feeding initia-
tion similar to homeostatic models developed to account for
temperature regulation behavior (Satinoff, 1983). A great
deal of evidence suggests that under relatively constant con-
ditions, animals eat a regular amount each day and that the
amount is sensitive to manipulations such as enforced depri-
vation or stomach preloading (Le Magnen, 1992). However,
there are a number of problems with this analysis, and these
problems become more intractable the more lifelike the
experiment becomes. For example, initiation of feeding
behavior has been demonstrated to be sensitive to a number
of different factors including nutrient storage levels, food
palatability, and circadian influences (Panksepp, 1974). The
crucial factor in determining the influence of various manip-
ulations on feeding behavior seems to be the nature of the ex-
perimental procedure used.
The Importance of Procedure
Collier (1987) described three different procedures that have
been used to study feeding motivation. By far the most com-
monly used is the session procedure. Here, the animal is de-
prived of a required commodity for most of the day and is
given repeated brief access to this commodity during a short,
daily session. In such a procedure very few of the determi-
nants of behavior are free to vary, placing most of the control
of the animal’s behavior into the hands of the experimenter.
Features of behavior including the number of trials, the inter-
trial interval, session length, portion size, response contin-
gencies, and total intake are determined by the experimenter
and not the animal (Collier & Johnson, 1997). This kind of
procedure changes the response characteristics of the animals
by placing a premium on rapid initiation and performance of
the food-rewarded behavior and does not allow analysis of
feeding initiation and termination because these are also de-
termined by the experimenter, rather than the animal.
A second class of studies uses the free-feeding proce-
dure in which animals are offered continuous access to the
commodity and their pattern of feeding is recorded. Unlike
the session procedure, there is no explicit deprivation, and the
animal is free to control various parameters of food consump-
tion, including meal initiation and termination. This proce-
dure has led to the dominant depletion/repletion model of
feeding motivation. This model hypothesizes that postinges-
tive information about the nutrient content of the meal is
compared against nutrient expenditure since the last meal to
determine the nutrient preference and size/duration of the
next meal (Le Magnen & Devos, 1980). Correlations between
length of food deprivation and subsequent meal size or the
rate of responding for subsequent feeding (Bolles, 1975; Le
Magnen, 1992) provide support for this interpretation. How-
ever, these correlations are influenced by a number of other
factors, including the availability of other behaviors (Collier,
Johnson, & Mitchell, 1999), and do not provide a complete
account of feeding initiation (Castonguay, Kaiser, & Stern,
1986). Even more important, the feeding initiation and subse-
quent meal patterning of free-feeding animals seem to be
such that they never undergo nutrient depletion: Free-feeding
animals never have empty stomachs (Collier, Hirsch, &
Hamlin, 1972), meaning that a near-constant stream of nutri-
ents enters the animal. This behavior suggests either that feed-
ing initiation must be unrelated to depletion or that it must
occur prior to, but not as a consequence of, nutrient depletion.The Cost of FeedingOne major parametric influence on feeding behavior not in-
cluded in the free-feeding procedure is the cost of procuring
food. In the laboratory foraging procedure (Collier, 1983) the
animal is not food deprived in the conventional sense—it has
constant access to food resources—but food availability is
restricted by making delivery contingent on the completion
of a response contingency. Unlike the session procedure, the
animal is free to control the various parameters of feeding
behavior. Unlike the free-feeding procedure, the animal must
not only work to gain access to the commodity, but it must
balance the demands of gaining access to food with other
biologically important activities such as drinking and sleep-
ing. In these studies, the cost of food procurement, and not
the repletion/depletion calculation, has been demonstrated to
be the crucial determinant of feeding initiation (e.g., Collier
et al., 1972). Experiments manipulating the cost of food pro-
curement have demonstrated that the number of meals an an-
imal takes in a day is directly related to the cost of initiating
a meal. By varying the number of lever presses required
to initiate a meal, Collier et al. (1972) demonstrated that
the daily number of meals initiated by the animal is a linear
function of the log of the response requirement. The number