40 Motivation
not until 10 min to 1 hr after ingestion, and not specifically to
food. Further studies (Hogan, 1973b) indicated that neither
satiation nor hunger was responsible for this delayed increase
and suggested instead that this effect was due to learning
reinforced by the postingestive consequences of food
consumption. Hogan (1977) demonstrated that only experi-
ence that involved pecking led to the development of dis-
crimination between food and sand and that this required a
postingestive delay of 2 min to 3 min, indicating that the dis-
crimination is most likely based on short-term metabolic
feedback. Hogan suggested that the behavioral control of
pecking and the development of metabolic feedback develop
independently, but experience is necessary for these two sys-
tems to become coordinated.
The Structure of Appetitive Behavior
The focus on using instrumental procedures to study appeti-
tive behavior in psychology has, to a large extent, blinded it to
the unlearned, underlying structure of appetitive behavior. Far
from being undifferentiated activity, close examination of mo-
tivated behavior has demonstrated that appetitive behavior is
organized into chains of behaviors that serve to increase the
likelihood of the terminal act. The classic demonstration of
this is Tinbergen’s (1951) analysis of the mating behavior of
the stickleback, although similar demonstrations have been
made for the organization of other appetitive behavior (e.g.,
Leyhausen, 1979). Despite the procedural difficulty in analyz-
ing the underlying organization of appetitive behavior in arbi-
trary response operant procedures, this organization has made
its presence felt through various phenomena variously de-
scribed as constraints on learning, misbehavior, and adjunc-
tive learning (Staddon & Simmelhag, 1970). The constraints
on learning phenomena demonstrate the underlying behav-
ioral organization of the animal through making some re-
sponses and stimuli easier to condition to various rewards than
others. One example of many is the relative inability of ani-
mals to learn an instrumental response chain that requires bar
pressing on a lever proximal to the feeder prior to pressing on
a lever distal to the feeder in order to be reinforced, whereas
the far-near sequence is learned rapidly (Silva, Timberlake, &
Gont, 1998). Perhaps the classic examples of the intrusion of
the underlying structure of appetitive behavior into operant re-
sponses are the reports of misbehavior made by the Brelands
(Breland & Breland, 1961, 1966) in which the typical feeding
behaviors of species began to intrude into well learned, arbi-
trary sequences of food-reinforced behavior.
Explicit examination of the organization of appetitive
behavior is a relatively recent phenomenon in learning
situations and has largely taken place through the study of
response topography in Pavlovian conditioning procedures
and the subsequent development of behavior systems theo-
ries (Domjan, 1994; Fanselow & Lester, 1988; Timberlake,
1983). The behavioral organization of predatory foraging and
feeding in the rat is the most extensively developed of these
behavior systems and is presented as a specific example later.
It is important to note that the precise behaviors and their or-
ganization would be expected to differ from species to
species and within species based on local factors such as rel-
ative prey selection. In addition, as has been shown through
operant conditioning, novel behaviors can readily be incor-
porated into the appetitive component of feeding behavior
chains. This simple addition of new behaviors into an appeti-
tively motivated chain of behavior can be contrasted with the
relative inflexibility of aversively motivated behavior chains
described in the section on aversively motivated response
organization later.A Feeding Response Organization: The Predatory
Behavior System of the RatTimberlake (1983, 1990, 1993, 1997, 2001; Timberlake &
Lucas, 1989; Timberlake & Silva, 1995) outlined a functional
behavior system that describes the predatory foraging and
feeding behavior of the rat in a hierarchical system that em-
phasizes the behavior-organizing role of motivational modes
within the system. The behavior system includes selective
stimulus processing mechanisms, timing and memory com-
ponents, functional motor programs, and organizing motiva-
tional structures that interrelate to serve a particular function.
Within that system, particular subsystems are defined by a
collection of stimulus predispositions and motor outputs or-
ganized to achieve a particular goal (see Figures 2.1 and 2.2).
In the case of the rat feeding system, activity in the predatory
subsystem is indicated by heightened responsiveness toFigure 2.1 A hungry rat en-
gages in focal search behavior
directed toward a moving artifi-
cial prey stimulus (ball bearing).