Adjunctive Behavior

On time-based or interval schedules, organisms may emit behavior patterns that are not required by the contingency of reinforcement (Staddon & Simmelhag, 1971). If you received $5 for pressing a lever once every 10 min, you might start to pace, twiddle your thumbs, have a sip of soda, or scratch your head between payoffs on a regular basis. Staddon (1977) has noted that during the time between food reinforcers, animals engage in three distinct types of behavior. Immediately after food reinforcement, interim behavior like drinking water may occur; next an organism may engage in facultative behavior that is independent of the schedule of reinforcement (e.g., rats may groom themselves). Finally, as the time for reinforcement gets close, animals engage in food-related activities called terminal behavior, such as orienting toward the lever or food cup. The first of these categories, interim or adjunctive behavior,2 is of most interest for the present discussion, because it is behavior that is not required by the schedule but is induced by reinforcement. Because the behavior is induced as a side effect of the reinforcement schedule, it is also referred to as schedule-induced behavior.

When a hungry animal is placed on a schedule of reinforcement, it may, if allowed to drink, ingest an excessive amount of water. Falk (1961, 1964, 1969) has suggested that this polydipsia, or excessive drinking, is adjunctive behavior induced by the time-based delivery of food. A rat that is working for food on an intermittent schedule may drink as much as half its body weight during a single session (Falk, 1961). This drinking occurs even though the animal is not water deprived. The rat may turn toward the lever, press for food, obtain and eat the food pellet, drink excessively, groom itself, and then repeat the sequence. Pressing the lever is required for reinforcement, and grooming may occur in the absence of food delivery, but polydipsia appears to be induced by the schedule. In general, adjunctive behavior refers to any excessive and persistent behavior pattern that occurs as a side effect of reinforcement delivery. The schedule may require a response for reinforcement, or it may simply be time based, as when food pellets are given every 30 s no matter what the animal is doing. Additionally, the schedule may deliver reinforcement on a fixed-time basis (e.g., every 60 s), or it may be constructed so that the time between reinforcers varies (e.g., 20 s, then 75, 85, 60 s, and so on).

Schedules of food reinforcement have been shown to generate such adjunctive behavior as attack against other animals (Flory, 1969; Hutchinson, Azrin, & Hunt, 1968; Pitts & Malagodi, 1996), licking at an airstream (Mendelson & Chillag, 1970), drinking water (Falk, 1961), chewing on wood blocks (Villareal, 1967), and preference for oral cocaine administration (Falk, D'Mello, & Lau, 2001; Falk & Lau, 1997). Adjunctive behavior has been observed in pigeons, monkeys, rats, and humans; reinforcers have included water, food, shock avoidance, access to a running wheel, money, and for male pigeons the sight of a female (see Falk, 1971, 1977; Staddon, 1977, for reviews). Muller, Crow, and Cheney (1979) induced locomotor activity in college students and retarded adolescents with fixed-interval (FI) and fixed-time (FT) token delivery. Stereotypic and self-injurious behavior of humans with developmental disabilities also have been viewed as adjuctive to the schedule

Induced behavior that immediately follows reinforcement has been called interim by Staddon (1977) and adjunctive by Falk (1961, 1964, 1969). The terms are interchangeable in this book.

FIG. 7.7. A bitonic relationship showing time between food pellets and amount ofad-junctive water drinking.

of reinforcement (Lerman, Iwata, Zarcone, & Ringdahl, 1994). Thus, adjunctive behavior occurs in different species, is generated by a variety of reinforcement procedures, and extends to a number of induced responses.

A variety of conditions affect adjunctive behavior, but the schedule of reinforcement delivery and the deprivation status of the organism appear to be the most important. As the time between reinforcement deliveries increases from 2 to 180 s, adjunctive behavior increases. After 180 s, adjunctive behavior drops off and reaches low levels at 300 s. For example, a rat may receive a food pellet every 10 s and drink a bit more than a normal amount of water between pellet deliveries. When the schedule is changed to 100 s, drinking increases; polydipsia goes up again if the schedule is stretched 180 seconds. As the time between pellets is further increased to 200, 250, and then 300 seconds, water consumption goes down. This increase, peak, and then drop in schedule-induced behavior is illustrated in Figure 7.7 and is called a bitonic function. The function has been observed in species other than the rat, and occurs for other adjunctive behavior (see Keehn & Jozsvai, 1989, for contrary evidence).

In addition to the reinforcement schedule, adjunctive behavior becomes more and more excessive as the level of deprivation increases. A rat that is at 80% of its normal body weight and is given food pellets every 20 s will drink more water than an animal that is at 90% weight and on the same schedule. Experiments have shown that food-schedule-induced drinking (Falk, 1969), airstream licking (Chillag & Mendelson, 1971), and attack (Dove, 1976) go up as an animal's body weight goes down. Thus, a variety of induced activities escalate when deprivation for food is increased and when food is the scheduled reinforcer.

Falk (1977) has noted that "on the surface" adjunctive behavior does not seem to make sense:

[Adjunctive activities] are excessive and persistent. A behavioral phenomenon which encompasses many kinds of activities and is widespread over species and high in predictability ordinarily can be presumed to be a basic mechanism contributing to adaptation and survival. The puzzle of adjunctive behavior is that, while fulfilling the above criteria its adaptive significance has escaped analysis. Indeed, adjunctive activities have appeared not only curiously exaggerated and persistent, but also energetically quite costly. (p. 326)

Falk (1977) goes on to argue that, in fact, induced behavior does make biological sense.

The argument made by Falk is complex and beyond the scope of this book. Simply stated, adjunctive behavior may be related to what ethologists call displacement behavior. Displacement behavior is seen in the natural environment and is "characterized as irrelevant, incongruous, or out of context____For example, two skylarks in combat might suddenly cease fighting and peck at the ground with feeding movements" (Falk, 1971, p. 584). The activity of the animal does not make sense given the situation, and the displaced responses do not appear to follow from immediately preceding behavior. Like adjunctive behavior, displacement activities arise when consummatory (i.e., eating, drinking, etc.) activities are interrupted or prevented. In the laboratory, a hungry animal is interrupted from eating when small bits of food are intermittently delivered.

Adjunctive and displacement activities occur at high strength when biologically relevant behavior (i.e., eating or mating) is blocked. Recall that male pigeons engage in adjunctive behavior when reinforced with the sight of (but not access to) female members of the species. These activities may increase the chance that other possibilities in the environment are contacted. A bird that pecks at tree bark when prevented from eating may find a new food source. Armstrong (1950) has suggested that "a species which is able to modify its behavior to suit changed circumstances by means of displacements, rather than by the evolution of ad hoc modifications starting from scratch will have an advantage over other species" (Falk, 1971, p. 587). Falk, however, goes on to make the point that evolution has probably eliminated many animals that engage in nonfunctional displacement activities.

Adjunctive behavior is another example of activity that is best analyzed by considering the biological context. Responses that do not seem to make sense may ultimately prove adaptive. The conditions that generate and maintain adjunctive and displacement behavior are similar. Both types of responses may reflect a common evolutionary origin, and this suggests that principles of adjunctive behavior will be improved by analyzing the biological context.

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