Motivation

Imprinting can be treated as an example of perceptual learning because it involves the development of object recognition by mere exposure. However, recognition works hand in hand with motivation to approach and remain in close proximity to the object. The following response is not itself the imprinting phenomenon, but it shows that imprinting is not purely a matter of learning, but also of motivation. The young bird is compelled to perform any response that minimizes distance to the imprinting stimulus.

In an early demonstration, 1- to 2-day-old ducklings (/1/7<7S rubripes tristis and A. platythyn-chos) were placed in a box with two compartments separated by a translucent wall. The chick was in one of these compartments and a moving yellow cylinder (the imprinting stimulus) was in the other. Furthermore, a peck on a small key placed in one of the walls of the chick’s compartment illuminated the stimulus compartment and set the imprinting stimulus in motion for 40 s. Chicks rapidly learned to respond to the key (acquisition) and stopped pecking at the key when this behavior no longer activated the imprinting stimulus (extinction) . While these are properties of responses acquired through instrumental conditioning with conventional reinforcers such as food or water, this pecking response persisted even after hundreds of presentations of the imprinting stimulus (Peterson, 1960). Thus, there was no sign of satiety on a response reinforced by access to an imprinting stimulus, as it happens with responses reinforced by food or water.

A direct comparison of instrumental behavior reinforced by an imprinting stimulus vs. water provides further evidence of functional differences between reinforcers. Animals reinforced on a fixed-interval 60-s schedule exhibit a characteristic pattern of response distribution called the fixed-interval scallop (see Box 6.4, Chapter 6). Ducklings trained under a fixed-interval schedule reinforced with water rapidly developed the scallop. By contrast, a response reinforced by activation of an imprinting stimulus showed no evidence of the fixed-interval scallop (DePaulo & Hoffman, 1981).

An interesting demonstration of the dissociation between recognition learning and the following response was provided by an experiment based on the omission procedure (see Chapter 6 and Figures 6.6 and 6.7). Ducklings were first imprinted to a moving bottle and, once the following response was strong, they received omission training. During omission training, the imprinting stimulus was constantly moving unless the animal approached the section of the box that was closest to the stimulus, in which case the stimulus was turned off for 8 s. Ducklings rapidly adjusted to this situation by withdrawing from the closest area and sitting on a remote part of the apparatus. This allowed for the activation of the imprinting stimulus, which the ducklings followed from the distance. Eventually, the ducklings would again approach the object and restart the cycle (Hoffman, Stratton, & Newby, 1969a).

The motivational properties of imprinting are evident in other behaviors. For example, young birds imprinted on a stimulus exhibit distress vocalizations when the stimulus is removed. These vocalizations are brief “peeps” of 3-4 kHz that also occur in aversive situations (when food deprived or low body temperature). Interestingly, distress calls can also be influenced by instrumental conditioning, much like the following response. Using a master-yoked design, master animals could activate the imprinting stimulus by emitting distress calls. Such calls also activated the imprinting stimulus for yoked animals, but independently of their behavior. Master animals emitted more distress calls than yoked controls (Hoffman, Schiff, Adams, & Searle, 1966). In the natural environment, distress calls communicate to the mother that something potentially dangerous is occurring and elicit corrective actions from the mother.

When the imprinting stimulus is presented independently of the chick’s behavior and relatively frequently, few distress calls are recorded. Furthermore, eating and drinking tend to occur when the imprinting stimulus is activated (Hoffman, Stratton, & Newby, 1969b). This pattern of responding is clear when the imprinting stimulus is presented frequently, but it breaks down when presentations are spaced apart by 2 min. The presentation of the imprinting stimulus switches the young bird into an appetitive mode, facilitating eating and drinking. However, these behaviors tend to be suppressed when the imprinting stimulus is off and the animal is in an aversive mode, as suggested by distress calls. Moreover, the administration of electric shocks when the imprinting stimulus is off, making these periods more aversive, leads to a stronger following response (Ratner, 1976); whereas injections of morphine, a drug that reduces aversiveness, diminish the frequency of distress calls elicited by removal of the imprinting stimulus (Panksepp, Vilberg, Bean, Coh, & Kastin, 1978).