Reinforcer magnitude attenuates: apomorphine's effects on operant pecking

Abstract

When given to pigeons, the direct-acting dopamine agonist apomorphine elicits pecking. The response has been likened to foraging pecking because it bears remarkable similarity to foraging behavior, and it is enhanced by food deprivation. On the other hand, other data suggest the response is not related to foraging behavior and may even interfere with food ingestion. Although elicited pecking interferes with food capture, it may selectively alter procurement phases of feeding, which can be isolated in operant preparations. To explore the relation between operant and elicited pecking, we provided pigeons the opportunity to earn different reinforcer magnitudes during experimental sessions. During signaled components, each of 4 pigeons could earn 2-, 4-, or 8-s access to grain for a single peck made at the end of a 5-min interval. In general, responding increased as a function of reinforcer magnitude. Apomorphine increased pecking for 2 pigeons and decreased pecking for the other 2. In both cases, apomorphine was more potent under the component providing the smallest reinforcer magnitude. Analysis of the pattern of pecking across the interval indicated that behavior lost its temporal organization as dose increased. Because apomorphine-induced pecking varied inversely with reinforcer magnitude, we conclude that elicited pecks are not functionally related to food procurement. The data are consistent with the literature on behavioral resistance to change and suggest that the effects of apomorphine may be modulated by prevailing stimulus-reinforcer relationships.

Keywords: apomorphine; foraging; key peck; pigeon; reinforcer magnitude; resistance to change.

Fig. 1

Control rates of responding. The y-axis shows the rate of responding as a function of the reinforcer magnitude available at the end of the fixed interval. Points represent averages of the data collected following saline administrations every Thursday during dose-response determinations.

Fig. 2

Effects of apomorphine on key pecking. The y-axis shows the rate of key pecking expressed as a proportion of control rates (data from Thursdays) plotted against dose of apomorphine on the x-axis for each pigeon. Points above “S” indicate the effects of the saline vehicle. Apomorphine's rate-increasing effects are plotted in the left column; rate-decreasing effects are plotted in the right column. Note, both axes are log-spaced.

Fig. 3

Effects of prefeeding on key pecking. The y-axis shows the rate of key pecking expressed as a proportion of control. The test lasted one week. Data from Monday served as the control; increasing amounts of food were given presession on Tuesdays-Fridays. Note the y-axis is log-spaced. Other details are as in Figure 2.