Behavioral morphology of the pigeon's peck: ingestion, prehension and cognition.

In nature, pecking involves the transport of a prehensile effector organ (the beak) towards biologically significant targets, and the generation jaw movement patterns whose probability and topography are appropriate to the stimulus properties of the target (e.g., distance, direction, size) and the motivational state of the animals (hunger, thirst). Because conditioned pecking preserves essential topographic features of the ingestive peck, we have used the pigeon's conditioned pecking response as a "model system" for the study of motor control, response form modulation and cognitive processes. In the first study, conditioning procedures were used to bring pecking under the control of visual targets whose stimulus properties (size, location) were independently varied, while simultaneously monitoring head acceleration, jaw movements and terminal peck location. Comparison of the kinematic profiles of conditioned and ingestive pecks suggests that conditioned pecking is functionally analogous to human "pointing" rather than "grasping" behavior. In the second study, we manipulated reinforcer type and size to generate populations of "virtual" eating and drinking responses and used them to study the process of response form modulation. The data suggest that different response forms do not reflect "fixed" movement patterns but the modulation of movement patterns across a topographic continuum. In the third study we manipulated stimulus, response and deprivational variables to test a cognitive model for the study of "expectancy", "intention", and "choice" in pigeon.