Prehension in the pigeon

Summary

During eating, the pigeon's jaw functions as a prehensile organ, i.e., as an effector organ involved in the grasping and manipulation of objects. The preceding paper provided a descriptive account of the jaw opening movements associated with each phase of the eating behavior sequence. For two of these movements, Grasping and Mandibulation, the amplitude of jaw opening is adjusted to pellet size. In the present study a kinematic analysis of these movements was carried out to clarify the motor control mechanisms mediating these adjustments. The analysis was carried out within the conceptual framework provided by a “pulse-control” model of targeted movement. For each of the movements the extent to which opening amplitude, its first and second derivatives and its rise time are scaled to pellet size was determined. Relationships among these kinematic variables were then examained in order to distinguish between “pulse-height” and “pulse-width” strategies. In addition, the possibility that “corrective adjustments” to the trajectory are made during its execution was also explored using a multiple regression analysis developed by Gordon and Ghez (1987a, b). For both movements, peak opening amplitude, acceleration and velocity are scaled to pellet size and these variables account for most of the variance in opening amplitude. The kinematic analysis suggests that critical parameters of the trajectory are determined (“programmed”) prior to its initiation. Moreover, pigeons, like cats and humans, appear to utilize a “pulse-height” strategy for the control of amplitude scaling during targeted movements. Finally, the multiple regression analysis suggests that, like humans, pigeons modulate rise time during the decelerative phase of the jaw opening response to correct for errors in its initial programmed trajectory. In view of the differences in species and methodology between the present study and previous work on motor control mechanisms in mammals, the findings suggest that the conceptual framework provided by “pulse-control” theory has considerable generality. They also confirm the utility of the pigeon's prehensile behavior as a “model system” for the study of motor control.