Barn owls use stereovision to detect small prey in dimly lit conditions. In barn owls the eyes are largely fixed in the skull, therefore, saccades to objects of interest are performed by head movements. However, in addition to the rapid head saccades, barn owls exhibit a variety of other stereotypic head movements. The function of these movements for vision is unknown. Reconstructing head movements and scan paths of the owls during carefully planned visual tasks will enable us to learn more about the function of the conspicuous head movements that contain both angular and translational components. Our hypothesis is that these movements play a role in distance estimation.
Another particularity of barn owls is the asymmetric position of ears on either side of their head. The capability of barn owls to track preys by auditory signals only has been observed, and it may also play an important role in distance estimation.
To investigate these movements we measure head kinematics in perching, freely moving barn owls. Head motion tracking is achieved via the Vicon™ motion capture system. We have designed a special head mounted device attached to the owl's head (Figure 1). This tracking device, weighing 17 grams, is equipped with five reflectors tracked by the Vicon system, a miniature wireless CCD camera and a battery pack for the CCD camera. The CCD camera is fixed and is oriented along the visual axis of the owl. The kinematic measurements are combined and synchronized with video images recorded from the miniature head-mounted camera. This setting allows us to measure detailed kinematics of head motion while simultaneously analyzing the visual scene and recovering the fixation point. We plan to use screw theory to describe the instantaneous head motion of the owl. The screw coordinates provide information on the nature of the head motion, i.e. pure rotation/ pure translation or a helical motion, and on the instantaneous biomechanical axis of the head motion. This data will then be analyzed for a deeper understanding of the kinematic nature of the owl head motion and used for clustering typical head motion behavior of the owl.