Visuomotor adaptation to displacing prisms by adult and baby barn owls

The capacity of barn owls to adapt visuomotor behavior in response to prism-induced displacement of the visual field was tested in babies and adults. Matched, binocular Fresnel prisms, which displaced the visual field 11 degrees, 23 degrees, or 34 degrees to the right, were placed on owls for periods of up to 99 d. Seven baby owls wore the prisms from the day the eyelids first opened; 2 owls wore them as adults. Prism adaptation was measured by the accuracy with which a target was approached and struck with the talons, a behavior similar to pointing behavior used commonly to assess prism adaptation in primates. Baby and adult owls exhibited a limited capacity to adapt this visuomotor behavior. Acquisition of adapted behavior was slow, taking place over a period of weeks, and was never complete even for owls that were raised viewing the world through relatively weak (11 degrees) displacing prisms. When the prisms were removed from adapted owls, they struck to the opposite side of the target. The recovery of strike accuracy following prism removal was rapid; 7 of 9 owls recovered normal accuracy within 30 min of prism removal, despite having worn the prisms for months. This limited capacity for adaptation contrasts dramatically with the extensive and rapid adaptation exhibited by adult primates exposed to comparable prismatic displacements. The mechanism of adaptation used by the owls was to alter the movements employed for approaching targets. Instead of moving straight ahead, the head and body moved diagonally relative to the orientation of the head. Thus, in contrast to prism adaptation by humans that can involve reinterpretation of eye, head, and limb position, prism adaptation by owls is based on changes in the motor commands that underlie approach behavior.