DYNAMICS OF VISUALLY GUIDED AUDITORY PLASTICITY IN THE OPTIC TECTUM OF THE BARN OWL

1. In the optic tectum of normal barn owls, bimodal (auditory-visual) neurons are tuned to the values of interaural time difference (ITD) th at are produced by sounds at the locations of their visual receptive f ields (VRFs). The auditory tuning of tectal neurons is selectively gui ded by visual experience during development: in the tectum of adult ow ls reared with an opitcally displaced visual field, neurons are tuned to abnormal values of ITD that are close to the values produced by sou nds at the locations of their optically displaced VRFs. In this study we investigated the dynamics of this experience-dependent plasticity. 2. Owls were raised from shortly after eye-opening (14-22 days of age) with prismatic spectacles that displaced the visual field to the righ t or left. Starring at similar to 60 days of age, multiunit recordings were made to assess the tuning of tectal neurons to ITD presented via earphones. In the earliest recording sessions (ages 60-80 days), ITD tuning was often close to normal. even though the majority of the owls previous experience was with an altered correspondence between ITD va lues and VRF locations. Subsequently, over a period of weeks, response s to the normal range of ITDs were gradually eliminated while response s to values of ITD corresponding with the optically displaced VRF were acquired. 3. At intermediate stages in this process, the ITD tuning a t many sites became abnormally broad, so that responses were simultane ously present to both normal values of ITD and to values corresponding with the optically displaced VRF. At this stage the latencies and dur ations of newly acquired responses systematically exceeded the latenci es and durations of the responses to normal values of ITD. 4. Dynamic changes in ITD tuning similar to those recorded in the optic tectum al so occurred in the external nucleus of the inferior colliculus (ICX), which provides the major source of ascending auditory input to the tec tum. 5. These results suggest the hypothesis that the neural selectivi ty for ITD in the barn owl's tectum is first established by vision-ind ependent mechanisms and only subsequently calibrated by visual experie nce. This calibration involves both the elimination of responses to no rmal values of ITD and the visually guided acquisition of responses to novel values and can be accounted for by plasticity at the level of t he ICX.