Fig. 6.

Functional and structural brain plasticity in the central auditory system of the barn owl. (A) Functional plasticity. Sensitive period for the visual calibration of the auditory system’s map of interaural time difference (ITD) in the midbrain. The data indicate mean adaptive shifts in the tuning of neurons in the optic tectum to ITD in 6 owls (different symbols) that resulted from experience with chronic displacement of the visual field with prismatic spectacles, beginning at different ages. Data are from Brainard and Knudsen (51). (B) Structural plasticity. Adaptive elaboration of axons and synapses in the brains of juvenile owls in response to experience with prism spectacles. Axon and synapse labeling in the external nucleus (ICX) after a tracer injection into the central nucleus (ICC) in a normal adult (typical) and in an owl that had acquired a learned ITD map as a juvenile (trained). Data are from DeBello et al. (52). (C) Increased adult plasticity. Early training leaves a memory trace that increases the capacity for functional plasticity in the adult brain. The data compare shifts in the ITD map in two typically reared adults (blue open symbols) with shifts in the ITD maps in three adults that had learned the alternative ITD map previously as juveniles (red filled symbols). Data are from Knudsen (53). Recent experiments indicate that the increased functional plasticity in previously trained adults is due to the persistence of altered architecture (see B) acquired during juvenile learning (54).