Submitted to the Harvard-mit Division of Health Sciences and Technology in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy in Health Science and Technology

Understanding intensity coding and auditory learning are basic concerns of research on the auditory central pathway. There is no unifying model of intensity coding but several mechanisms have been proposed to play a role. The first aim of this thesis was to determine the mechanisms of intensity coding in the central auditory pathway from the cochlear nucleus to the auditory cortex. The Fos labeling method was used to assess neuronal activation in the central auditory system. This technique allows one to study large regions of the brain in awake animals. Increasing sound pressure level led to: (1) spreading of labeling towards neurons with higher best frequencies; (2) spread of labeling orthogonal to the tonotopic axis; (3) and increased density of labeling within the tonotopic band. In addition to encoding the physical features of a stimulus, it is fundamental for survival that we learn about the meaning of sounds and put them in a behavioral context. The second aim of this thesis was to study how learning, in particular auditory fear conditioning, changes the pattern of neuronal activation of neurons, as measured with Fos labeling, in the central nervous system. Conditioning led to an increase in Fos labeling in central auditory nuclei. This increase in labeling was similar to the effects of increasing sound intensity. The present results support the idea that auditory fear memories are stored in the auditory pathway. Thesis Supervisor: M. Christian Brown Title: Associate Professor, Dept. of Otology and Laryngology, Harvard Medical School