Developmental plasticity in the auditory cortex of the cat

grantor: University of TorontoThe consequences of an abnormal pattern of sensory input during development on the organization of the auditory cortex and the thalamocortical pathway were examined. Two different experimental paradigms were used to change the peripheral pattern of neural input to the system: (1) auditory deprivation: partial cochlear lesions were induced by treating newborn kittens with the ototoxic aminoglycoside drug amikacin and (2) auditory augmentation: newborn kittens were reared in an altered acoustic environment, consisting predominantly of a continuous 8 kHz FM tone. Standard microelectrode recording techniques were used to examine the functional organization of primary auditory cortex and revealed an altered cortical frequency map as a consequence of these experimental manipulations. Scanning electron microscopy (SEM) was used to examine the cochlea, and auditory brainstem responses (ABR audiogram) were used to measure frequency-specific threshold changes in ascending neural activity. Retrograde tracers were introduced in AI, and were used to examine the organization of the thalamocortical projection. As a result of neonatal partial cochlear lesions the cochleotopic organization of primary auditory cortex was altered, with the deprived high frequency region of AI devoted instead to the representation of low frequencies. Furthermore, the deafferented region of the cortical map displayed an abnormally large cortical area (expansion) with neurons having common characteristic frequencies. The range of characteristic frequencies within this monotonic cortical region corresponded to both the high frequency border of the hearing loss and the edge of the cochlear lesion. However, retrograde tracer injections into different regions in AI produced a normal pattern of labelling in the medial geniculate body of the thalamus. These results suggest that the cochleotopic organization of the thalamocortical projection is not disrupted in deafened cats, despite the extensive physiological reorganization of the cortical frequency map observed in these animals. As a consequence of rearing newborn kittens in an altered acoustic environment, the cochleotapic representation in AI also develops abnormally. Exposure to a continuous 8 kHz FM signal during a period from birth to three months of age produced a significant expansion of the 6-12 kHz frequency region of the cortical map in mature cats. These studies have shown that manipulating the pattern of cochlear activity during the neonatal period induces changes in the functional organization of the cochleotopic map in primary auditory cortex of the cat. In conclusion, the cochleotopic map within auditory cortex is altered in a manner which reflects the pattern of sensory input from the periphery during development.Ph.D