Neuronal responses in chinchilla auditory cortex after postnatal exposure to frequency-modulated tones.

Early postnatal exposure to an abnormal acoustic environment has been shown to significantly influence the behaviour of neurons in the auditory cortex. In the present study, we ask if sustained neonatal exposure to an FM sweep affects the development of responses to tonal and FM stimuli in chinchilla auditory cortex. Newborn chinchilla pups were exposed continuously to an upward linear FM sweep (0.1-20 kHz) at 0.05 kHz/ms for 4 weeks. Neuronal responses to pure tones and bidirectional linear FM sweeps (range: 0.1-20 kHz; speeds: 0.05-0.82 kHz/ms) were assessed in anesthetized animals following the exposure period as well as in age-matched controls (P28). We hypothesized that constant FM exposure would increase the response selectivity of cortical neurons to the environmental FM sweep. However, our results show that while tonal response latencies increased after the exposure period (p < 0.0001, one-way ANOVA), the exposure stimulus had minimal effect on neuronal direction sensitivity and decreased neuronal selectivity for any of the presented FM sweep speeds (p < 0.05, one-way ANOVA). We therefore suggest that the development of FM direction sensitivity is experience-independent while normal acoustic experience may be required to maintain FM speed tuning.