Title : Responses of Neurons in Chinchilla Auditory Cortex to Frequency - Modulated Tones

Frequency-modulated (FM) stimuli have been used to explore the behaviour of neurons in the auditory cortex of several animal models; however, the properties of FM-sensitive auditory cortical neurons in the chinchilla are still unknown. Single-unit responses to FM stimulation were obtained from the auditory cortex of anesthetized adult chinchillas (Chinchilla laniger). Upward and downward linear FM sweeps spanning frequencies from 0.1 to 20 kHz were presented at speeds of 0.05 to 0.82 kHz/ms. Results indicated that over 90% of sampled neurons were responsive to FM sweeps. The population preference was for upward FM sweeps and for medium to fast speeds (≥ 0.3 kHz/ms). Few units (3%) were selective for downward FM sweeps while less than 22% of units preferred slow speeds (≤ 0.1 kHz/ms). Velocity preference and direction sensitivity were positively correlated for downward sweeps only (r = 0.37, p = 0.0005, t-test). Three types of firing rate patterns were observed in the FM response peristimulus time histograms: a single peak at sweep onset/offset (‘onset’) and a single peak (‘late’) or multiple peaks (‘burst’) during the sweep. ‘Late’ units expressed the highest mean values for direction sensitivity and speed selectivity; ‘onset’ units were selective only for direction and ‘burst’ units were not selective for either direction or speed. The robust responsiveness of these neurons to FM sweeps suggests a functional role for FM detection such as the identification of FM sweeps present in vocalizations of other organisms within the chinchilla’s natural environment.