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

¹Departments of Physiology and ²Otolaryngology/Head and Neck Surgery, University of Toronto; and ³Auditory Science Laboratory, Neurosciences and Mental Health Division, The Hospital for Sick Children, Toronto, Ontario, Canada

Submitted 18 August 2008; accepted in final form 9 February 2009

Frequency-modulated (FM) stimuli have been used to explore the behavior 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 >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, whereas <22% of units preferred slow speeds (0.1 kHz/ms). Velocity preference and direction sensitivity were positively correlated for upward sweeps only (r = 0.40, P = 0.0021, 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.