Sexual dimorphism of auditory activity in the zebra finch song system.

While the tracheosyringeal motor neurons of anesthetized male zebra finches fire in response to acoustic stimuli, the same motor neurons in females show no such response. Females masculinized by estradiol implants on Days 1 or 2 after hatching may develop auditory responses in their tracheosyringeal motor neurons; the presence of the response is directly related to the degree of masculinization of the estradiol-treated females' telencephalic song centers. In male zebra finches, neurons in HVc (Hyperstriatum Ventrale pars caudalis) respond to sound, and the HVc is necessary for the tracheosyringeal auditory response. Multiunit auditory activity was demonstrated in the HVc of female zebra finches. A single 20-microA pulse delivered to the male HVc elicits a large volley in the tracheosyringeal nerve; microstimulating the female HVc does not evoke a response in the motor nerve. This failure of both auditory and HVc stimulation to elicit a response in the female tracheosyringeal nerve is attributed to the lack of a functional HVc-nucleus Robustus Archistriatalis projection in females. If, as has been suggested, the tracheosyringeal auditory response may be important for the processing of song, female zebra finches might not process song in the same manner as do males.