The song nucleus high vocal center (HVC) sends neural signals for song production and receives auditory input. By using electroencephalography (EEG) to objectively identify wake͞sleep state, we show that HVC auditory responses change with physiological states. Comparison of EEG and HVC records revealed that HVC response to auditory stimuli is greatest during slow-wave sleep. During slow-wave sleep, HVC neurons responded preferentially to the bird's own song. Strikingly, both spontaneous and forced waking during sleep caused HVC auditory responses to cease within milliseconds of an EEG-measured state change. Statedependent phenomena in downstream nuclei, such as robustus archistriatalis, are likely to be derivatives of those in HVC.T he song system of birds contains neurons that respond to auditory stimuli, particularly the individual bird's own song (BOS) (1). Song-specific responses occur even within the pathway that conveys neural signals to the muscles of the vocal organ (2). Recent studies suggest that auditory responses within this pathway occur primarily or are greatly enhanced under anesthesia or in sleep (3-5). These artificial and natural physiological states seem to lift the gate that controls auditory input. These findings raise two important issues: one concerns the definition of relevant physiological states; the other concerns the primary site of the gate. To address the first question, we used electroencephalography (EEG) to investigate the state dependence of high vocal center (HVC) auditory responses and, further, to examine the temporal dynamics of these state changes. For the second question, we discuss the results from HVC with reference to the published data from a follower nucleus, robustus archistriatalis.
MethodsAnimals and Surgery. General methods were previously described by Schmidt and Konishi (3). Eighteen adult (Ͼ120 days posthatching) male zebra finches (Taeneopygia guttata) were obtained from our breeding colony or from a local vendor (Magnolia Bird Farm, Los Angeles, CA).Birds were anesthetized with 60-90 l intrapectoral 3 mg/ml ketamine hydrochloride (Phoenix Pharmaceuticals, St. Joseph, MO) and 1.5 mg/ml xylazine hydrochloride (Lloyd Laboratories, Shenandoah, IA) in 0.9% sodium chloride 45 min before surgery. After local injection of lidocaine hydrochloride (1% xylocaine, Astra Pharmaceutical, Worcester, MA), the skull was exposed through an incision in the scalp. Small openings were made in the skull over the right HVC, over the right hyperstriatum accessorium anterior to HVC for placement of the reference electrode, and over the right and left hyperstriatum ventrale anterior and lateral to HVC for placement of the EEG electrodes.HVC recording electrodes were made of formvar-insulated nichrome wires (66-m diameter; AM Systems, Everett, WA), the tips of which were coated with rhodium to lower impedance. EEG electrodes were silver wires coated with Teflon (AM Systems). Teflon was stripped from the last Ϸ3 mm (75-m tip diameter). The reference electrode was also 75-m silver wire c...