Extracellular recording

Methods for surgery, stimulus delivery, and data collection have been described previously (Fischer et al., 2007). Briefly, four barn owls (Tyto alba) were anesthetized with intramuscular injections of ketamine (20 mg/kg; Ketaject; Phoenix Pharmaceuticals, St. Joseph, MO) and xylazine (2 mg/kg; Xyla-Ject; Phoenix Pharmaceuticals). Extracellular recordings of single ICcl neurons (n = 77) were made with tungsten electrodes (1 MΩ, 0.005-in.; A-M Systems, Carlsborg, WA). All recordings took place in a double-walled sound-attenuating chamber (Industrial Acoustics, Bronx, NY). Acoustic stimuli were delivered by a stereo analog interface [DD1; Tucker Davis Technologies (TDT), Gainesville, FL] through a calibrated earphone assembly. Stimuli for both intracellular and extracellular recordings consisted of broadband noise (0.5–12 kHz) 100 ms in duration with 5-ms linear rise and fall ramps. Stimulus ILD was varied in steps of 3–5 dB.