Sm 21

The responses of rat inferior collicular neurons to acoustic stimuli were recorded in a sound-insulated double-walled room. Glass electrodes (1.0–2.0 MΩ impedance, filled with 2 M NaCl) were advanced from the cortex to the inferior colliculus at an angle of 10° from the frontal plane using a remote controlled microdrive (SM-21, Narishige, Japan). The signal from the electrode was amplified (1000×) and filtered (0.3–3.0 kHz) by a DAM80 pre-amplifier (WPI, United States), digitized by a RZ-5 Bioamp data processor (TDT3, United States), and then stored in the computer for both online and off-line analyses. Simultaneously, the electrode signal was also sent to a digital oscilloscope (TDS 2024, USA) and an audio speaker for online monitoring.

A parylene-coated tungsten electrode (2 MΩ, WPI, USA) was inserted into the cortex and lowered to a depth of 480–650 μm below the pia by a powered manipulator with depth reading (SM-21 Narishige, Japan). A free-field electrostatic speaker was placed 10 cm away from the animal and facing the contralateral ear of the recording site. Pure tones (0.5–64 KHz at 0.1 octave intervals, 25 ms duration, 3 ms ramp) at 8 intensities (varying from 0 to 70 dB, 10 dB interval) were repeated in a pseudorandom order at least five times to measure the receptive field. Recorded signals were amplified and collected by a TDT System 3 (Gain: 5000, sampling rate: 100 KHz, TDT, USA). The high/low pass filter was set at 300/10000 Hz for spiking activities and the threshold for detection was set at three times the standard deviation from the baseline. The multiunit spiking rate and spiking time were automatically analyzed online and recorded for offline analysis by Brainware (TDT Inc., USA). The location of A1 was verified by the typical “V” shaped receptive field, the onset latency (10–20 ms) and the distribution of the characteristic frequency (CF) in the tonotopic map (Figure 1C).

Whole-cell patch electrodes were introduced into the cortex using a motorized micromanipulator (Narishige SM-21), with the electrode impedance monitored continuously with a 50% duty cycle, 10 Hz current pulses alternating between 0 and −1.11 nA (this value was chosen to allow rapid decade reduction of current amplitude). The manipulator was typically adjusted so that the pipette entered the cortex at an angle between 20 and 30 degrees off the perpendicular. For the initial descent, contact with the cortical surface and penetration, the electrode was advanced in continuous mode (∼750 µm/second), with a strong positive pressure of 100–300 mmHg applied to the interior of the pipette. After penetration of the cortical surface was detected by a transient deflection of the measured voltage, the electrode advance was continued until a predetermined depth (100–2000 microns) was reached. The electrode was then immediately retracted 100–200 microns in continuous mode, and the positive pressure quickly reduced to 40–60 mmHg (the step pressure P_step) for advancing stepwise into the tissue. The electrode resistance was then determined by visual inspection and fully compensated by the bridge circuit of the amplifier, and voltage offset adjusted according to an estimated tip offset potential (−14 mV with the K-gluconate based solution used here).