The procedures for single-unit recordings have been described previously (11, 12, 24, 25, 40). Wires were screened daily for activity; if no activity was detected, rats were removed from the testing box and the electrode assembly was advanced 40 to 80 µm. If activity was detected, the session occurred as usual and the electrode assembly was advanced 40 to 80 µm at the end of testing. Neural activity was recorded using four identical Multichannel Acquisition Processor systems (Plexon). Signals from electrode wires were amplified 20× by an op-amp headstage located on the electrode array. Immediately outside the testing chamber, signals were passed through a differential preamplifier (Plexon) where single units were amplified 50× and filtered at 150 to 9,000 Hz. Single-unit signals were then sent to the Multichannel Acquisition Processor box, where they were further filtered at 250 to 8,000 Hz, digitized at 40 kHz, and amplified at 1 to 32×. Waveforms (>2.5:1 signal to noise) were extracted from active channels and recorded to disk by an associated workstation with event timestamps provided by the behavioral computer.
