Schematic diagram of the VNS-tone train pairing procedure. A 0.5 second, 30 Hz train of 100 μs wide biphasic pulses was delivered to the left vagus nerve via a cuff electrode. Each group received VNS at one of four different intensities: 0.4 mA, 0.8 mA, 1.2 mA, or 1.6 mA. Rats received VNS paired with a 9 kHz tone every 30 s, 300 times during each 2.5 hour session for 20 days. Cortical recordings were made 24 hours after the last pairing session.