Single-unit electrophysiological recordings in the LHb were performed with a slight modification as described previously42. In brief, rats (n = 7) were placed in a stereotaxic frame under pentobarbital anesthesia (60 mg/kg, i.p.). Anesthesia level was maintained by intravenous infusion of sodium pentobarbital (15 mg/kg/hr) during the recording. A single microcarbon (7 μm diameter) filament-filled glass electrode (carbon fiber electrode, impedence 0.4–1.2 MΩ, Kation Scientific, USA) was positioned into the LHb (anterior, −3.5 mm; lateral ± 0.7 mm; deep, −4.9 mm) with one-axis water hydraulic micromanipulator (Narishige, USA). Neuronal signals were amplified 10,000 times, filtered at 0.3 to 10 kHz and digitized by using an Iso80 amplifier and a microCED-1401. Single-unit activity was isolated by Spike2 software. Once stable baseline activity was achieved for at least 10 min, mechanical stimulation was applied to the ulnar site for 20 sec. Firing rates were evaluated for 20 sec before, 20 sec during and 20 sec after mechanical stimulation.