Surgical and histological procedures were as described previously (Johnson and Redish, 2007; Schmitzer-Torbert and Redish, 2004a; van der Meer and Redish, 2009). Briefly, rats were chronically implanted with a movable electrode array consisting of 12 tetrodes and 2 reference electrodes (Kopf, Tujunga, CA, USA) targeting ventral striatum (coordinates: AP +1.2, ML ±2.3–2.5 mm relative to bregma). Tetrodes were constructed from 0.0127-mm diameter wire insulated with polyamide (Kanthal Precision Wire, Palm Coast, FL, USA) and gold-plated to impedances in the 250–300 kΩ range (measured at 1 kHz, BAK Electronics IMP-1). Reference electrodes consisted of tetrodes with all four channels connected at the interface board level (before the headstage pre-amps), gold-plated to 200–250 kΩ impedance. After the completion of experiments, the location of each tetrode was marked by passing a small amount of current through each tetrode (10 μA for 5–10 s) for subsequent histological localization.
