On the day following Phase 1 conditioning, animals were food and fluid deprived for 120 minutes. Subsequently, they were confined, by means of an acrylic barrier, to the sandpaper-lined side of the place preference apparatus, as described earlier. During confinement to this compartment, animals were intermittently stimulated with 10% sucrose (four 9-μl pulses, interstimulus interval: 55 s). During the 4 min second order conditioning phase, animals were videotaped to allow later examination of behavioral responsiveness. The following variables were registered: duration of wall climbing, frequency of head-shakings and general locomotion. Wall climbing duration was registered when animals stood on their rear limbs with the forepaws placed on and treading the walls of the chamber. Head shaking was measured when observing rapid side-to-side movements of the head. A head movement of approximately 90° off midline to each side was necessary to generate a positive count, with these head shakes individually counted in real-time. Finally, time spent in forward locomotion (i.e., the combined movement of the four paws in a horizontal plane) was also registered in real-time. A researcher blind to the experimental conditions measured these behaviors.
