Figure 1.

Experimental apparatus, with components presented to scale. The inner left and right walls consisted of opaque plastic. The restraint tubes were cylinders composed of clear Plexiglas. Experimenters accessed each of the three chambers through clear Plexiglas ceiling hatches that acted as a roof when closed. Not shown are two indicator lights set into the wall immediately above each of the levers. Unless otherwise noted, components were composed of aluminum. In Conditions 1–4, pellets were delivered into the pellet tray from a dispenser outside the apparatus. In Conditions 5–7, food was dispensed into the right restraint tube.