Figure 1

The layout of the panel in the hole-box is illustrated by the various symbols. The holes are designated numerically by the couplet (r, c), with the holes in the upper row as seen from the animal's working space designated (1, 1), (1, 2),...,(1, 5), row two (2.1), (2, 2),...,(2, 5). Nose pokes in the target hole (2, 3) produced water in a water cup on the opposite wall according to fixed interval schedules, while pokes in the other holes either had no consequences or produced a brief flickering of the houselight or a sound stimulus. The center of each hole was 4 cm from its nearest neighbor.