Cumulative records of lever pressing under different conditions.

<p>Data from 4 mice are shown from left to right: 2 receiving 10% sucrose solution (sucrose), and 2 receiving 10% sucrose plus 20% alcohol (alcohol). Each graph shows data from the last session (12.5%). The identity of the reward (sucrose = 10% sucrose solution; alcohol = 10% sucrose plus 20% ethanol) and the animal number are shown on top.