Figure 4.

Contingency-independent lever bias developed when response outcomes were equal and rarely when they differed. A, C57BL/6 mice (n = 5) form a clear (contingency-independent) bias when both levers delivered one pellet. B, Most WT mice (circles, n = 4) acquired preference for the high-reward (3 pellet) lever. One subject (1 of 5) persisted in preference for the less profitable (1 pellet) lever (contingency-independent bias) (squares, n = 1). Percent preference was calculated from responses made during choice trials (Block 2). Percent preference = no. of responses on 1 pellet lever/total no. of responses on both levers.