Figure 1.

Schematic drawing of the TPL maze used by Van der Zee et al. (2008). Food is placed behind a metal grid on which mice have to stand to reach it, providing the possibility to administer a mild foot shock (A). Schematic representations of the learning curve: wild-type and Per1,2 mutant mice readily learn to avoid the “time-of-day dependent” negatively reinforced location. In contrast, Cry1,2 knockout mice cannot master the task and remain at the 33% chance level (B).