Fig 2. The number of visits to the operant chamber and trials made by each rat during the initial 24 h of the habituation/exploration phase.

Each vertical black line indicates a visit to the operant chamber by an individual rat (R). Grey lines (upper: rats 1–6, lower: rats 7–12) signify completed trials and delivered rewards. Rats 1–6 and 7–12 were in separate groups. The vertical black line at 11 h indicates the shift from the dark to light phase. On the x-axis, “00” represents the beginning of the training phase for all rats (all introduced simultaneously to the system). The time of day corresponding to experimental time “00” was 15:00 h.