Figure 1

Schematic diagram showing equivalence between ID and PD protocols at the mathematical indifference point (odds against reinforcing = 4). Arrows represent nose-poking for large reward by rats. In the PD protocol, there are an average of four "unlucky" events (omissions) before a "lucky" event (5-pellet food delivery). In the ID protocol, the delay interval should be at least four times the time-out (delay = 15 × 4 = 60 seconds) to reproduce a 5-pellet delivery with approximately the same temporal characteristics as in the PD protocol. To turn this ideal situation into a more realistic account, four times the spontaneous waiting observed between two consecutive trials (further 10 × 4 = 40 seconds) shall be added to delay duration (see Table 1). To optimize performance for long-term benefit, choice should be LL/L before the mathematical indifference point, and SS beyond it.