In yet other procedures, only internal signals indicate when responses will be reinforced (e.g. internal signals that indicate the passage of time in a differential reinforcement of low-rate responding procedure; DRL). Tasks that measure delay aversion (delay discounting) are characterized by choices between small rewards available immediately or with a minimal delay, and larger rewards available after a delay. To quantify sensitivity to delay, some procedures vary the length of the delay to the larger reward and examine changes in preference (adjusting delay procedure, e.g. Perry et al. 2005; the within sessions procedure, e g., Evenden & Ryan 1999; the between session procedure, e.g. Poulos, Le & Parker 1995), while others vary the size of the small reward (adjusting amount procedure, e.g. Richards et al. 1997).
