Progressive Ratio

Progressive Ratio

Overview

The progressive ratio schedule was first described by Hodos (1961) as a method to quantify the motivational strength of reinforcers. Unlike fixed ratio schedules, where the number of responses per reward is constant, the PR schedule increases the response requirement after each successive reward. For example, in a PR1 schedule, the animal must make 1 poke for the first pellet, 2 pokes for the second, 3 for the third, and so on. The point at which the animal ceases responding, known as the breakpoint, is considered a robust measure of motivation and willingness to expend effort.

In FED-based closed economy applications, the PR program can run continuously in the home cage. To allow animals to resume responding after disengagement, a reset rule is included: if no poke is made within 30 minutes, the ratio requirement resets back to 1:1. This ensures animals are not permanently locked out at very high ratio demands, while still allowing long-term measurement of motivational processes.

Theory

PR performance reflects both the reinforcing efficacy of the reward and the subject's willingness to tolerate escalating cost. Dopaminergic signaling in striatal circuits is central to sustaining responding under rising effort demands, while prefrontal circuits contribute to persistence and cost–benefit evaluation. Drugs or manipulations that enhance dopamine signaling increase breakpoints, whereas antagonists, lesions, or metabolic challenges typically lower them. Because the task forces animals to reveal the maximum effort they will invest for a reward, it is widely used as a sensitive assay of motivation.

Considerations

• Step size: The progression (e.g., PR1 linear increase, PR2, or exponential schedules) should be chosen to balance sensitivity and feasibility.
• Reset rule (FED-specific): If no poke is made for 30 minutes, the ratio resets to FR1 to enable re-engagement.
• Reinforcer type: Different food types or palatability levels can shift breakpoints, making PR useful for comparing reward value.
• Animal welfare: Escalating effort can suppress intake; body weight and health must be monitored closely.
• Interpretation: Breakpoints index motivation, but can also be constrained by motor capacity or satiety.

Key Features

• Progressive reinforcement schedule: each successive reward requires more responses (e.g., 1 → 2 → 3 → … pokes per pellet).
• Breakpoint: the final ratio completed before responding ceases.
• FED-specific reset: if no poke is made within 30 minutes, the requirement resets to FR1.
• Measures motivational drive, effort tolerance, and reinforcer value.
• Sensitive to pharmacological, genetic, and environmental manipulations.
• Supports long-term measurement in naturalistic closed economy conditions