Fig 4. For group data and the first four reinforcers, length of the first visit following a reinforcer (presses per visit) on the lever that produced the first reinforcer of a component (first-reinforced alternative: P; left panel) and length of visits following a reinforcer on the not-first-reinforced lever (N; right panel) as a function of reinforcer-source sequence.

The unfilled symbols indicate visits in which a switch followed a reinforcer. Points plotted at zero on the x-axis represent data before any reinforcers were delivered.