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Single-session response rates from Experiment 1, averaged across pigeons and displayed as a function of fixed-ratio value. The top panel shows response rate for Birds 3, 5, 14, and 15 recorded using a microswitch (circles) or a Gerbrands (disks) key. The bottom panel shows the different functions obtained from the first half of a session alone versus the second half of a session alone. The 95% confidence intervals for the regressions within each panel overlap.
