Comparisons between one-key and two-key versions of the sinewave schedule for pigeons.

When the rate of reinforcement for pigeons' key pecking varied over time following a sine waveform, performances were more consistent and reliable if a constant-rate reinforcement schedule was concurrently available on a second key than if only the sinewave-varying reinforcement schedule was available. In the two-key version, response rates clearly followed varying reinforcement rates with the same frequency, with no phase lag, and without breaks. In both versions, pecking rate was a power function of reinforcement rate. Sinewave-schedule performance waveforms qualified for engineering methods of frequency analysis and met criteria for a standard measurement system.