Three psychophysical principles of timing in pigeons

In each of two phases, pigeons were trained on a 9-s fixed interval schedule with one signal, and a 30-s fixed interval schedule with another signal. Across two phases, the intervals associated with the signals were reversed. On occasional empty trials, one of the signals alone, or both signals simultaneously were presented for a long time with no reward dispensed. The distributions of responses in time for these three types of tests showed the following three properties: 1) Distributions were nearly symmetric about the time of peak responding, implying that the internal clock runs linearly in time. 2) The width of the distribution relative to the height on tests with single signals was a constant proportion of the fixed interval the birds were timing, supporting Weber's Law. 3) When both signals were presented simultaneously, single-peaked distributions were found, with the time of peak responding in the majority of cases some time between 9 s and 30 s. This implies that the birds were using both signals in timing. The criterion durations of the arrival of reward associated with each signal (around 9 s and 30 s, respectively) were averaged in a weighted fashion to arrive at one criterion for timing. The same three phenomena in the psychophysics of two domains of spatial search are discussed.