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In a discrete-trial choice situation, 12 pigeons were trained to discriminate which of two different fixed ratios they had completed. Psychometric functions were obtained at three ratio requirements (i.e., with the larger ratio set at 10, 20, or 30 responses) by gradually reducing the size of the smaller value. Although different response biases developed across subjects, in each case accuracy decreased systematically with ratio difference regardless of absolute ratio requirements. Above-chance performances were maintained even at relative ratio differences of 10% or less. Estimates of the Weber fraction showed that, in general, discriminability improved with absolute ratio size up to 30 responses, and beyond, when the results of other studies are considered. A similar trend held for rats studied by other investigators in fixed-ratio "counting" tasks at lower requirements. In terms of a signal-detection analysis, performance was similar to that reported for other species and dimensions. Taken together, the results suggest that for this somewhat novel dimension the same psychophysical relations hold as are commonly observed for exteroceptive stimuli.
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