THE DISCRIMINATION OF RELATIVE FREQUENCY BY PIGEONS

Five experiments addressed the issue of how pigeons learn to discrimin ate the relative frequency of stimuli. During a sampling period, three different stimuli (keylights) were presented serially, in mixed order , and with different frequencies. During a choice period, the stimuli were presented simultaneously, and reinforcement was arranged for choo sing the stimulus that was presented the least number of times during the sample. The results showed that (a) the overall proportion of corr ect choices was always above chance levels; (b) the likelihood of a co rrect choice decreased with the serial position of the correct stimulu s, a negative recency effect; (c) when the last three stimuli of the s ample were constrained to be one of each kind, the negative recency ef fect decreased but errors became more likely when the correct stimulus occurred early in the sample, a negative primacy effect; (d) accurate performance generalized to new and larger samples; and (e) under some conditions the probability of a correct choice was independent of the serial position of the correct stimulus. The serial position curves s uggest that in a least frequent discrimination task, two processes det ermine how the least frequent stimulus controls behavior: a passive de cay process (the stimulus loses its effectiveness with time since its last occurrence), and a residual salience process (when the stimulus o ccurs in the first position it may decay to a higher asymptote than wh en it occurs in later positions).