Evidence against a constant-difference effect in concurrent-chains schedules

Differences and ratios in a nonsymbolic 'Artificial algebra': Effects of extended training

Grace et al. (2018) showed that humans could estimate ratios and differences of stimulus magnitudes by feedback and without explicit instruction in a nonsymbolic 'artificial algebra' task, but that responding depended on both operations even though only one was trained. Here we asked whether control by the trained operation would increase over several sessions, that is, if perceptual learning would occur. Observers (n = 16) completed four sessions in which feedback was based on either ratios or differences for stimulus pairs that varied in brightness (Experiment 1) or line length (Experiment 2). Results showed that control by the trained and untrained operations increased and decreased, respectively, over the sessions, indicating perceptual learning. For about two thirds of individual sessions, regressions indicated significant control by both differences and ratios, suggesting that the perceptual system automatically computes two operations. The similarity of results across experiments with both intensive (brightness) and extensive (line length) stimulus dimensions suggests that differences and ratios are computed centrally, perhaps as part of a general system for processing magnitudes (cf. Walsh, 2003).

Stimulus control in fixed interfood intervals

The control exerted by various portions of fixed-time and fixed-interval schedules was assessed with a trace-conditioning procedure. The intervals were segmented into 10 bins. In all but 1 of those bins, the stimuli were presented in different random orders on each trial. In 1 bin, the stimulus was the same on each trial. The position of this trace stimulus was varied across phases. The results indicated that a trace stimulus can come to control behavior and that differential control can extend to even the second tenth of an interfood interval. The results were interpreted as indicating that traditional explanations of the rate loss in earlier portions of an interfood interval are inadequate and that models such as Palya's (1993) bipolar model or Miller and Schachtman's (1985) comparator model may provide a principled framework with which to understand within-trial effects.