Effects of stimulus spacing on steady state gradients of inhibitory stimulus control

Abstract

When the response of pigeons is maintained to a number of stimulus wavelengths, but extinguished to one (S−), the birds peck more rapidly at stimuli near the S− than at more distant stimuli. The present study explores this “dimensional contrast” effect as a function of the number and spacing of test wavelengths. A fixed portion of the wavelength continuum was spanned by 5, 9, 13, or 49 stimuli, which appeared in random sequence behind a standard pecking key. At the end of each 20-sec trial, pecks to test stimuli produced a conditioned reinforcer (sometimes followed by food), while pecks to the S− stimulus produced only darkness. Dimensional contrast “shoulders” developed to test stimuli on either side of the S−; these shoulders were of approximately the same height and wavelength position for all but the 5-stimulus (widely spaced) condition, and were comparable to the original contrast results with 25 stimuli. The results strongly suggest that the extent and locus of contrast shoulders are largely independent of the number and spacing of test stimuli.