EFFECTS OF FOOD-PELLET SIZE ON RATE, LATENCY, AND TOPOGRAPHY OF AUTOSHAPED KEY PECKS AND GAPES IN PIGEONS

Four pigeons responded under autoshaping contingencies in which differ ent conditional stimuli (red or green keylights) were associated with unconditional stimuli of different magnitudes (large or small food pel lets) over successive trials within a session. Both topography (beak o pening or gape) and strength (rates and latencies of key pecks and gap es) of responding during the conditional stimuli depended on the magni tude of the correlated unconditional stimulus. Key-peck and gape rates were higher and latencies were shorter in large-pellet trials than in small-pellet trials. Gape amplitudes varied directly with pellet size , although conditional and unconditional gapes were larger than either pellet. These findings were replicated when the key colors were prese nted either on one or two keys and after reversals of the color-size c orrelations. Because the unconditional stimulus was varied through pel let size, magnitude was not confounded with food-access duration or qu ality These results demonstrate the effects of the magnitude of the un conditional stimulus, in that rates and latencies of both key pecks (w hich are directed movements toward the key) and gapes (which are indep endent of the bird's position and key properties) varied with pellet s ize. Gape measures were unique in that two dimensions (response streng th and topography) of a single response class varied simultaneously wi th magnitude.