Reaction times in discriminations of varying difficulty: Decision modulated by arousal

Pigeons discriminated the hue of a spot of light that appeared in discrete trials. A green spot always signaled food (“green S+”) and so did a red spot of constant hue (“red S+”), but on most trials a different red hue appeared and no food was given for pecks (“red S−”). The hue of red S− stayed the same during blocks of up to twelve sessions, but it changed from block to block. During a final group of sessions red S− was omitted and the percent reinforcement to the two S+ stimuli was varied. Major findings were that (1) percent response (Pct(R)) to S− varied with S+/S− similarity, describing a typical ogival psychometric function; (2) Reaction times (RTs) to both red and green S+s were minimal when the red discrimination was impossible (that is, when red S+ and S− were identical); (3) RTs to red S+ were greatest during discriminations of intermediate difficulty; (4) as Pct(R) declined during the learning of a difficult discrimination, RTs increased to red S+ as well as red S−. Most aspects of the data were reproduced by simulations with a quantitative model that incorporated reinforcement-based decision and arousal processes.