Preparing to react in the absence of uncertainty: II. Stimulus uncertaintyand response compatibility in tactile reaction time

Stimulus uncertainty and stimulus response compatibility were manipulated i n three reaction time (RT) experiments. Key-release responses were summoned either by direct tactile or visual cuing of the response finger or by obli que cuing of the contralateral finger. Comparisons of standard simple react ion time (SRT) with 2:1 convergent RT (CoRT), in which participants were re quired to execute the same response to either of two stimuli, invariably re vealed a stimulus uncertainty effect. Extended practice and high compatibil ity diminished, but did not eliminate, this effect. The effect was not conf ined to trials in which the stimulus differed from the previous one, trials following an error, or to a longer tail on participants' CoRT distribution . CoRT remained longer than SRT when tactile imperative stimuli were replac ed by visual ones, establishing that the stimulus uncertainty effect was no t a peculiarity of the bimanual tactile RT set-up. The latencies of CoRT we re invariably shorter than for binary Go/No-Go RT pr standard choice RT (CR T), indicating a role for factors other than stimulus uncertainty. A remark able feature of the tactile paradigm was the varying impact of compatibilit y on different tasks: for example, CRT was actually shorter than Go/No-Go R T when participants responded with the finger stimulated but considerably l onger when responding with the contralateral finger.