The traffic light paradigm: a reaction time task to study laterally directed arm movements.

Patients with unilateral brain damage may show slowed or hypometric arm movements toward the contralesional space, as compared to movements directed towards the side of the brain lesion. The present article describes a reaction time paradigm devised to study accuracy and latency of directional arm movements in normal human subjects and brain-damaged patients. Experimental paradigms hitherto used to explore directional motor disorders often do not reliably disentangle between perceptual and motor factors, because they employ lateralized perceptual stimuli. The traffic light paradigm, instead, consists of visual stimuli presented on the vertical midline (like a traffic light) and hand responses to be produced in either hemispace. Thus, participants have to produce lateralized arm responses to central visual stimuli. Performance on this 'motor' paradigm can be contrasted with performance on a 'perceptual' reaction time task, consisting of similar, but lateralized visual stimuli and central motor responses. Results obtained with these paradigms on normal participants and brain-damaged patients are presented and discussed. These results give empirical support to the claim that the traffic light paradigm is suitable to study directional motor disorders in relative isolation from perceptual biases.