Age-related increases in reaction time result from slower preparation, not delayed initiation

Recent work indicates that healthy younger adults can prepare accurate responses faster than their voluntary reaction times indicate, leaving a seemingly unnecessary delay of 80-100ms before responding. Here we examined how the preparation of movements, initiation of movements, and the delay between them are affected by age. Participants made planar reaching movements in two conditions. The "Free Reaction Time" condition assessed the voluntary reaction times at which participants responded to the appearance of a stimulus. The "Forced Reaction Time" condition assessed the minimum time actually needed to prepare accurate movements by controlling the time allowed for movement preparation. The time taken to both initiate movements in the Free Reaction Time and to prepare movements in the Forced Response condition increased with age. Notably, the time required to prepare accurate movements was significantly shorter than participants' self-selected initiation times; however, the delay between movement preparation and initiation remained consistent across the lifespan (~90ms). These results indicate that the slower reaction times of healthy older adults are not due to an increased hesitancy to respond, but can instead be attributed to changes in their ability to process stimuli and prepare movements accordingly, consistent with age-related changes in brain structure and function.

Changing the “When” and “What” of Intended Actions

Humans often have to modify the timing and/or type of their planned actions on the basis of new sensory information. In the present experiments, participants planned to make a right index finger keypress 3 s after a warning stimulus but on some trials were interrupted by a temporally unpredictable auditory tone prompting the same action ( experiment 1) or a different action ( experiment 2). In experiment 1, by comparing the reaction time (RT) to tones presented at different stages of the preparatory period to RT in a simple reaction time condition, we determined the cost of switching from an internally generated mode of response production to an externally triggered mode in situations requiring only a change in when an action is made (i.e., when the tone prompts the action at a different time from the intended time of action). Results showed that the cost occurred for interruption tones delivered 200 ms after a warning stimulus and remained relatively stable throughout most of the preparatory period with a reduction in the magnitude of the cost during the last 200 ms prior to the intended time of movement. In experiment 2, which included conditions requiring a change in both when and what action is produced on the tone, results show a larger cost when the switched to action is different from the action being prepared. We discuss our results in the light of neurophysiological experiments on motor preparation and suggest that intending to act is accompanied by a general inhibitory mechanism preventing premature motor output and a specific excitatory process pertaining to the intended movement. Interactions between these two mechanisms could account for our behavioral results.

Anticipatory Heart-Rate Changes: Environmental vs Self-Produced Mediation of the Time between a Warning Stimulus and the Required Response

9 Ss participated in an experiment comparing anticipatory heart-rate changes to two different kinds of reaction times. Ss were paid for pushing a button within 500 msec, of a signal (simple reaction time) and were paid for pushing a button within ± 250 msec, of 5 sec. after a signal (internally mediated reaction time). For all Ss and under both conditions, the heart-rate was observed to decelerate during at least the 2 sec. prior to the response being made. However, return to an accelerating phase was delayed by approximately 2 sec. for the internally mediated reaction time condition.