AbstractReaction time is the interval from stimulus onset to movement initiation. It is relatively straightforward to measure stimulus onset to within a few milliseconds. Movement initiation, however, is more difficult to measure. Common strategies to measure movement initiation include mechanical events (e.g. release of a home button), absolute thresholds (e.g. velocity > 50 mm/s), and relative thresholds (e.g. velocity > 5% of peak velocity). We compared these three measures in this methodology-based study. Forty-one participants completed a 2-choice reaction time task where they held down a home button until a short- or long-distance target button illuminated. There was a load cell under the home button and an LED and accelerometer on the participant's finger. Participants were instructed to press the target button as quickly as possible. Reaction time based on button release was 282 ms, [273, 291]. The velocity at button release was 147 mm/s, [129, 165]. It follows that reaction time based on a lower absolute velocity threshold of 50 mm/s was significantly shorter (255 ms, [246, 265]). Reaction time was comparable with a threshold of 5% of peak velocity (253 ms, [245, 261]), likely because 5% of peak velocity was 49 mm/s, [45, 53]. Interestingly, this was the only measure with a significant difference between short- and long-amplitude movements (249 ms [247, 250] vs. 257 ms, [255, 259]), a response complexity effect. Correlations between the measures were all larger than .9, but the significantly largest correlation was between absolute and relative velocity thresholds (.985, [.976, .994]).
Acknowledgments: Department of Kinesiology & Sport Management, Texas Tech University