In Experiment 1 we found that for very simple actions, reactive actions are indeed executed faster than initiated actions. In Experiment 2 we removed the “shoot-out” setting, to investigate whether this framing plays a critical role. Furthermore, we added a two-step condition [similar to that used in Welchman et al. (2010)], allowing us to study the effects of action complexity on the reactive advantage.
Ten naïve volunteers between the ages of 22–49 (average 32.6 years) participated. The experiment consisted of 6 blocks of 30 trials and took approximately 10 min. Participants started each trial with one finger holding down the 5-key of the numlock pad. The one-step action was to lift this finger from the 5-key and press the 4-key as quickly as possible. The two-step action was to lift the finger from the 5-key and then press the 4-key and the 7-key in succession. Participants were only permitted to use one finger for the experiment. Participants were instructed to carry out the movements as quickly and accurately as possible. They were specifically instructed that we were only interested in inter-button time. Reactive versus initiated actions were varied across blocks. Each trial started with the participant holding down the 5-key, at which point the screen turned black until the participant performed the instructed action. In the act blocks, participants could release the 5-key and start the sequence of button-release and button-press(es) whenever they wanted. In the react blocks, participants were instructed to start the sequence in response to the trigger: a briefly flashed white disk, presented at a randomly selected moment between 500 and 4,000 ms after the start of the trial. The 0.4° diameter disk (CIE: x,y: 0.284, 0.3210 luminance: 51.95 cd/m2) was presented at the center of the screen for 13 ms (i.e., a single monitor refresh). The one-step action was run in the first four blocks, and the two-step action in the last two blocks. Blocks alternated between act and react blocks, with the order counterbalanced across participants.
The first half of the first block was for practice, and these trials were excluded from the analysis. On approximately 1% of the react trials, participants released the key before or while the trigger was presented. These trials were also not considered in the analysis. Trials with IBTs more than 3 standard deviations away from the mean (per participant, per condition) were also excluded from analysis. This resulted in the exclusion of 0.55% of the trials.
See Fig. 3 for an overview of the IBT and error results. A planned comparison between the act and the react condition for both types of motion revealed that participants were significantly faster in executing the motion when they were reacting rather than acting for one-step actions (t(9) = 2.27, p < 0.05, η2 = .364), but not two-step actions (t(9) < 0.5, p > 0.75, η2 = .01). Furthermore, participants were significantly faster in performing the first step of the two-step actions when they were reacting (t(9) = 2.47, p < 0.05, η2 = .403), but there was no difference in execution speed for the second step (t(9) = 0.82, p > 0.4, η2 = .07). Note that the total time for executing the two-step action is the sum of the IBTs for the first step and the second step plus the resting time after the first and before the second action (resting times did not differ significantly, t(9) = 1.43, p > 0.15, η2 = .184).
For the one-step actions, the trend was towards a speed-accuracy tradeoff, though the difference between act and react conditions was not significant (t(9) = 1.65, p = 0.13, η2 = .232). Errors in the two-step condition followed the overall IBTs, though again the difference was not significant (t(9) = 1.71, p = 0.12, η2 = .247). Note that although there is no significant difference between resting times and second step, these tend to be somewhat faster in the act than in the react condition, which explains why the speed advantage for the reactive movements is not observed for the two-step actions.
With one-step actions, we again replicated the reactive advantage. However, this advantage for reacting only held for the first step of a two-step action. Why might this be? One possibility is that the execution of the second step becomes more similar across conditions. For example, the second step might be triggered by the first step, making it functionally an initiated action, independently of the trigger for the first step. This would serve to reduce the difference between the two conditions. If we look at the IBTs for the two steps separately, the IBTs for the first step closely resemble those for the one-step case, with a 25-ms reactive advantage. For the second step, both conditions yield IBTs that are equivalent to the IBT for the one-step initiated condition. In other words, when an action is comprised of multiple actions, it seems that only the first action shows a reactive speed advantage; the other actions are equally fast for both reactive and initiated actions. Thus, the overall reactive speed advantage should decrease as more steps are added.
Note that Welchman et al. (2010) did find a significant speed advantage for reactive movements, even with two-step actions. Their experiment may simply have been more sensitive. Additionally, the speed-accuracy tradeoff may have increased the size of the IBT advantage in their experiment, whereas our data did not show a speed-accuracy tradeoff for two-step actions.
Importantly, we find that framing the task as a shoot-out does not affect the speed benefit for reactive actions (although IBTs were faster overall in Experiment 1 than in Experiment 2, 80 vs. 113 ms., t(32) = 2.74, p < 0.01, η2 = .1053; there was no significant difference in reactive speed advantage, t(32) = 1.02, p > 0.3, η2 = .016).
Thus, it seems to be a fundamental feature of the motor system that simple actions are carried out faster when they are performed in response to an external trigger. Why should this be the case? As we noted in the introduction, several studies have shown that reactive actions generate less activity in SMA and pre-SMA areas than initiated actions. One highly speculative way to interpret these data would be to suggest that during initiative actions, the motor system is in some sense “overthinking.” For example, some studies suggest that SMA and pre-SMA areas are essential for switching between different options (e.g., Isoda & Hikosaka, 2007; Rushworth, Hadland, Pans, & Sipila, 2002). If the pre-SMA is involved in selecting among several courses of action (e.g., Soon et al., 2008), then perhaps the additional SMA activity is simply unnecessary for the ballistic actions we have been studying.
To return to the gunslinger motif, reacting might be faster than acting in a standard shootout, when you face a single adversary, but if an innocent barmaid strays into the scene and you have to be sure to choose the correct target, it might be better to be the villain and initiate your own shot. In the next experiment, we tested the hypothesis that the reactive advantage would be eliminated in a choice task.