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Reaction beats intention A. E. Welchman et al. 7
Faster movement speed for reactive movements has a
parallel to known deficits in Parkinson’s disease. In par-
ticular, Parkinson’s patients are especially compromised
in speed when making intentional, rather than reactive,
reaching arm movements (Majsak et al. 1998). Differ-
ences between reactive and intentional movement
systems may thus become more apparent in Parkinson’s
as the basal ganglia makes a greater contribution to inten-
tional actions (Roland et al. 1982; Jones 1987). Testing
Parkinson’s patients with our paradigm would be of inter-
est as concerns about high-level speed-accuracy decisions,
or strategies for different experimental (i.e. self-paced
versus stimulus-driven) conditions could be ruled out.
We interpret our results as reflecting the operation of
different processing routes for intentional versus reactive
movements; however, might the results rather reflect a
deliberate strategy by participants, and thus not imply
different neural architectures? In particular, perhaps
participants deliberately change their movements according
to whether a reactive or an intentional movement is
required, optimizing their actions by speeding up on reac-
tive trials when there is less chance of ‘winning’ the duel,
and slowing down on intentional trials to minimize
energetic cost. This would be possible had participants
known ahead of time whether an intentional or a reactive
movement was required. However, under our paradigm,
the dynamic nature of the competition meant that
participants have little opportunity to change their move-
ments deliberately, as on any given trial they might be the
initiator or the reactor, and reaction times were low
(ca 200 ms).
It could also be argued that individuals learn about
their opponent’s behaviour across trials, thereby develop-
ing a strategy based on the probability of making an
intentional or reactive movement on a particular trial.
We believe this is unlikely as there was no feedback at
the end of each trial and participants were thus very
frequently unaware of who had won the duel (detecting
the small temporal offsets between one’s own actions
and that of the opponent in the context of a rapid
competition was not easy). Moreover, under some cir-
cumstances (experiment 3), participants were effectively
playing against themselves, making it difficult to argue
that participants exploited differences between their own
behaviour and that of their opponent to maximize their
chance of winning the duel. However, we tested whether
there was a systematic relationship between the prob-
ability of being a ‘reactor’ and the reactive advantage.
(Data were pooled across experiments to maximize stat-
istical power, and the reactive advantage was expressed
as a percentage to minimize the influence of between-
subject differences in movement times.) We found no
evidence of a relationship between the probability of
reacting and the increased speed of reactive movements
(r¼ 0.11, F1,39 , 1, p¼ 0.485).
As a general survival strategy, the evolution of a move-
ment system capable of producing quick (and possibly
dirtier) movements that support faster responses to the
environment seems reasonable. However, within the con-
text of a gunfight, a strategy based purely on reaction
seems unlikely to increase evolutionary fitness as the
advantage produced by reacting is far outweighed by the
time taken to react to the opponent. Anecdotal reports
suggest that Bohr tested his original idea with colleague
George Gamow using toy pistols, with the ‘reactive’
Bohr apparently winning every duel (Cline 1987). Our
data make it unlikely that these victories can be ascribed
to the benefits associated with reaction. Rather, they
suggest that Bohr was a crack shot, in addition to being
a brilliant physicist.
All participants gave written informed consent and local
ethics committees approved the experiments.
This work was supported by grants from the Biotechnology
and Biological Sciences Research Council (C520620,
EO09565), the Wellcome Trust (069439), a post-doctoral
stipend from the Max Planck Gesellschaft and the WCU
(World Class University) programme through the
National Research Foundation of Korea funded by the
Ministry of Education, Science and Technology (R31-
2008-000-10008-0). We thank the Max Planck
electronics workshop, Reinhard Feiler, Jonathan Winter
and Dagmar Fraser for technical assistance; Tom Olphin
and Richard Charlton for data collection. Emma Gowen
for discussions. Marva Felchlin for her knowledge of the
American West. Peter Praamstra and Alan Wing for
comments on the manuscript.
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