ther. Some persons can
never bring their record much below the figures stated, while a few
can get the time down to .10 sec, which is about the limit of human
ability. Every one is bound to vary from trial to trial, at first
widely, after practice between narrow limits, but always by a few
hundredths of a second at the least. It is curious to find the
elementary fact of variability of reaction present in such a simple
performance.

What we have been describing is known as the "simple reaction", in
distinction from other experiments that demand more of the subject. In
the "choice reaction", there are two stimuli and the subject may be
required to react to the one with the right hand and to the other with
the left; for example, if a red light appears he must respond with the
right hand, but if a green light appears, with the left. Here he
cannot allow himself to become keyed up to as high a pitch as in the
simple reaction, for if he does he will make many false reactions.
Therefore, the choice reaction time is longer than the simple reaction
time--about a tenth of a second longer.

The "associative reaction" time is longer still. Here the subject must
name any color that is shown, or read any letter that is shown, or
respond to the sight of any number by calling out the next larger
number, or respond to any suitable word by naming its opposite. He
cannot be so well prepared as for the simple, or choice reaction,
since he doesn't know exactly what the stimulus is going to be; also,
the brain process is more complex here; so that the reaction time is
longer, about a tenth of a second longer, at the best, than the choice
reaction. It may run up to two or three seconds, even in fairly simple
cases, while if any serious thinking or choosing has to be done, it
runs into many seconds and even into minutes. Here the brain process
is very {24} complex and involves a series of steps before the
required motor response can be made.

These laboratory experiments can be paralleled by many everyday
performances. The runner starting at the pistol shot, after the
preparatory "Ready! Set!", and the motorman applying the brakes at the
expected sound of the bell, are making "simple" reactions. The boxer,
dodging to the right or the left according to the blow aimed at him by
his adversary, is making choice reactions, and this type is very
common in all kinds of steering, handling tools and managing
machinery. Reading words, adding numbers