Mental Chronometry

Johannes Müller had speculated in his Handbuch that the
speed of transmission of a nerve impulse was greater than the
speed of light. Helmholtz tested that hypothesis by measuring
the time to react (“reaction time”) to stimuli applied to motor
nerves of different lengths in a frog and found the time to be
much slower than the speed of light (Boring, 1950; Hall,
1901). He extended this research to sensory nerves by measuring
the time to respond by a human to a touch on the toe and a
touch on the thigh and demonstrated that he time to respond
was slower for the impulse that had longer to travel. Helmholtz
extended the use of time to measure a sensory-motor response
to include spoken responses to words, providing a measure of
the time necessary to associate words or ideas.
The determination of reaction times to measure the speed of
mental processes was investigated by the Dutch physiologist
F. C. Donders (1818–1889). Donders began with the time to
make a motor response to a stimulus (simple reaction time)
and then added more stimuli, each with a different response.
By subtracting simple reaction time from the time taken to
make the correct response to one of several stimuli, Donders
believed that he had measured the time required to make a
choice (Boring, 1950;Woodworth, 1938). He then recognized
that his experimental procedure required not only that an observer
choose a response from among the several responses
possible but also that an observer detect which stimulus had
been presented from among the several possible stimuli (discrimination
reaction time). Using the subtractive method that
he devised, Donders estimated the time for a simple reaction,
the time taken to discriminate one stimulus from others, and
the time taken to choose a response. The possibility of measuring
the time required by mental processes appeared to have
been realized, and the reaction-time experiment as well as the
subtractive procedure became part of the science of psychology
(for modern adaptations, see Posner & Raichle, 1994;
Sternberg, 1969).