Measuring the Speed of Mental Processing

At one time it was believed that thought was instantaneous and therefore attempts to measure its speed were futile. This belief was overthrown in 1850 when Herman von Helmholtz measured the speed of neural transmission and showed that nerve impulses travel at speeds that are not particularly fast (around 30 m/s or about 100 kilometers per hour). Because the physical substrate of thought is presumably based in the nervous system, Helmholtz’s measurement led people to believe that thoughts might also require a measurable amount of time to take place. With his measurement, known as the subtraction method, Helmholtz also introduced an important idea to psychological research: comparisons between carefully selected conditions might provide relatively direct measurements of otherwise inaccessible processes. Although contemporary psychologists now usually prefer a modification to the subtraction method known as the additive factors method (introduced by Ulric Neisser, Saul Sternberg and other cognitive psychologists in the 1950s and 1960s) , the subtraction method still plays a key role in brain 1