Helmholtz's "psychological" time experiments - continued...
In comparison with his frog experiments, Helmholtz's work on humans presented special challenges. For obvious reasons, the experiments could not be conducted on preservable preparations separated from the rest of the organism. Helmholtz tried to circumvent this difficulty by measuring what today's psychologists would call 'simple reaction times'. Helmholtz described the basic structure of his experiments:
In a human being, a very weak electric shock is applied to a limited space of skin. When he feels the shock, he is asked to carry out a specific movement with the hand or the teeth interrupting the time measurement as soon as possible. (Helmholtz, 1883 , p. 878)
When all parameters remained constant, Helmholtz had found in the results of these experiments a 'surprising constancy'. As he reported to the Berlin Physical Society on December 15, 1850, the results of repeated experiments in different subjects oscillated between a mean of 0.12 and 0.20 seconds.
Within each series, however, there was only a 'probable error' of 0.003 seconds (Helmholtz, 1850a, p. 2) The relative homogeneity of measurements seemed to be a good precondition for deducing the duration of single aspects of the reaction process from the 'sums' of reaction times Helmholtz obtained. In other words, the Königsberg physiologist could start systematically to change single parameters of the experimental set up, holding the other parameters constant or at least assuming that they remained constant.
Reference: Schmidgen, Henning. 2003. Helmholtz's "Psychological" Time Experiments.. The Virtual Laboratory (ISSN 1866-4784), http://vlp.mpiwg-berlin.mpg.de/references?id=art10&page=p0002