Helmholtz's "psychological" time experiments - continued...

The concrete procedure Helmholtz then used was to stimulate the human body in different places (e.g. in the foot and the neck) and to derive the speed of the propagation of the stimulus within the sensory nerves from the differences in the results obtained. Helmholtz assumed that, as to their duration, the brain processes of perceiving and willing do not depend essentially from the position of the skin part that was hit. In this perspective, it seemed to be plausible that the stimulation of different skin regions of the human body only altered 'the first link' of the reaction chain (i.e. the propagation in the sensory nerves) (ibid.).

Helmholtz, 1850a, p. 3 (Detail with Transcription)

The results Helmholtz obtained showed that the propagation of stimulations in the human sensory nerves were approximately twice as fast as those in the frog. As he explained in his communication to the Physical Society, the difference between the time needed to react to a stimulation of the big toe and the time needed to react to a stimulation of the sacrum suggested a stimulation speed of 62.1 meters per second. Comparing the reactions to finger stimulations with those to stimulations of the neck gave a result of 61.0 meters per second (Helmholtz, 1850a, p. 3). Helmholtz concluded that in humans, the 'message of an impression' propagates itself 'to the brain with a speed of circa 60 Meter (180 feet) [per second] and does not differ noticeably at various times' (Helmholtz 1883 [1850], p. 878).