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

Helmholtz had to admit, however, that his experiments with humans involved a factor apt to threaten the required constancy of all others, namely the attention of the subject under experimentation. 'Slight feelings of sickness' and 'fatigue' of the experimental subject could significantly disturb the precision measurements, as well as distractions of all kinds: 'If at the time of perceiving the signal the thoughts are occupied with something else, and if the mind has to recall to itself what kind of movement one must carry out, it [the reaction] takes much more time.' (Helmholtz, 1850a, p. 4) At this point, Helmholtz had definitely reached psychological ground, and some sixty years later, attention had become a major topic in experimental psychology and was tested.

Apparatus for testing attention (ca. 1910)