REACTION TIME. One of the cardinal problems of psychophysics is the measurement of the duration of mental processes. The men tal processes themselves are always inaccessible to direct temporal measurement, but certain physiological-psychological complexes are easily available. These consist in the entire aggregate of phenomena which occur between the recep tion of some stimulus and the conscious, vol untary response to the stimulus. The duration of the aggregate is known as the reaction of time and is measured by various types of chronoscope (q.v.). A simple reaction times is one where the response follows directly upon the stimulation. The length of a simple reaction depends on the sense and mode of reaction employed, and on whether the attention is di rected toward the stimulus or the reaction. The following table gives an idea of the order of magnitude of reaction times: Attention toward Attention towards stimulus. reaction.
Reaction to light .27o seconds .18o seconds Reaction to sound .225 seconds .12o seconds Reaction to pressure .210 seconds .1I0 seconds Besides these simple reactions, reaction ex periments may be performed which demand a recognition or a discrimination before the sub ject can press the reaction key. These experi
ments will require .03 to .05 seconds more than a simple reaction experiment of the same sort with the attention directed toward the stimulus. It may be thought that this is the actual time of the process of recognition. This interpretation is dependent on interpreting the reaction as an ordinary sensory reaction inter rupted by an act of discrimination or cognition. This is in all probability an over-simplification of the facts. Reaction-experiments have also been used to determine the duration of an act of choice and of various types of association. In this latter connection they have been used by some investigators to determine the mental con tent of a subject who is concealing something. Consult James, W., Principles of Psychology' (New York 1890) ; Kiilpe, 'Outlines of Psy chology' (trans. London 1895) ; Pilzecker, A., Lehre von der sinnlichen Aufmerksam keit' (1889); Titchener, (Experimental Psy chology' (New York 1905) ; (Text-book of Psychology' (New York 1909-11) ; Wundt, 'Grundzuge der physiologischen Psychologie (6th ed., Leipzig 190S 111