The effect of drugs upon the higher mental functions can only be ascertained satisfactorily in man. These functions vary in complexity from simple choice to the highest efforts of genius.

The effect of drugs upon the time required for mental processes is observed by ascertaining, first, the time required for the performance before and after the administration of a drug, and comparing these two times with one another.

The processes generally investigated are, (a) the time required for simple reaction; (b) for discrimination; (c) for decision. The simple reaction is ascertained by marking on a chronograph the time when a signal is made, such as, for example, the exhibition of a coloured flag. As soon as this is seen by the individual experimented upon he marks the time upon the same chronograph by placing a ringer upon a key which is connected with the registering electro-magnet. The difference of time between the exhibition of the flag and the time registered by the electro-magnet is equal to the time required for the transmission of the sensory impulse to the brain, for its transmission from the sensory to the motor tracts of the brain, for its passage down the motor nerves, and the latent period of the muscles.

The time required for selection is ascertained in the same way, but either a red or blue flag may be shown, and the person experimented upon has to discriminate between them, and only to press when the one previously agreed upon is shown. The difference between the time of this experiment and the former gives the time required for discrimination.

The time required for decision is ascertained in the same way as the previous one, excepting that a different signal is to be made on the appearance of the red and of the blue.

Simple reaction has been found by Kraepelin 1 to be little affected by nitrite of amyl: sometimes it is a little quicker and sometimes a little slower than normal. It is rendered slower by ether and much slower by chloroform, although exceptionally it may be quickened by chloroform, probably when used in small doses.

The time required for discrimination is not definitely affected by nitrite of amyl, being sometimes increased and sometimes diminished. It is generally increased, though it may be diminished, by small doses of ether and also by chloroform.

The time for decision is sometimes increased and sometimes diminished by nitrite of amyl. It is increased by ether and also by chloroform; and if the quantity given be great, the increase may be very large.

1 Kraepelin, Ueber die Einwirkung einiger medicamentosen Stoffe auf die Dauer einfacher psychischer Vorgange, 1882. Abstract in Rivista Sperimentale di Freniatria, anno ix. 1883, p. 124.

The influence of alcohol upon psychical processes is curious; for while it renders them much slower, the individual under its influence believes them to be much quicker than usual.