One of the earliest experiments to be introduced into psychology was that on reaction time, conducted as follows: The experimenter tells his "subject" (the person whose reaction is to be observed) to be ready to make a certain movement as promptly as possible on receiving a certain stimulus. The response prescribed is usually a slight movement of the forefinger, and the stimulus may be a sound, a flash of light, a touch on the skin, etc. The subject knows in advance exactly what stimulus is to be given and what response he has to make, and is given a "Ready!" signal a few seconds before the stimulus. With so simple a performance, the reaction time is very short, and delicate apparatus must be employed to measure it. The "chronoscope" or clock used to measure the reaction time reads to the hundredth or thousandth of a second, and the time is found to be about .15 sec. in responding to sound or touch, about .18 sec. in responding to light.

Even the simple reaction time varies, however, from one {23} individual to another, and from one trial to another. Some persons can never bring their record much below the figures stated, while a few can get the time down to .10 sec, which is about the limit of human ability. Every one is bound to vary from trial to trial, at first widely, after practice between narrow limits, but always by a few hundredths of a second at the least. It is curious to find the elementary fact of variability of reaction present in such a simple performance.

What we have been describing is known as the "simple reaction", in distinction from other experiments that demand more of the subject. In the "choice reaction", there are two stimuli and the subject may be required to react to the one with the right hand and to the other with the left; for example, if a red light appears he must respond with the right hand, but if a green light appears, with the left. Here he cannot allow himself to become keyed up to as high a pitch as in the simple reaction, for if he does he will make many false reactions. Therefore, the choice reaction time is longer than the simple reaction time--about a tenth of a second longer.

The "associative reaction" time is longer still. Here the subject must name any color that is shown, or read any letter that is shown, or respond to the sight of any number by calling out the next larger number, or respond to any suitable word by naming its opposite. He cannot be so well prepared as for the simple, or choice reaction, since he doesn't know exactly what the stimulus is going to be; also, the brain process is more complex here; so that the reaction time is longer, about a tenth of a second longer, at the best, than the choice reaction. It may run up to two or three seconds, even in fairly simple cases, while if any serious thinking or choosing has to be done, it runs into many seconds and even into minutes. Here the brain process is very {24} complex and involves a series of steps before the required motor response can be made.

These laboratory experiments can be paralleled by many everyday performances. The runner starting at the pistol shot, after the preparatory "Ready! Set!", and the motorman applying the brakes at the expected sound of the bell, are making "simple" reactions. The boxer, dodging to the right or the left according to the blow aimed at him by his adversary, is making choice reactions, and this type is very common in all kinds of steering, handling tools and managing machinery. Reading words, adding numbers, and a large share of simple mental performances, are essentially associative reactions. In most cases from ordinary life, the preparation is less complete than in the laboratory experiments, and the reaction time is accordingly longer.