A Beginner's Psychology

by Edward Bradford Titchener

[§ 55]. The Reaction Experiment.—The reaction experiment comes to us, of all unlikely things, by the road of astronomy. In the old days, before electrical instruments were invented, astronomers used to time the passage of a star across the meridian of their observatory by means of the eye-and-ear method. You can easily imagine the procedure. You have your eye at the ocular of a telescope, the field of which is evenly divided by a number of fine vertical lines. The star enters the field from the right, and crosses to the left; your task is to determine the instant at which it traverses the midmost vertical line, which corresponds with the meridian. A clock is behind you, beating seconds; and you count these seconds, one, two, three, from a given starting-point. If the star passes the meridian exactly on a beat, well and good; you know the time of its passage; if, as ordinarily happens, it passes somewhere between two beats, then you must estimate the time of passage to the nearest tenth of a second. That is the principle of the eye-and-ear method; you watch and listen, and so make your observation.

In the year 1796, the astronomer in charge of the Greenwich Observatory found himself obliged to dismiss an otherwise competent assistant, who in the previous year had fallen into the habit of recording his transits some half-second too late, and had now increased his error to almost a whole second. This unfortunate man was the originator of what came to be known as the personal difference. For it was found that no two astronomers exactly agreed in their recording of times; every observer differed from every other by a certain average amount. So it became customary to take some highly skilled observer as standard, and to refer other observers to him; and that is the origin of the personal equation; the formula A-B = 0.8 sec. means, for instance, that the observer A records a transit, on the average, four-fifths of a second later than the more skilled observer B. What B’s error may have been nobody knows.

We cannot trace the history of the personal difference in detail. It is enough to say that the astronomers, having discovered it, were naturally anxious to get rid of it; and they presently found a way to relieve the observer of the task of listening; he simply pressed a key when the star crossed the line of the meridian, and the time of pressing was recorded automatically. This device did not eliminate the personal difference; but it was methodically of great importance. For the eye-and-ear method had now become, essentially, a method of response to stimulus by movement; and in that form it settled down permanently in the psychological laboratory. The stimulus for the astronomer was the star on the meridian, and the response was the pressure of his finger on a key. But it is clear that the stimulus need not be visual; the observer might just as well respond to a sound or a touch or a taste. It is clear, further, that the response need not be a movement of the hand; the observer may respond, just as well, by movement of the organs of speech, or of the foot, or of lip or eyelid. It is clear, finally, that if we know the actual time at which the stimulus is presented, and the actual time at which the movement of response takes place, we can measure the interval between the two. A little ingenuity makes this possible. If, for instance, the flash of light which serves as stimulus makes an electrical circuit, and the finger-movement in response breaks the circuit; and if an electrical clock is placed in the same circuit; then the clock-hands will begin to move when the flash comes, and will stop when the movement occurs, and we can read off the reaction time from the dial.

In its simplest form, then, the reaction experiment takes shape as follows. We subject the observer to some prearranged form of stimulation (a flash of light, a sharp noise), to which he is to reply by some prearranged movement (perhaps, the slipping of his finger from the button of a telegraph key); and the instruments which we employ are so connected that we can measure the time elapsing between the exhibition of stimulus and the performance of answering movement. The experiment thus has two sides. It gives us numerical results, the reaction times measured in units of our clock, in hundredths or thousandths of a second; but it gives us also a complete impulsive action, which we can observe as often as is necessary for analysis.

For consider the course of the reaction experiment in the light of our typical formula of action! The observer sits down with the intention of moving when he has perceived the stimulus; and he has an idea of the result of his movement, namely, the performance of a reaction experiment. The stimulus is presented; he perceives the object of movement; and slips his finger from the key. He thus perceives the movement itself, and also, by the movement, realises in perception his idea of result. He has performed a complete impulsive action, but an action which, on the mental side, has been thinned out to a manageable degree of simplicity. The mental accompaniment is there; but the intention to move bears upon a single finger, the idea of result is just the idea of completing the experiment, the perception of object is the perception of a simple stimulus, the movement itself is a slight local displacement of a single member; nothing is left out, although the action is reduced to a skeleton. It has thus been made manageable; the mental accompaniments of the movement are not so complex that they baffle observation; and the technique of the experiment is an outline which can be filled in and further complicated in all manner of ways. We may hope that that Greenwich assistant found further employment; but we can hardly, as psychologists, regret that he timed his transits later than he should!

[§ 56]. Sensory and Motor Reaction. —Suppose that you are performing the simple reaction experiment, and that you tell your observers beforehand to react as soon as they perceive the stimulus. You soon find that this instruction is differently interpreted. One observer will prepare to react as soon as he perceives the stimulus; and another, to react as soon as he perceives the stimulus. The difference of emphasis may be brought out by a homely illustration. When the lights are turned on in the evening, it is not uncommon, even in the best regulated families, for a clothes-moth to start up from some corner. You say ‘There’s a moth!’ and clap your hands to kill it. But it escapes; and henceforth you do not trouble to identify it; you clap your hands at anything mothlike that flits across the field of vision; you are set or disposed for the movement. So in the two forms of the simple reaction: some observers tend naturally to make sure of the stimulus, before they move, and others tend naturally to move, as soon as any stimulus has appeared.

We cannot rely, however, upon the natural tendency of the observer, because his attitude is likely to change as the experiment proceeds, and a change of attitude means a disturbance of the experimental conditions. Moreover, there are observers of intermediate tendency, who accent both the ‘perception of stimulus’ and the ‘reaction as soon as,’ and may accent them in different degree. Hence it is necessary to instruct the observers at the outset that they are to perform either a sensory or a motor reaction, that is, that they are to look forward either to the perception of the stimulus or to the execution of the movement. With this preliminary instruction, the sensory reaction takes, on the average and for practised reactors, a tenth of a second longer than the motor, whether the stimulus be a sight, a sound, or a touch. The longer time points, of course, to a more complicated nervous path; and that in turn raises the presumption of a richer mental accompaniment. Observations show, in fact, that only the sensory reaction represents a complete impulsive action; the motor reaction does not fall under our formula.

The main difference—and we have no space for detailed analysis—is this. The instruction for the motor reaction sets up kinæsthetic sensations of strain in the reacting member, principally in the finger; these are contextual processes (p. 118), which carry the meaning ‘You are to react as quickly as possible’; and they are accordingly known as ‘sensations of intended movement.’ They imply that the instruction is already in part fulfilled; the muscles are, from the very first, prepared for the movement that shall end the experiment. Indeed, an observer who is thus instructed will sometimes react prematurely, before the stimulus has appeared, and is also liable to accept as the stimulus any chance stimulus that intervenes, and so to react wrongly. The instruction for the sensory reaction, on the other hand, sets up an expectation of the stimulus; the organism is thus prepared especially for perception; premature and wrong reactions do not occur. The intention to move is present, to be sure, but it is in the background, carried only by the feel of the finger as it lies upon the key, or in more diffuse form by the feel of the extended arm upon the table. We might therefore say that, in the motor reaction, the formula tends to close up on itself, like a telescope; idea of result is always approaching perception of result, and intention of movement is always approaching perception of movement; the perception of object gets squeezed between the two extremes, as these draw together; whereas, in the sensory reaction, the formula is followed in extenso; the mental processes are thinned out, as we have put it, but they are all present, following one another in their regular order. The reaction experiment thus renders the impulsive action manageable, puts it at our disposal for scientific analysis, but also shows that an action, even in its simplest form, will vary with every shift of emphasis in the suggestion (p. 213) which calls it forth.

Let us look, now, at the reaction times, and see if they can be turned to scientific account. So many experiments have been made that we know the average times of reaction, both sensory and motor, to light, sound and touch; and we also know what their average constancy or regularity will be, if the reactor keeps his attitude to the experiments unchanged. The times themselves, and the numerical statement of their constancy, may therefore be used as indexes to the type of reaction, sensory or motor, and to the stability or instability of the reactor’s attitude. They embody, as if in short-hand, the results of oft-repeated observation, and they may henceforth take the place of direct psychological observation when we are asked to decide on the type of reaction or the reliability of the reactor. The psychological observation must, however, come first; we cannot take the reaction-times of children or South Sea Islanders, and at once put them down as sensory or motor or mixed; we must know what the reactors were trying to do, how they understood the instructions given them.

[§ 57]. The Degeneration of Action: From Impulsive to Reflex.—We have now to trace the course of impulsive action, downward to automatic, and upward to deliberative action. If we start out on the downward path, we note that impulsive action by frequent repetition degenerates, first, to what is called sensorimotor or ideomotor action: sensorimotor, if the object is still perceived, as it is in the impulsive action proper (p. 235), and ideomotor, if the perception is replaced by an idea of object. Here the predetermination is a nervous set without any mental correlates; the intention to move has dropped away; and the idea of result is, so to say, incorporated in the perception or idea of object; so that movement follows at once upon this perception or idea. When we sit down at table, for instance, we take up our knife as a thing to cut food with; and when we are dressing, we close our fingers round a button as a thing to fasten a garment with; the movements that we make are predetermined, but not premeditated; the actions are sensorimotor. When, again, it occurs to us, in the midst of our reading, that the mail must have arrived, we ideate the packet of letters as something to be fetched from the mail-box; and when, as we watch the shower, it occurs to us that the cellar hatchway is open, we ideate the hatchway as something to be closed; we act without further thought, and the actions are ideomotor.