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the experimental method itself. Most such experiments, however, would afford very little information about the character of the retardation, whether concerned with the lower, motor, or higher, associative processes. A purely intellectual retardation we cannot measure directly by any psychological methods at present known, for we always require some form of motor response in addition to the intellectual or associative process. We may first turn attention therefore to the functions which are more primarily motor, attempting to make our measure of retardation, so far as possible, independent of the higher mental processes.

To experimentally isolate mental retardation from motor retardation is no easy task. Almost every experiment in the time relations of voluntary movement involves a conscious process of greater or less complexity, followed by a motor response. While this is not absolutely true (as in simple reaction time in late practice) it is essentially so in the situations with which we have to deal clinically. For rough and ready purposes, some idea can be gained from the comparative degrees of retardation in responses involving different degrees of purely mental effort. Thus if a subject could calculate but slowly, and yet could respond quickly to commands, the indication would be that the retardation was mainly in the nature of a thinking disorder. On the other hand, the more the simple and complex responses were equal in slowness, the greater the likelihood of a retardation principally in the lower, motor centres. The presence of a certain amount of ordinary motor activity does not prove the absence of a motor retardation. The essential point here would be whether the reactive movements are quick or slow, and this does not appear in the ordinary observation of a patient's behavior. The phenomena which seem to be associated with motor retardation in these experiments have appeared most strikingly in cases in which little if any retardation would have been evident to ordinary clinical examination.

But, as has been said, the time of reaction in most psychological experiments is an unsatisfactory measure, since there is not sufficient differentiation between the mental and motor aspects of the symptom. Even such a measure as simple reaction to sound is, at all ordinary stages of practice, seriously vitiated by this difficulty in interpretation. Obviously, the simpler and more independent of the higher centres we make our experimental process, the more exclusively it becomes concerned with the purely motor factors, and a measure of time relations in the motor sphere.

Of those reactions to external situations which are so automatic as to be largely independent of control of the higher centres, the eye movements are the most important. Theoretically, I should consider the ocular pursuit reaction (Diefendorf and Dodge) as altogether the best criterion of retardation by the method of reaction to external stimulus. The present criterion of retardation possesses a good quality in that it is not immediately dependent on reaction to external stimulus, as well as certain minor technical advantages.

The psychological process involved in the successive movements of the tapping test is not clearly made out. It is, however, the verdict of introspection that the successive taps at maximum rate are not each the product of a separate conscious innervation. Nor can we conceive of the process as one of successive simple reactions to the various kinæsthetic sensations that the movements of the test present. The normal initial rate of tapping (about 7 per second) cannot be executed without 14 co-ordinated movements per second, and in reality the process is much more complex. Only the simplest reflexes have such a rate as this; no movement in which a voluntary process shares. A series of taps at the maximum rate is therefore a product of but few volitional processes, which merely give a “set” to which the organism responds by a continued series of reflex responses.

What then determines the rate at which these reflex responses take place? It is limited by the refractory phase of the neuronepaths, but, save in late practice, it is much slower than this; it is a matter mainly of efficiency in co-ordination. Beyond this we must judge purely empirically, by noting the external conditions which affect it. In normal individuals it does not seem to be especially influenced by conditions which are open to introspection. On the other hand, any previous activities tending to produce the Erregung," of Kraepelin, seem to increase the rate. If carried to the point of physical fatigue, the rate is decreased; but it does not seem that what is ordinarily called “ intellectual fatigue" has any marked effect. If the tapping be continued, there results the decrease in efficiency ordinarily described as fatigue. This can hardly be dependent on muscular factors, since it normally is a matter of about 14 per cent within 180 taps, and the isolated muscle would hardly fatigue to rapidly succeeding stimuli in this way.

Nor has it any but a very indirect relation to the sensations of fatigue, which are presumably of muscular origin. The process can only be described as a decreased efficiency of coordination (possibly also to a proportionate lengthening of the period of the refractory phase). Both the absolute rate and the fatigue phenomena of the tapping test seem thus to be determined by obscure conditions of neural tone.

Once the general volition to tap as rapidly as possible is obtained, the minute features of the process go on in relative independence of conscious influence; it is not therefore a function that we should expect to see especially influenced in the presence of a pure thinking disorder. This might affect the time taken to respond to the signal to begin to tap; but once the tapping is begun, its rate is rather a matter of raised or lowered Willensspannung, using this term in a wholly objective sense.

The efficiency of this purely motor responsiveness of the organism, the maximum rate of repeated voluntary movements measures in a relatively unequivocal way. For the rest, it is an ordinary work-curve, such as may be obtained in any continuous psychological experiment, being subject to and reflecting all the influences which affect the curve of work. Fatigue, and the reflex inhibitions resulting from sensations of fatigue, tend to produce a decrease in efficiency. Factors having a favorable influence are the so-called "warming up" processes, as well as any more or less external influences that may spur the organism to greater effort. The work-curve that we finally obtain is the complex of all such influences, and their separate analysis is impossible save on a purely empirical basis. We are not concerned, however, with the measurement of such abstractions as absolute fatigue, warming up, or impulse effects, but with the immediate question of how the individual responds to any experimental conditions demanding the continual exercise of a certain voluntary effort. In every individual and in every measure the factors that determine the course of the Arbeitskurve have a certain way of balancing each other, and the way in which this balance varies in different individuals and under different conditions is the essence of the problem with which we are attempting to deal.

In a previous report, attention was called to certain anomalies in the work-curve especially associated with the retardation of manic-depressive depression.' In the present study certain of the more important of these cases, together with certain additional material, will be more fully presented from the standpoint of clinical analysis, and the correlation of the experimental results with the clinical pictures. Certain points in differential symptomatology also appear.

While the method and general experimental conditions are the same as those described in a previous contribution,' a brief review of them may not be out of place here. The subject begins at a given signal and taps at the maximum rate until a record of 30 seconds is obtained, when he receives a signal to stop. Then follows a 2-minute and 30-second pause, after which a second series is executed, then another 2-minute and 30-second pause, and so on until five series of 30 seconds, each with an intervening pause of 2 minutes and 30 seconds, have been obtained. A similar record of five 30-second series is then made with the other hand. When more than one experiment is made, the hands alternate in precedence from experiment to experiment, except as otherwise specified. In this form the experiment requires about half an hour to make, the greater part of which time is occupied in the pauses. This time can, of course, be much shortened by having the hands alternate from series to series, with a very much shorter pause, say 30 seconds, intervening between the different hands. For the first five or six daily experiments (50 or 60 series) the writer does not believe that this shorter form would, in normal subjects, yield different results from the form with the 2-minute and 30-second pause. In the present subjects, however, many of the points which appear of essential significance would have been much obscured by such a routine, so that this apparently uneconomical form of experiment seems to have more than justified itself.

The experiment is evaluated by counting the number of times

1

Studies in Retardation, Am. J. Psych., XX, 1909, pp. 38-59. ? Am. J. Psych., XIX, 1908, pp. 437-439.

the key is struck and the circuit closed during each of the six 5-second intervals in each 30 seconds' tapping. The precise method of dealing with the data may be illustrated by quoting in full the figures of a single record (five series) with the right hand of a normal individual at the beginning of practice.

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Each of the 30 two-place integers gives the number of taps executed in an interval of 5 seconds. Reading the top line from left to right, we obtain a gradual decrease in the size of the figures, indicating a slowing up in the tapping rate. The right hand figure of three digits, 198, gives the sum of the figures for the six intervals, i. e., the number of taps for the whole 30 seconds. Each successive line may be read in the same way. Considering the two-place integers in column, we obtain in the six figures of two digits and a decimal on the bottom line (36.2, 34.6, 33.8, 33.2, 32.6, 32.6) the average number of taps executed during the first 5 seconds, the second 5 seconds, etc. Naturally, a regular decrease is noted. The figure in heavy-faced type, 203.0, gives the sum of these six interval figures, i. e., the average number of taps executed each time during the five 30-second series. This figure is taken as indicating the “ total efficiency of the function in that record, subject, of course, to the variability of the single series it represents. Six successive 5-second intervals form a series, five 30-second series a record, and the two records of right and left hands constitute the single standard experiment.

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