The Dancing Mouse: A Study in Animal Behavior

by Robert Mearns Yerkes

CHAPTER XVI

THE DURATION OF HABITS: MEMORY AND RE-LEARNING

The effects of training gradually disappear. Habits wane with disuse. In the dancer, it is not possible to establish with certainty the existence of memory in the introspective psychological sense; but it is possible to measure the efficiency of the training to which the animal is subjected, and the degree of permanency of habits. The materials which constitute this chapter concern the persistence of unused habits, and the influence of previous training on the re-acquisition of a habit which has been lost or on the acquisition of a new habit. For convenience of description, I shall refer to certain of the facts which are to be discussed as facts of memory, with the clear understanding that consciousness is not necessarily implied. By memory, wherever it occurs in this book, I mean the ability of the dancer to retain the power of adaptive action which it has acquired through training.

I first discovered memory in the dancer, although there was previously no reason for doubting its existence, in connection with the ladder-climbing tests of Chapter XII. In this experiment two individuals which had perfectly learned to escape from the experiment box to the nest-box by way of the wire ladder, when tested after an interval of two weeks, during which they had remained in the nest-box without opportunity to exercise their newly acquired habit, demonstrated their memory of the method of escape by returning to the nest-box by way of the ladder as soon as they were given opportunity to do so. As it did not lend itself readily to quantitative study, no attempts were made to measure the duration of this particular habit. At best the climbing of a wire ladder is of very uncertain value as an indication of the influence of training.

Similarly, the persistence of habits has been forced upon my attention day after day in my various experiments with the mice. It is obvious, then, that the simple fact of memory is well established, and that we may turn at once to an examination of the facts revealed by special memory and re- learning experiments.

The visual discrimination method, which proved invaluable as a means of measuring the rapidity of habit formation, proved equally serviceable in the measurement of the permanency or duration of habits. Memory tests for discrimination habits were made as follows. After a dancer had been trained in the discrimination box so that it could choose the correct electric-box, white, red, blue, or green as it might be, in three successive daily series of ten tests each, it was permitted to remain for a certain length of time without training and without opportunity to exercise its habit of visual discrimination and choice. At the expiration of the rest interval, as we may designate the period during which the habit was not in use, the mouse was placed in the discrimination box under precisely the same conditions in which it had been trained and was given a series of ten memory tests with the box to be chosen alternately on the right and on the left. In order that the entire series of ten tests, and sometimes two such series given on consecutive days, might be available as indications of the duration of a habit, the mouse was permitted to enter and pass through either of the electric-boxes without receiving a shock. Had the shock been given as punishment for a wrong choice, it is obvious that only the first test of the memory series would be of value as an indication of the existence of a previously acquired habit. Even under the conditions of no shock and no stop or hindrance the first test of each memory series is of preeminent importance, for the mouse tends to persist in choosing either the side or the visual condition (sometimes one, sometimes the other) which it chooses in the first test. If the wrong box is chosen to begin with, mistakes are likely to continue because of the lack of punishment; in this case the animal discriminates, but there is no evidence that it remembers the right box. Likewise, if the right electric- box is chosen in the first test, correct choices may continue simply because the animal has discovered that it can safely enter that particular box; again, the animal discriminates without depending necessarily upon its earlier experience. I have occasionally observed a series of ten correct choices, made on the basis of an accidental right start, followed by another series in which almost every choice was wrong, because the animal happened to start wrong.

As the results of my tests of memory are of such a nature that they cannot advantageously be averaged, I have arranged in Table 48 a number of typical measurements of the duration of visual discrimination habits. In this table I have indicated the number and age of the individual tested, the habit of discrimination which had been acquired, the length of the rest interval, the result of the first test (right or wrong), and the number of errors made in each series of ten memory tests.

TABLE 48

MEASUREMENTS OF THE DURATION OF A HABIT

Memory

ERRORS NO. AGE NAME OF TEST REST FIRST FIRST SECOND INTERVAL CHOICE SERIES SERIES

1000 25 weeks White-black 4 weeks Right 0
5 27 White-black 4 Right 5 7
210 15 White-black 8 Right 5
220 15 White-black 8 Right 4
230 15 White-black 8 Wrong 5
215 15 White-black 8 Right 5
225 15 White-black 8 Right 2
235 15 White-black 8 Right 7
410 15 White-black 8 Wrong 4
415 15 White-black 8 Wrong 6
420 15 White-black 8 Wrong 3
425 15 White-black 8 Right 3
2 28 Black-white 4 Wrong 9
7 17 Black-white 2 Wrong 1
7 21 Black-white 6 Right 1
7 27 Black-white 10 Right 1 6
998 18 Black-white 2 Wrong 3
998 22 Black-white 4 Right 0
998 28 Black-white 10 Right 5 5
13 10 Black-white 4 Right 3
14 10 Black-white 4 Right 3
15 10 Black-white 4 Right 2
16 10 Black-white 4 Right 4
1000 25 Light blue-orange 4 Right 4
2 28 Light blue-orange 2 Wrong 5
5 28 Light blue-orange 6 Wrong 4 6
3 25 Light blue-orange 4 Wrong 8
10 24 Light blue-orange 2 Right 8
10 26 Light blue-orange 2 Right 5
11 25 Light blue-orange 2 Right 6
11 27 Light blue-orange 2 Wrong 5
151 13 Green-red 2 Right 1 0
152 13 Green-red 2 Right 5 1

This quantitative study of the duration of simple habits of choice showed that in the majority of cases a perfectly acquired habit persists for at least two weeks. To be perfectly fair to the animal I must restrict this statement to visual conditions other than colors, for the dancer exhibited little ability either to acquire or to retain a habit of distinguishing spectral colors. Altogether, I made a large number of white-black and black-white memory tests after rest intervals of four, six, eight, or ten weeks. The results for the four-week interval show extreme individual differences in memory. Number 1000, for example, was able to choose correctly every time in a series of white-black tests after a rest interval of four weeks, whereas No. 5 was wrong as often as she was right after the same interval. I have placed the results for these two individuals at the head of the table because they suggest the variations which render averages undesirable. Number 1000 had a perfect habit at the end of four weeks of disuse; No. 5 had no habit whatever. I shall reserve further discussion of age, sex, and individual differences in the permanency of habits for the next chapter.

With Nos. 7 and 998 memory tests were made after three different rest intervals. At the end of two weeks the black-white habit was present in both individuals, although it was not perfect. After six and four weeks, respectively (see Table 48), it still persisted; in fact, it apparently had improved as the result of additional training after the earlier memory tests. At the expiration of ten weeks it had wholly disappeared. In her first series of memory tests after the ten-week interval No. 7 made only one error, but a chance choice of the black (right) in the first test and the subsequent choice of the box in which no shock had been received serve to account for results which at first appear to be indicative of memory. That this explanation is correct is proved by the fact that a second memory series, in which the first choice happened to be wrong, resulted in six mistakes. Evidently she had lost the habit.

In no instance have memory tests definitely indicated the presence of a habit after a rest interval of more than eight weeks. It is safe, therefore, to conclude from the results which have been obtained that a white-black or black-white discrimination habit may persist during an interval of from two to eight weeks of disuse, but that such a habit is seldom perfect after more than four weeks.

The measurements of memory which were made in connection with color discrimination experiments are markedly different from those which were obtained in the brightness tests. As might have been anticipated (?), in view of the extreme difficulty with which the dancer learns to discriminate colors, the habit of discriminating between qualitatively different visual conditions does not persist very long. I have never obtained evidence of a perfect habit after an interval of more than two weeks, and usually, as is apparent from Table 48, the tests indicated very imperfect memory at the end of that interval. It seems probable that even in these so-called color tests discrimination is partly by brightness difference, and that the imperfection of the habit and its short duration are due to the fact that the basis of discrimination is inadequate. This is the only explanation which I have to offer for the difference which has been demonstrated to exist between the duration of brightness discrimination habits and color discrimination habits.

The duration of a discrimination habit having been measured with a fair degree of accuracy, I undertook the task of ascertaining whether training whose results have wholly disappeared, so far as memory tests are in question, influences the re-acquisition of the same habit. Can a habit be re-acquired with greater facility than it was originally acquired? Is re- learning easier than learning? To obtain an answer to the question which may be asked in these different forms, ten individuals were experimented with in accordance with a method whose chief features are now to be stated. In each of these ten individuals a perfect white-black habit was established by the use of the standard series of tests the order of which is given in Table 12. At the expiration of a rest interval of eight weeks precisely the same series of tests were repeated as memory and re-training tests. In this repetition, the preliminary series, A and B, served as memory tests, and the subsequent training series, as re-training series.

[Illustration: FIGURE 32.—Error curves plotted from the data given by ten dancers in white-black discrimination tests. The solid line ([Symbol: solid line]) is the error curve of the original learning process; the broken line (———) is that of the re-learning process, after an interval of eight weeks.]

The striking results of this investigation of re-learning are exhibited in the curves of learning and re-learning of Figure 32. These curves make it appear that the mice re-acquired the white-black discrimination habit much more readily than they had originally acquired it. But in addition to furnishing the basis for some such statement as the foregoing, the curves suggest a serious criticism of the experiment.

In the original tests, the preliminary series indicated a strong preference for black. In series A it was chosen on the average 5.8 times in 10, and in series B, 5.7 times. This preference was rapidly overcome by the training series, and at the end of 130 tests discrimination was perfect. All this appears in the curve of learning (solid line of figure). On the other hand, these preliminary series when repeated as memory tests, after a rest-interval of eight weeks, gave markedly different results. Series A indicated preference for white (5.6 times in 10) instead of black, and series B indicated only a slight preference for black. In brief, series A and B show that the preference for black was considerably stronger at the beginning of the training than at the beginning of the re-training.

In the light of these facts it is fair to claim that the effects of the white-black training had not wholly disappeared as the result of eight weeks of rest, and that the experiment therefore fails to furnish satisfactory grounds for the statement that re-learning occurs more rapidly than learning. I accept this criticism as pertinent, although not necessarily valid, and at the same time I freely admit that the results have a significance which I had not anticipated. But they are not less interesting or valuable on that account. Granting, then, that at least some of the ten individuals which took part in the experiment had not completely lost the memory of their white-black training at the end of eight weeks, it is still possible that an examination of the individual results may justify some conclusion concerning the question which was proposed at the outset of the investigation. Such an examination is made possible by Tables 49 and 50, in which I have arranged separately the results for the males and the females.