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A First Report on A Test of Mcdougall’s Lamarckian Experiment on the Training of Rats

1935; The Company of Biologists; Volume: 12; Issue: 3 Linguagem: Inglês

10.1242/jeb.12.3.191

1477-9145

W. E. Agar, F. H. Drummond, O. W. Tiegs,

Gene Regulatory Network Analysis

This paper presents a first report on an experiment closely similar to McDougall’s well-known experiment on the inheritance of the effects of training in rats, which has furnished what is probably the most important evidence of Lamarckian inheritance yet produced. McDougall’s reports on this experiment, which is still in progress, extend to the 34th generation of training, and his figures appear to afford strong evidence of an increasing facility in learning the particular task as the generations go on. McDougall’s three reports (the third in collaboration with Rhine) were published in 1927, 1930 and 1933, and will be referred to hereafter as I, II and III.The experiment is so well known that it is only necessary to recall its nature very briefly. The rats are placed in a tank containing water, from which they can escape by either of two sloping gangways leading to a platform above the water level. In each trial, one of the two gangways is brightly illuminated by an electric light behind it, and the rat has to learn to escape always by the other (less brightly illuminated) exit, the incentive to do so being provided by the fact that it receives an electric shock every time it climbs the illuminated gangway. Following McDougall, we will refer to these as the bright and dim gangways. The apparatus and procedure are described below in connection with our own experiments.Facility in learning is measured by the number of errors (choices of the bright gangway) made by the rat before it learns to avoid this and always choose the dim one.In McDougall’s experiment the average number of errors per rat, and the number of errors made by the best and worst rat, of each generation show a marked, though somewhat irregular, decline from about generation 13 to 34 (the last generation so far reported upon). The evidence is summarised in more detail at the end of this paper.Our own experiment has only reached the 6th generation at present, but owing to the great interest which has naturally been aroused by McDougall’s experiment, and the possibility of other persons undertaking to repeat it, we have thought it well to publish this first report covering the first five generations in detail, setting out our methods and the general results so far obtained. We should welcome any criticisms or suggestions as to future procedure.We are aware of one other attempt to repeat the experiment—that of Crew (1932) at the University of Edinburgh. A preliminary report dealing with the first five generations of this experiment was published during the course of our own experiments, and shows no Lamarckian effect so far.Our tank is of the same general type as that described and figured by McDougall in his last two reports. It measures about 50 · 48 cm. (exclusive of the landing platforms), one end being curved. It is divided into three parallel passages by two partitions which stop short of the far (curved) end of the tank. At this end, therefore, the three passages communicate. From the blind (near) ends of the two side passages, steeply sloping wire gangways lead to a platform above the water-level. Behind each gangway is a sheet of ground glass, and behind each sheet is an electric lamp which illuminates the gangway and shines through it down the whole length of the passage so that the rat while still in the central passage can see which side passage is illuminated. The gangways are connected with an alternating current in such a way that the rat receives the current on mounting the gangway. By means of a double switch, the lighting and shocking currents can be thrown together into either gangway, so that the bright gangway is always also the shocking one.A rat is placed in the blind end of the central passage. On finding itself in the water, it seeks to escape. Swimming down the central passage it turns at the end either into the right or left side-passage, swims back along this to the gangway, climbs it and so escapes from the water. Each rat is given six trials a day, except during the first five days when it receives only four. In this respect our procedure differs from McDougall’s, who gives six trials a day throughout. We found, however, that six runs seemed to tax the strength of the very young rats. Crew gives six trials a day from the start, but begins training when the rats are seven weeks old, instead of four, the age at which McDougall and we start training.On the first trial of each day, the left-hand passage is illuminated and electrified, on the second the right, and so on alternately for the six (or four) trials. The rat has to learn to avoid taking the bright gangway and to choose the dim one. McDougall trained his rats in batches, generally of six, there being frequently as many as three rats in the water at once (I, 286 and cf. III, 226). Throughout our experiment, however, we have never had more than one rat in the tank at a time, thus avoiding the chance of the behaviour of one rat being influenced by that of another, and also allowing every run of every rat to be recorded on a separate scoring card for each rat.We have adopted McDougall’s criterion of learning—namely, the performance of 12 consecutive runs correctly, that is to say, to the dim gangway.McDougall found (II, 211) that the number of experiences required to learn the task was much fewer with a strong than with a weak shock. It is therefore essential to keep the shock as uniform as possible. McDougall used an interrupted current from a secondary coil, but as he states, and as is well known, the current from such a source is variable owing to the variable behaviour of the interrupter, etc. (II, 211). McDougall indeed evidently readjusted the strength of the shock empirically from time to time. He says (III, 214): “After much consideration of this problem of standardising the shock delivered to the rat, I am of the opinion that the best one can do is to adjust the secondary coil in such a way that the current is strong enough to tetanise the muscles of the rat’s legs and hold him fast, in, as nearly as possible, 50 per cent, of all contacts made by any batch of rats.”To diminish as far as possible disturbance due to variation of current we decided to employ the alternating current supplied to the laboratory through the electric mains. At first we cut down the 230 volts at which it is delivered to 26 by means of a transformer. We were, however, met by a difficulty which McDougall also encountered, though we suffered in higher degree than he, doubtless owing to higher amperage. This trouble was a tendency to electrocute the rats, or sometimes to paralyse their hindlegs. Like McDougall, we found that this is apt to happen when a rat makes contact with the gangway with its nose.As the result of a considerable mortality from this cause, we decided to abandon what we had meant to be the first generation of our experiment, and to begin again with an improved shocking apparatus.It is clear that if the rats differ in innate ability to master the task, a mortality from such a cause will act selectively, since the more stupid rats are exposed to the risk more frequently. McDougall states (II, 204) that from the 14th generation onward (i.e. to the 23rd, the last generation dealt with in the second report) among the rats that entered upon training in each generation were a small number (not more than three of any generation) whose training was not completed owing in nearly every case to the rat receiving a shock which incapacitated him. Later, the mortality from this was evidently less, for he states in his third report (p. 233) that of the last 208 rats trained only three were killed, and “This is a fair sample of the whole.” Taking also into account that electrocution occurs almost without exception in the very early stages of training, so that the quicker learners do not escape the danger period (III, 233), it seems unlikely that this was a significant source of selection in McDougall’s experiments. Crew also lost a number of rats from this cause : “Danger attends every trial in the case of the rats during the first week of training” (p. 124). For instance, he lost in this way two out of the first 24 rats trained.For many reasons it is obviously important both to avoid injury to the rats and to keep the severity of the shock constant, and therefore throughout the experiment we have employed a different form of current. The alternating current from the mains is employed as in our abandoned experiment, but this time at its full 230 volts ; the amperage, however, is reduced to 1 · 2 milliamps. Crew uses the same voltage, and 2 milliamps., but does not aim at constancy of effect, for he uses a resistance which can be thrown in to reduce the shock for a weak rat. Our shock has never caused any injury to the rats. They have not learnt so quickly as Crew’s, no doubt owing to the lower amperage of our shocks. It is hard to compare the rate of learning of our rats with that of McDougall’s, since most of our rats seem to learn more quickly than most of McDougall’s, but our slowest ones are slower than his.As with such a low amperage it is necessary to minimise leakage of current, the gangways are constructed as follows. Each is formed of a wooden framework on which is stretched galvanised iron-wire netting of 15 mm. mesh. Each gangway is attached by wooden extensions to the wooden sides of the superstructure of the tank apparatus at two points about 30 cm. above water-level. Below this point it touches nothing, a gap of about a centimetre being left between it and the sides of the apparatus, the landing platform and the water. As the points of attachment are out of danger of being wetted, satisfactory insulation is obtained.One electric wire is soldered on to the wire netting of the gangway, and the other to the metal side of the tank, the circuit being therefore closed when the rat is on the gangway so long as any part of its body remains in the water.Another source of variation in severity of punishment is the length of time during which the rat is subjected to it. In our preliminary experiments we found, as did McDougall, that often a rat will succeed in dashing very quickly up the electrified gangway; on other occasions (about 50 per cent, in McDougall’s case, III, 214) its legs are tetanised and it hangs squealing on the gangway. To quote McDougall: “When a rat is thus held by the current, I count three slowly before breaking the current and releasing the rat.” In view of the great effect which McDougall found strength of shock to exert on the rate of learning, and the obvious relation between the severity of punishment and its duration as well as its intensity, we decided that it was necessary to eliminate variation of duration as far as possible. This is accomplished as follows :After the breaking of the circuit, the glass plate is removed and the rat climbs out.We feel that in this way we have eliminated the most serious sources of variation in the intensity and duration, and therefore in the severity, of the punishment, and made it practically independent of the judgment of the operator. This latter point is specially important when an experiment is the work of collaborators, as is hardly avoidable in a long time-consuming experiment such as this.Throughout our experiment we have encountered a feature in the behaviour of our rats which must introduce an element of great uncertainty into the interpretation of McDougall’s method of estimating the rate of learning by the number of errors made. This is the strong tendency exhibited by the great majority of our rats to form the habit of turning out of the central passage always in the same direction — some rats forming the habit of turning to the right, others to the left.Although McDougall encountered this phenomenon, it may have played a smaller part in his experience than in ours. He states (III, 215) that less than 50 per cent, of his rats form the habit of turning regularly right or left, rather more than half of them taking the right or left turn at random, and about equally often.1 These figures, however, are estimates only, since he did not keep records of individual rats, but only of the total behaviour of batches of about six. Even with our individual records it is difficult to give an actual measure of the frequency of this habit, since the determination of the point at which an excess of right or left turns constitutes a habit is arbitrary. Any reasonable criterion, however, would give a much larger percentage than 50 in our rats.If we adopt as the criterion of the formation of the habit the choice of the same passage 20 or more consecutive times, we find that of the 328 rats of generations 2 − 5, 261 formed a right or left habit, 67 failed to do so. The latter were mostly rats that learnt very quickly, and include none that required more than 47 shocks. The early learning and the failure to form the habit quickly are undoubtedly causally connected. Our rats are albinos, and, like those used by McDougall and Crew, of pure Wistar origin. (We are indebted to the University of Sydney for providing the few pairs from which we started our colony.) We have, however, also tested a number of rats from a different source, consisting of pink-eyed creams (31), hooded (1) and black (3), some being trained in the usual way, others tested with the alternating light but no shocks. Of these 35 rats, all but one (which learnt early) showed a pronounced right or left habit.The great majority of our rats have shown a preference for the right-hand passage. We have not been able to discover any asymmetry in the construction of our tank to account for this ; moreover, those rats that form the habit of turning to the left form it as early, and retain it as persistently, as those which prefer the right turn. Of the 261 rats which formed the right or left habit, 212 were right-handers, 44 left-handers, and five after forming the habit in one direction, changed over and took to turning always to the other side. Among the cream and coloured rats, 23 were right-handers and 11 left-handers. Crew mentions that among his rats, also, there were rather more right-than left-handers.The right or left habit may not become strongly established till after a few days of irregular orientation; once established, however, it is very strong, and many of the rats give the impression that they do not know, or in consequence of their prolonged habit of going to the one side, have forgotten, that there is an alternative way of escape. Long after fear of the bright passage has developed, they continue to take whichever passage they have formed the habit of entering, though their behaviour is markedly different on the alternate occasions when this is illuminated and when it is not. While the behaviour of the rats is characteristically individual, a very common type of behaviour at a certain stage of training is as follows. On the occasion when the familiar passage is not illuminated the rat will swim steadily and quickly down the central passage, turn into the side passage, swim back along it and climb the gangway. When it is illuminated, the rat hesitates in the central passage, perhaps swims up to the entrance of the illuminated passage but, instead of entering it, turns back into the central passage again. As much as 25 min. has been spent in this way before the rat makes up its mind to face the shock.Fortunately, few rats which pass through this “hesitating” stage exhibit it in as marked degree as this, or the experiment would be impracticable on anything but a very small scale. As it is, we have found it necessary in the case of a small proportion of our rats—perhaps one in 50—to stimulate them to leave the central passage by gently submerging them for a few seconds.None of our rats have shown the phase mentioned by McDougall in II, 216. In the phase of hesitation “both gangways evoke the impulse of retreat, and the rat oscillates, looks first at one, then turns about and looks at the other, in some cases approaching nearly to each gangway again and again.” Hesitation in our rats has practically been confined to the case of rats with a strong right or left bias, refusing to leave the central passage on the occasions when its accustomed passage is illuminated. The same rat usually shows no hesitation on the alternate occasions when it is not illuminated.Thus we are unable for two reasons to agree fully with McDougall when he states (III, 215) : “The number of errors...made by any rat in the total process of training is accepted as the measure of his facility in learning to effect the discrimination.” Firstly, McDougall is referring to discrimination between the bright and dim passages as presented simultaneously to the rat’s choice. In the case of that large majority of our rats which definitely adopt a right or left habit, the first discrimination to be achieved is the discrimination between the occasions when their chosen passage is illuminated and when it is not—for they are paying.no attention to the other passage. The impression made on the observer is that the alternatives presented to the rat’s mind are not between escape by a bright or a dim passage, but between taking the shock which it knows is awaiting it, or staying in the water. Subsequently, of course, when the process of learning has got past the stage of breaking the right or left habit, the rat has to make the other type of discrimination— and this it generally does very quickly. A rat with a pronounced right or left habit which has once reached the stage of breaking through this habit and escaping by the other passage often makes no further error, and generally only a very few. A striking example of this is rat A 564, which went to the right without a single exception during the first 42 days of training. On the 43rd day, it broke through its right-hand habit at its fourth trial, and did not make another error.Secondly, the period between effecting the discrimination (between the occasions when the accustomed passage is illuminated and when it is not—that is to say, forming the association between the light and the shock) and the achievement of “learning” in the sense of always choosing the dim passage is often very prolonged. Many of our rats which had not learnt after 150 shocks (errors) were recorded as showing the discrimination after 15− 25 shocks. Sometimes the difference between a rat which “learns” after a few errors and one which makes a great many appears to depend upon a trifling chance. For instance, a rat in the hesitating stage may swim down the central passage, but on reaching the point of entry to its accustomed passage (say the right) it swerves away to the left. If it makes a wide swerve, this will carry it into the dim left passage, which it swims down and out by the gangway. Once this has happened, it is rare that more than half a dozen more errors will be made. On the other hand a narrower swerve may bring it back again down the central passage, and it may make 100 more errors before it finds the left gangway.We feel that a large part of the enormous difference in the performances of individual rats is to be explained on these lines. Rats which do not quickly form the right or left habit tend to learn much sooner than those that do. When a rat has formed the habit, factors other than facility in learning influence the time which elapses between discovering the relation between the light and the shock, and attaining the accepted criterion of learning.In McDougall’s 17th generation, out of 11 rats the best made only 9 errors, the worst 147; in the 23rd generation, out of 26 rats the best made 3 and the worst 71 errors ; in the 31 st generation 2 out of the 38 rats made 3 errors apiece, and 1 made 100. In about 350 rats Crew found a variation between o and at least 140 errors. We have found even greater variation in our rats, and our impression is, that this variation is largely due to the factors, not closely related to learning capacity, which we have indicated above.In order to test this interpretation of the rat’s behaviour, we tried out the following procedure in our preliminary experiments. Before each day’s trials, the rat was given a few runs in the apparatus with neither gangway illuminated or electrified. At each run, a clear glass partition was introduced into one of the entrances from the central into the side passages, so that in one run the entrance to the left passage was blocked, in the next that to the right. In this way the rat could not avoid finding out and remembering that there were two alternative routes of escape from the water. It was at once apparent that rats subjected to this “special training” would learn after far fewer shocks than those trained in the standard manner. In the main experiment we have trained 19 rats in this fashion in the 5th generation of controls in line A. The numbers of errors made were as follows :4, 9, 9, 10, 10, 12, 12, 12, 14, 16, 16, 16, 16, 18, 20, 20, 21, 23, 26.In terms of the classes in Tables I and II, this gives the mean class 1·42, which, it will be seen, is much less than any of the means in the tables.We intend, as the experiment proceeds, to make periodic tests of Lamarckian inheritance by this method of training, in addition to the standard method, since there can be no doubt that it gives a better measure of learning capacity.It became apparent during our preliminary experiments that a few of the rats would take an excessive time before achieving the criterion of learning without the assistance of this “special training”. (McDougall mentions (II, 202) that in his earlier generations lack of time prevented him from completing the training of some rats.) The experiment would have been complicated by using, as parents, rats which had failed to learn, and yet not to have done so would have been to exercise selection. Consequently it was decided to use the above method of “special training” on all rats that had not learnt after 52 days of training—that is to say, after 302 trials (four per day for the first five days, six per day after that). As these slow learners were, without exception, rats with a strongly developed right or left habit, this means about 151 errors or shocks. At this stage, therefore, the rats were forced to discover, and become accustomed to, the unaccustomed exit.In the first two generations, “special training” was not applied till after 60 days’ ordinary training (about 175 shocks) with failure to learn, but as only one out of 18 rats which passed the 52nd day without learning learnt before the 60th day, it became apparent that the “special training” might be introduced after the 52nd day without making a significant difference, and this was therefore adopted as the standard practice. The effects of this “special training” (and also, incidentally, the persistence of the right-hand habit) may be illustrated by the history of a particular rat.Rat A 326, in the third generation of the control subline of line A, chose the right and left passages irregularly, and on an equal number of occasions, during the first five days of training. Then it adopted the right-hand passage, and took this every time for the next 47 days—an unbroken sequence of 282 escapes by the righthand passage. By this time it had climbed the bright gangway, and received the shock 152 times. Already on the nth day of training the rat had achieved discrimination between the occasions when its adopted passage was illuminated and when it was not. On the occasions when the light was in the left passage the rat would swim quietly and steadily down the central passage, turn in to the righthand passage, swim back along it and climb the gangway. When the light was on the right, however, it showed a disinclination to leave the central passage, and on one trial on the nth day it turned back twice when on the point of entering the right-hand passage. At intervals throughout the rest of its training, records appear on its score card opposite the occasions when the light was on the right, such as “turned away from light,” “hesitated,” and so on. Nevertheless it did not succeed in rediscovering the left passage which it had used as freely as the right during its first five days of training, but not for some days before it had shown evidence of associating the light with the shock. On the 53rd day, the preliminary compulsory alternating turns were introduced, the rat being forced into the left-hand passage twice. After this it did not make another mistake, choosing always the dim passage whether this was right or left.The effect of this “special training” has in all cases been very rapid. Many rats subjected to it, like A326, have not made another error after having been forcibly made to take the other passage. The first 25 rats which received this “special training”, with an average of 152·16 shocks before being forced into the unaccustomed passage, required an average of only 3·60 additional shocks before achieving the criterion of learning.It appears therefore that the total number of errors made by a rat which has developed a strong right or left habit before it attains the standard of 12 consecutive correct runs depends upon two factors :The principal factors influencing these two main factors seem to be the following :It must not be supposed that the learning process always runs the course so far described. Two other types of behaviour which are fairly frequent are as follows:In both these classes, A and B, the number of errors (shocks received) before fulfilling the criterion of learning does therefore nearly coincide with the number of errors before achieving discrimination.We have found, as did McDougall, that once a rat has made 12 consecutive runs correctly it seldom makes another error. It is clearly a good test of the rat’s understanding of the situation. Our practice has been to give any rat which has reached this standard only two trials a day thereafter (once with the light on the left and once on the right) till the day on which it is mated, except that on any day on which it makes an error in either of these trials it is given the full six. The first 50 of the trained rats which had in this way to fill in a longer or shorter period between learning and mating were given an aggregate of 3948 runs after learning, and made a total of only 70 errors or less than 2 per cent.—one rat being responsible for 11 of them, and two others for six each.We have tested the retention of the effects of training in another way. Twenty rats after mating and producing litters were retested, the interval since their last experience of the tank varying from 93 to 115 days. Nine of these rats showed perfect retention, making their 12 consecutive runs without an error. Six made one error, two made three, one made four, one made five, and one made seven errors, before reaching this point.Our rats have a slight but unmistakable initial bias to the light. This can be shown in the following way.The number of choices of the bright and dim passages was counted for every rat of generations 2− 5, for each of the first three days of training, when they were getting four trials a day.Combining all generations of the trained sublines of lines A and B into one group—the trainees—and combining similarly the controls into another group, we get a total of 176 trainees and 152 controls. If the rats were running completely at random, we would expect on each day 352 choices of the bright gangway by the trainees, and 304 by the controls. The actual numbers of times the bright gangway was chosen by the trainees on the first, second and third days respectively were 396, 384, 384, and by the controls 331, 325, 319—in every case an excess of choices of the bright gangway over expectation. Combining both groups for the whole three days, the expectation, if choice is random, is 1968 choices of the bright gangway. The number found is 2139. Of the 328 rats, 104 went equally often to the bright and dim gangways, 61 went more often to the dim and 163 more often to the bright.The bias to the light is, however, undoubtedly stronger than these figures represent, for it is counteracted by two factors. Firstly, by the third day a very few of the quickest learners are already avoiding the bright gangway from experience. Secondly, the right- and left-hand habit, which in many rats is strongly developed from the beginning, tends to increase the number of rats which take the bright and dim passages equally often, to a figure far above that to be expected on purely random choice. This factor reduces the absolute excess of choices of the bright passage.McDougall also found an initial bias to the bright gangway. He gives his rats six preliminary immersions, before training proper begins, with alternating light but no shocks. Confining himself to these first six immersions, he finds (I, Table V) that 122 control rats took the dim gangway 337 times and the bright gangway 395 times. This gives 54 per cent, of choices of the bright gangway. This is the same as our figure, which, however, is, as we have seen, an understatement.Crew, who also gives his rats six preliminary runs with alternating light but no shocks, concludes from the data for these runs that there is no reason to suppose his rats tend to avoid or seek the light.Although McDougall made a few comparisons between the performances of rats in the trained line and of control rats (without trained ancestry), he used no systematic control, judging his results mainly by the change in the rate of learning which may have taken place over the period of the experiment. This extends in his third report to 34 generations, covering 13 or 14 years. Clearly, however, the only satisfactory way to control the experiment is to train each generation contemporaneously with a batch of rats similar in all other respects except that their ancestors have not been