Harvard Psychological Studies Part 76

Number 5 7.1 " 25 " Female.

Number 6 7.8 " 42 " Male.

For most of these frogs a one-cell stimulus was near the threshold, and consequently the reaction time is extremely variable. In Table X.

an a.n.a.lysis of the reactions according to the number of repet.i.tions of the stimulus requisite for a motor reaction has been made. Numbers 1 and 5 it will be noticed reacted most frequently to the first stimulus, and for them 48 satisfactory records were obtained; but in case of the others there were fewer responses to the first stimulus, and in the tabulation of series 1 (Table XI.) averages are given for less than the regular sets of 24 reactions each.

TABLE X.

a.n.a.lYSIS OF REACTIONS TO ONE-CELL STIMULUS.

Frog. Reactions to To 2d. To 3d. To 4th. To 5th. More. Total No.

first Stimulus. of Reactions.

1 53 2 1 0 0 1 57 2 20 12 5 5 4 12 58 4 31 15 1 0 2 8 57 5 51 11 1 2 0 1 66 6 45 15 6 3 1 5 75 Totals, 200 55 14 10 7 27 313

Table XI. is self-explanatory. In addition to the usual averages, there is given the average for each half of the sets, in order that the effect of fatigue may be noted. In general, for this series, the second half is in its average about one third longer than the first half. There is, therefore, marked evidence of tiring. The mean reaction time for this strength of stimulus is difficult to determine because of the extremely great variations. At one time a subject may react immediately, with a time of not over a fifth of a second, and at another it may hesitate for as much as a second or two before reacting, thus giving a time of unusual length. Just how many and which of these delayed responses should be included in a series for the obtaining of the mean reaction time to this particular stimulus is an extremely troublesome question. It is evident that the mode should be considered in this case rather than the mean, or at least that the mean should be gotten by reference to the mode. For example, although the reaction times for the one-cell stimulus vary all the way from 150[sigma] to 1000[sigma] or more, the great majority of them lie between 200[sigma] and 400[sigma]. The question is, how much deviation from the mode should be allowed? Frequently the inclusion of a single long reaction will lengthen the mean by 10[sigma] or even 20[sigma].

What is meant by the modal condition and the deviation therefrom is ill.u.s.trated by the accompanying curve of a series of reaction times for the electric stimulus of group I.

__________________________________________________________________________ _8_|______________________________________________________________________ _7_|_____________________________________|________________________________ _6_|_____________________________________|________________________________ _5_|_____________________________________|________________________________ _4_|________________________________|____|____|___________________________ _3_|____________|___________________|____|____|___________________________ _2_|_______|____|____|_________|____|____|____|____|______________________ _1_|__|____|____|____|_________|____|____|____|____|____|____|____|____|__ 100 110 120 130 140 150 160 170 180 190 200 210 220 230

The column of figures at the left indicates the number of reactions; that below the base line gives the reaction times in cla.s.ses separated by 10[sigma]. Of thirty-one reactions, seven are here in the cla.s.s 170[sigma]. This is the model cla.s.s, and the mean gotten by taking the average of 31 reactions is 162[sigma]. If the mode had been taken to represent the usual reaction time in this case, there would have been no considerable error. But suppose now that in the series there had occurred a reaction of 800[sigma]. Should it have been used in the determination of the mean? If so, it would have made it almost 30[sigma] greater, thus removing it considerably from the mode. If not, on what grounds should it be discarded? The fact that widely varying results are gotten in any series of reactions, points, it would seem, not so much to the normal variability as to accidental differences in conditions; and the best explanation for isolated reactions available is that they are due to such disturbing factors as would decrease the strength of the stimulus or temporarily inhibit the response. During experimentation it was possible to detect many reactions which were unsatisfactory because of some defect in the method, but occasionally when everything appeared to be all right an exceptional result was gotten. There is the possibility of any or all such results being due to internal factors whose influence it should be one of the objects of reaction-time work to determine; but in view of the fact that there were very few of these questionable cases, and that in series I, for instance, the inclusion of two or three reactions which stood isolated by several tenths of a second from the mode would have given a mean so far from the modal condition that the results would not have been in any wise comparable with those of other series, those reactions which were entirely isolated from the mode and removed therefrom by 200[sigma] have been omitted. In series I alone was this needful, for in the other series there was comparatively little irregularity.

The results of studies of the reaction time for the one-cell electric stimulus appear in Table XI. The first column of this table contains the average reaction time or mean for each subject. Nos. 2 and 4 appeared to be much less sensitive to the current than the others, and few responses to the first stimulus could be obtained. Their time is longer than that of the others, and their variability on the whole greater. Individual differences are very prominent in the studies thus far made on the frog. The one-cell stimulus is so near the threshold that it is no easy matter to get a mean which is significant. Could the conditions be as fully controlled as in human reaction time it would not be difficult, but in animal work that is impossible. No attempt has thus far been made to get the reaction time in case of summation effects except in occasional instances, and in so far as those are available they indicate no great difference between the normal threshold reaction and the summation reaction, but on this problem more work is planned.

There are large mean variations in Table XI., as would be antic.i.p.ated.

Since the reactions were taken in sets of 24, the means of each set as well as that of the total are given, and also, in columns 4 and 5, the means of the first half and the last half of each set.

A comparison of Tables XI., XII. and XIII. makes clear the differences in reaction time correlated with differences in the strength of an electric stimulus. For Table XI., series I, the relative value of the stimulus was I; for Table XII., series 2, it was 2, and for Table XIII., series 3, it was 4. Throughout the series from I to 3 there is a rapid decrease in the reaction time and in the variability of the same. The reaction time for stimulus I, the so-called threshold, is given as 300.9[sigma]; but of the three it is probably the least valuable, for reasons already mentioned. The mean of the second series, stimulus 2, is 231.5[sigma] while that of the third, stimulus 4, is only 103.1[sigma]. This great reduction in reaction time for the four-cell stimulus apparently shows the gradual transition from the deliberate motor reaction, which occurs only after complex and varied central neural activities, and the purely reflex reaction, which takes place as soon as the efferent impulse can cause changes in the spinal centers and be transmitted as an afferent impulse to the muscular system.

TABLE XI.

ELECTRICAL STIMULUS REACTION TIME. SERIES 1.

Average Average of Average Average Mean Var Frog. of all. Mean Var. Sets. of 1st h. of 2d h. of Sets.

1 238.5* 33.3* 216.0* 205.6* 226.7* 33.2*

261.0 248.0 274.1 33.3 2 458.0 219.0 458.0 270.4 643.8 219.0 4 273.4 59.9 273.4 245.7 301.1 59.9 5 263.9 50.5 268.6 244.7 292.5 44.9 259.2 236.0 282.4 56.1 6 271.1 65.1 322.6 273.2 372.0 87.9 219.6 208.5 230.6 42.3 Gen Av. 300.9 85.5 300.9 244.8 356.8 85.5

Totals.

For No. 1 the averages are for 2 sets of 24 reactions each, 48 " 2 " " one set of 12 " " 12 " 4 " " one set of 24 " " 24 " 5 " " two sets of 24 " " 48 " 6 " " two sets of 24 and 12 reactions, respectively, 36

*Transcriber's Note: All values in [sigma], 1/1000ths of a second.

TABLE XII.

ELECTRICAL STIMULUS REACTION TIME. SERIES 2.

Average Average of Average Average Mean Var Frog. of all. Mean Var. Sets. of 1st h. of 2d h. of Sets.

1 227.3* 33.7* 229.4* 209.1* 249.6* 25.5*

225.2 207.3 243.0 42.1 2 240.1 30.9 239.0 222.3 255.1 29.0 241.3 220.2 262.4 32.8 4 270.3 56.5 298.5 285.3 311.4 62.8 242.2 206.0 278.4 50.2 198.5 26.2 195.0 197.5 193.0 33.5 202.0 195.2 209.0 18.8 6 224.4 24.4 221.6 209.7 233.7 23.6 227.2 213.5 241.0 25.1 Gen. Av. 231.5 34.3 231.0 216.6 246.6 34.3

For No. 5 the averages are for two sets of 18 each; for all the others there are 24 in each set.

*Transcriber's Note: All values in [sigma], 1/1000ths of a second.

TABLE XIII.

ELECTRICAL STIMULUS REACTION TIME. SERIES 3.

Average Average Average Average Mean Var.

Frog. of all. Mean Var. of all. of 1st h. of 2d h. of Sets.

1 93.6* 13.5* 91.8* 93.2* 90.4* 13.5*

95.4 91.8 99.0 13.5 2 99.9 12.8 92.2 89.4 95.0 17.4 107.5 105.9 109.0 8.2 4 125.2 16.3 113.5 106.5 120.5 13.6 136.0 135.7 138.2 19.0 5 94.4 8.0 88.6 90.5 88.6 8.2 100.2 97.8 102.7 7.9 6 102.5 12.2 104.2 98.6 109.9 12.8 100.9 101.0 108.3 11.6 Gen. Avs. 103.1 12.5 103.1 101.0 105.9 12.5

For each animal there are two sets of 24 reactions each.

*Transcriber's Note: All values in [sigma], 1/1000ths of a second.

The spinal reflex for a decapitated frog, as results previously discussed show, is approximately 50[sigma]; and every time the four-cell stimulus is given this kind of a reaction results. There is a slight twitch of the legs, immediately after which the animal jumps.

Now for all these series the thread was slackened by one eighth of an inch, but the reflex time was determined without this slack.

Calculation of the lengthening of the reaction time due to the slack indicated it to be between 20 and 30[sigma], so if allowance be made in case of the reactions to the four-cell stimulus, the mean becomes about 70[sigma], or, in other words, nearly the same as the spinal reflex. The conclusion seems forced, therefore, that when a stimulus reaches a certain intensity it produces the cord response, while until that particular point is reached it calls forth central activities which result in much longer and more variable reaction times. It was said above that the series under consideration gave evidence of the gradual transition from the reflex to the volitional in reaction time.

Is this true, or do we find that there are well-marked types, between which reactions are comparatively rare? Examination of the tables VII., VIII., IX., XI., XII. and XIII. will show that between 70[sigma]

and 150[sigma] there is a break. (In tables XI., XII. and XIII., allowance must always be made for the slack in the thread, by subtracting 30[sigma].) All the evidence furnished on this problem by the electrical reaction-time studies is in favor of the type theory, and it appears fairly clear that there is a jump in the reaction time from the reflex time of 50-80[sigma], to 140 or 150[sigma], which may perhaps be taken as the typical instinctive reaction time. From 150[sigma] up there appears to be a gradual lengthening of the time as the strength of the stimulus is decreased, until finally the threshold is reached, and only by summation effect can a response be obtained.

The most important averages for the three series have been arranged in Table XIV. for the comparison of the different subjects. Usually the reaction time for series 3 is about one half as long as that for series 2, and its variability is also not more than half as large. In the small variability of series 3 we have additional reason for thinking that it represents reflexes, for Table IX. gives the mean variation of the reflex as not more than 8[sigma], and the fact that the means of this series are in certain cases much larger is fully explained by the greater opportunity for variation afforded by the slack in the thread.

TABLE XIV.

MEANS, ETC., FOR EACH SUBJECT FOR THE THREE SERIES. (TIME IN [sigma])

Mean First Second Mean Frog.

Half. Half. Variation.

Series 1 238.5 226.8 259.4 33.3 Series 2 227.3 208.2 246.3 33.7 No. 1 Series 3 93.6 92.5 94.7 13.5

Series 1 458.0 270.4 643.8 219.0 Series 2 240.1 221.2 258.8 30.9 No. 2 Series 3 99.9 97.6 102.0 12.8

Series 1 273.4 245.7 301.1 59.9 Series 2 270.3 245.6 294.9 56.5 No. 4 Series 3 125.2 121.1 129.3 16.3

Series 1 263.9 240.4 287.4 50.5 Series 2 198.5 196.4 201.0 26.2 No. 5 Series 3 94.4 94.2 94.7 8.0

Series 1 271.1 240.8 301.3 65.1 Series 2 224.4 211.6 237.3 24.4 No. 6 Series 3 102.5 99.8 109.1 12.2