Harvard Psychological Studies Part 21

If the rod and the disc rotate in opposite directions, the phenomena are changed only in so far as the changed geometrical relations require. For the ratio 1:3 between the two rates, the strobic system has four bands of each color; for 1:2, three bands of each color; while when the two rates are equal, there are two bands of each color, forming a diameter. As would be expected from the geometrical conditions, a system of one band of each color cannot be generated when rod and disc have opposite motions. For of course the rod cannot now hide two or more times in succession a sector at any given point, without hiding the same sector just as often at the opposite point, 180 away. Here, too, the cycle of strobic movements is different. It is reversed. Let the disc be said to rotate forward, then if the rate of the rod is slightly less than one fourth, etc., that of the disc, the system will rotate forward; if greater, it will rotate backward.

So that as the rate of the rod increases, any system on its appearance will move forward, then stand still, and lastly rotate backward. The reason for this will be seen from an instant's consideration of where the rod will hide a given sector.

It is clear that if, instead of using as 'rod' a single radial sector, one were to rotate two or more such sectors disposed at equal angular intervals about the axis, one would have the same strobic phenomena, although they would be more complicated. Indeed, a large number of rather narrow sectors can be used or, what is the same thing, a second disc with a row of holes at equal intervals about the circ.u.mference.

The disc used by the writer had a radius of 11 inches, and a concentric ring of 64 holes, each 3/8 of an inch in diameter, lying 10 inches from the center. The observer looks through these holes at the color-disc behind. The two discs need not be placed concentrically.

When produced in this way, the strobic illusion is exceedingly pretty.

Instead of straight, radial bands, one sees a number of brightly colored b.a.l.l.s lying within a curving band of the other color and whirling backward or forward, or sometimes standing still. Then these break up and another set forms, perhaps with the two colors changed about, and this then oscillates one way or the other. A rainbow disc subst.i.tuted for the disc of two sectors gives an indescribably complicated and brilliant effect; but the front disc must rotate more slowly. This disc should in any case be geared for high speeds and should be turned by hand for the sake of variations in rate, and consequently in the strobic movement.

It has been seen that this stroboscope is not different in principle from the illusion of the resolution-bands which this paper has aimed to explain. The resolution-bands depend wholly on the purely geometrical relations between the rod and the disc, whereby as both move the rod hides one sector after the other. The only physiological principles involved are the familiar processes by which stimulations produce after-images, and by which the after-images of rapidly succeeding stimulations are summed, a certain number at a time, into a characteristic effect.

STUDIES IN MEMORY.

RECALL OF WORDS, OBJECTS AND MOVEMENTS.

BY HARVEY A. PETERSON.

Kirkpatrick,[1] in experimenting with 379 school children and college students, found that 3-1/3 times as many objects were recalled as visual words after an interval of three days. The experiment consisted in showing successively 10 written names of common objects in the one case and 10 objects in the other at the rate of one every two seconds.

Three days later the persons were asked to recall as many of each series as possible, putting all of one series together. The averages thus obtained were 1.89 words, 6.29 objects. The children were not more dependent on the objects than the college students.

[1] Kirkpatrick, E.A.: PSYCHOLOGICAL, REVIEW, 1894, Vol. I., p.

602.

Since the experiment just described was performed without laboratory facilities, Calkins[2] repeated it with 50 college women, subst.i.tuting lantern pictures for objects. She obtained in recall, after two days, the averages 4.82 words, 7.45 pictures. The figures, however, are the number of objects or words remembered out of ten, not necessarily correctly placed. Kirkpatrick's corresponding figures for college women were 3.22 words, 5.44 objects. The two experiments substantially agree, Calkins' higher averages being probably due to the shortening of the interval to two days.

[2] Calkins, M.W.: PSYCHOLOGICAL, REVIEW, 1898, Vol. V., p.

451.

a.s.suming, thus, that objects are better remembered than names in deferred recall, the question arises whether this holds true when the objects and names are coupled with strange and arbitrary symbols--a question which is clearly of great practical interest from the educational point of view, as it is involved in the pedagogical problem whether a person seeking to acquire the vocabulary of a foreign language ought to connect the foreign words with the familiar words or with the objects themselves. And the further question arises: what are the facts in the case of movements instead of objects, and correspondingly in that of verbs instead of nouns. Both questions are the problems of the following investigation.

As foreign symbols, either the two-figure numbers were used or nonsense-words of regularly varying length. As familiar material, nouns, objects, verbs and movements were used. The words were always concrete, not abstract, by which it is meant that their meaning was capable of demonstration to the senses. With the exception of a few later specified series they were monosyllabic words. The nouns might denote objects of any size perceptible to the eye; the objects, however, were all of such a size that they could be shown through a 1412 cm. aperture and still leave a margin. Their size was therefore limited.

Concerning the verbs and movements it is evident that, while still being concrete, they might be simple or complicated activities consuming little or much time, and further, might be movements of parts of the body merely, or movements employing other objects as well. In this experiment complicated activities were avoided even in the verb series. Simple activities which could be easily and quickly imaged or made were better for the purpose in view.

THE _A_ SET.

The _A_ set contained sixteen series, _A_^{1}, _A_^{2}, _A_^{3}, etc., to _A_^{16}. They were divided as follows:

Numbers and nouns: _A_^{1}, _A_^{5}, _A_^{9}, _A_^{13}.

Numbers and objects: _A_^{2}, _A_^{6}, _A_^{10}, _A_^{14}.

Numbers and verbs: _A_^{3}, _A_^{7}, _A_^{11}, _A_^{15}.

Numbers and movements: _A_^{4}, _A_^{8}, _A_^{12}, _A_^{16}.

The first week _A_^{1-4} were given, the second week _A_^{5-8}, etc., so that each week one series of each of the four types was given the subject.

In place of foreign symbols the numbers from 1 to 99 were used, except in _A_^{13-15}, in which three-figure numbers were used.

Each series contained seven couplets, except _A_^{13-16}, which, on account of the greater difficulty of three-figure numbers, contained five. Each couplet was composed of a number and a noun, object, verb, or movement.

Certain rules were observed in the composition of the series. Since the test was for permanence, to avoid confusion no number was used in more than one couplet. No two numbers of a given series were chosen from the same decade or contained identical final figures. No word was used in more than one couplet. Their vowels, and initial and final consonants were so varied within a single series as to eliminate phonetic aids, viz., alliteration, rhyme, and a.s.sonance. The kind of a.s.sonance avoided was ident.i.ty of final sounded consonants in successive words, _e.g._, lane, vine.

The series were composed in the following manner: After the twenty-eight numbers for four series had been chosen, the words which entered a given series were selected one from each of a number of lists of words. These lists were words of like-sounded vowels. After one word had been chosen from each list, another was taken from the first list, etc. As a consequence of observing the rules by which alliteration, rhyme, and a.s.sonance were eliminated, the words of a series usually represented unlike categories of thought, but where two words naturally tended to suggest each other one of them was rejected and the next eligible word in the same column was chosen. The following is a typical series from the _A_ set.

_A_^{1}. Numbers and Nouns.

19 42 87 74 11 63 38 desk girl pond m.u.f.f lane hoop vine

The apparatus used in the _A_ set and also in all the later sets may be described as follows: Across the length of a table ran a large, black cardboard screen in the center of which was an oblong aperture 14 cm. high and 12 cm. wide. The center of the aperture was on a level with the eyes of the subject, who sat at the table. The aperture was opened and closed by a pneumatic shutter fastened to the back of the screen. This shutter consisted of two doors of black cardboard sliding to either side. By means of a large bulb the length of exposure could be regulated by the operator, who stood behind the table.

The series--consisting of cards 42 cm., each containing a printed couplet--was carried on a car which moved on a track behind and slightly below the aperture. The car was a horizontal board 150 cm.

long and 15 cm. wide, fixed on two four-wheeled trucks. It was divided by vertical part.i.tions of black cardboard into ten compartments, each slightly wider than the aperture to correspond with the visual angle.

A curtain fastened to the back of the car afforded a black background to the compartments. The couplets were supported by being inserted into a groove running the length of the car, 3 cm. from the front. A shutter 2 cm. high also running the length of the car in front of the groove, fastened by hinges whose free arms were extensible, concealed either the upper or the lower halves of the cards at the will of the operator; _i.e._, either the foreign symbols or the words, respectively. A screen 15 cm. high and the same length as the car, sliding in vertical grooves just behind the cards and in front of the vertical part.i.tions, shut off the objects when desired, leaving only the cards in view. Thus the apparatus could be used for all four types of series.

The method of presentation and the time conditions of the _A_ set were as follows:--A metronome beating seconds was used. It was kept in a sound-proof box and its loudness was therefore under control. It was just clearly audible to both operator and subject. In learning, each couplet was exposed 3 secs., during about 2 secs. of which the shutter was fully open and motionless. During this time the subject read the couplet inaudibly as often as he wished, but usually in time with the metronome. His object was to a.s.sociate the terms of the couplet. There was an interval of 2 secs. after the exposure of each couplet, and this was required to be filled with repet.i.tion of only the _immediately preceding couplet_. After the series had been presented once there was an interval of 2 secs. additional, then a second presentation of it commenced and after that a third. At the completion of the third presentation there was an interval of 6 secs. additional instead of the 2, at the expiration of which the test commenced.

_A_^{13-16} had five presentations instead of three. The test consisted in showing the subject either the numbers or the words in altered order and requiring him to write as many of the absent terms as he could. In the object and movement series the objects were also shown and the movements repeated by the subject if words were the given terms. The time conditions in the test were,

Exposure of a term 3 secs.

Post-term interval in A^{1-12} 4 secs.

Post-term interval in A^{13-16} 6 secs.

This allowed the subject 7 secs. for recalling and writing each term in A^{1-12} and 9 sec. in A^{13-16}. If a word was recalled after that time it was inserted, but no further insertions were made after the test of a series had been completed. An interval of 3 min. elapsed between the end of the test of one series and the beginning of the next series, during which the subject recorded the English word of any couplet in which an indirect a.s.sociation had occurred, and also his success in obtaining visual images if the series was a noun or a verb series.

As already indicated, four series--a noun, an object, a verb, and a movement series--given within a half hour, const.i.tuted a day's work throughout the year. Thus variations due to changes in the physiological condition of the subject had to affect all four types of series.

Two days later these series were tested for permanence, and in the same way as the tests for immediate recall, with this exception:

Post-term interval in A^{13-16} 8 secs.

Thus 11 secs. were allowed for the deferred recall of each term in A^{13-16}.

In the movement series of this set, to avoid hesitation and confusion, the operator demonstrated to the subject immediately before the series began, once for each word, how the movements were to be made.

The _A_ set was given to three subjects. The results of each subject are arranged separately in the following table. In the tests the words were required in A^{1-4}, in A^{5-16} the numbers. The figures show the number of terms correctly recalled out of seven couplets in A^{1-12} and out of five couplets in A^{13-16}, _exclusive_ of indirect a.s.sociation couplets. The figures in brackets indicate the number of correctly recalled couplets per series in which indirect a.s.sociations occurred. The total number correctly recalled in any series is their sum. The figures in the per cent. row give the percentage of correctly recalled couplets left after discarding both from the number recalled and from the total number of couplets given those in which indirect a.s.sociations occurred. This simply diminished the subject's number of chances. A discussion of the propriety of this elimination will be found later. In _A_^{1-12} the absent terms had to be recalled exactly in order, to be correct, but in _A_^{13-16}, on account of the greater difficulty of the three-place numbers, any were considered correct when two of the three figures were recalled, or when all three figures were correct but two were reversed in position, _e.g._, 532 instead of 523. _N_ means noun series, _O_ object, _V_ verb, and _M_ movement series. Series _A_^{1}, _A_^{5}, _A_^{9}, _A_^{13} are to be found in the first and third columns, _A_^{2}, _A_^{6}, _A_^{10}, _A_^{14} in the second and fourth, _A_^{3}, _A_^{7}, _A_^{11}, _A_^{15}, in the fifth and seventh, and _A_^{4}, _A_^{8}, _A_^{12}, _A_^{16} in the sixth and eighth columns.