THE RELATION OF STRENGTH OF STIMULUS TO RAPIDITY OF HABIT-FORMATION

Transcription

1 THE RELATON OF STRENGTH OF STMULUS TO RAPDTY OF HABTFORMATON BY ROBERT M. YERKES AND JOHN D. DODSON. (Fom the Havmd Psychological Laboatoy) WTH FVE FGURES. n connection with a study of vaious aspects of the modifiability of behavio in the dancing mouse a need fo definite knowledge concening the elation of stength of stimulus to ate of leaning aose. t was fo the pupose of obtaining this knowledge that we planned and executed the expeiments which ae now to be descibed. Ou wok was geatly facilitated by the advice and assistance of Docto E. G. MARTN, Pofesso G. W. PERCE, and Pofesso A. E. KENNELLY, and we desie to expess hee both ou indebtedness and ou thanks fo thei geneous sevices. The habit whose fomation we attempted to study quantitatively, with espect to the stength of the stimulus which favoed its fomation, may be descibed as the whiteblack discimination habit. Of the mice which seved as subjects in the investigation it was demanded that they choose and ente one of two boxes o passageways. One of the boxes was white; the othe black. No matte what thei elative positions, the subject was equied to choose the white one. Attempts to ente the black box esulted in the eceipt of a disageeable electic shock. t was ou task to discove () whethe the stength of this electic stimulus influences the apidity with which dances acquie the habit of avoiding the black passageway, and if so, () what paticula stength of stimulus is most favoable to the acquisition of this habit. As a detailed account of the impotant featues of the whiteblack visual discimination habit in the dance has aleady been published,' a bief desciption of ou method of expeimentation YERKES, ROBERT M. The dancing mouse. New Yok: The Macmillan Company. See especially p. 9, et seq. 98.

2 Jounal of Compaative Neuology and Psychology. will suffice fo the puposes of this pape. A sketch of the expeiment box used by us in this investigation appeas as fig., and a gound plan of the box with its electic attachments, as fig.. This appaatus consisted of a wooden box 9 cm. long; cm. wide; and. cm. deep (inside measuements), which was divided FG.. " A FG. z. FG.. Discimination box. W, electic box with white cadboads; B, electic box with black cadboads. FG.. Gound plan of discimination box. A, nestbox; B, entance chambe; W W, electic boxes; L, dooway of left electic box; R, dooway of ight electic box; E, exit fom electic box to alley;, swinging doo between alley and A; C, induction appaatus; C, electic battey; K, key in cicuit. into a nestbox, A, (fig. ) an entance chambe, B, and two electic boxes, W, W, togethe with alleys which connected these boxes with the nestbox. The dooways between the electic boxes and the alleys wee by cm. On the floo of each electic box, as is shown in the figues, wee the wies of an inteupted cicuit

3 YERKES AND DODSON, Habit Fomation. Tests+ Seies + A i B which could be completed by the expeimente, by closing the key K, wheneve the feet of a mouse ested upon any two adjacent wies in eithe of the boxes. n this cicuit wee an electic battey and a Pote inductoium. One of these electic boxes boe black cads, and the othe white cads similaly aanged. Each box boe two cads. One was at the entance on the out TABLE. Positions of white cadboads fo two pefeence seies and twentyfive taining 8 9 ' ' ' 8 '9 l l seies. 8 9 i side of the box and the othe on the inside, as fig. indicates. The latte consisted of thee sections of which two constituted linings fo the sides of the box and the thid a cove fo a potion of the open top of the box. n no case did these inside cads extend the entie length of the electic boxes. The white and black cads wee eadily intechangeable, and they neve wee left on the same electic box fo moe than fou consecutive tests. The

4 ounal of Compaative Neuology and Psychology. ode in which they wee shifted duing twentyfive seies of ten tests each, in addition to the pefeence seies A and B, is given in table. n case a mouse equied moe than twentyfive seies of tests ( tests), the same set of changes was epeated, beginning with seies. n the table the lettes and efe to the position of the white cads; indicates that they maked the electic box which was on the ight of the mouse as it appoached the entances of the electic boxes fom the nestbox; indicates that it maked the left electic box. The way in which this appaatus was used may be indicated by a bief desciption of ou expeimental pocedue. A dance was placed in the nestbox by the expeimente, and thence it was pemitted to pass into the entance chambe, B. The expeimente then placed a piece of cadboad between it and the dooway between A and B and gadually naowed the space in which the animal could move about feely by moving the cadboad towad the electic boxes. This, without in any undesiable way intefeing with the dance s attempts to disciminate and choose coectly, geatly lessened the amount of andom activity which peceded choice. When thus bought face to face with the entances to the boxes the mouse soon attempted to ente one of them. f it happened to select the white box it was pemitted to ente, pass though, and etun to the nestbox; but if, instead, it stated to ente the black box the expeimente by closing the key, upon which his finge constantly ested duing the tests, caused it to eceive an electic shock which as a ule foced a hasty eteat fom the black passageway and the enewal of attempts to discove by compaison which box should be enteed. Each of the foty mice expeimented with was given ten tests evey moning until it succeeded in choosing the white box coectly on thee consecutive days, that is fo thity tests. A choice was ecoded as wong if the mouse stated to ente the black box and eceived a shock; as ight if, eithe diectly o afte unning fom one entance to the othe a numbe of times, it enteed the white box. Whethe it enteed the white electic box o the black one, it was pemitted to etun to the nestbox by way of the white box befoe anothe test given. Escape to the nestbox by way of the black box was not pemitted. A male and a female, which wee housed in the same cage between expeiments, wee placed in the expeiment box togethe and given thei tests tun about

5 ~ YERKES AND DODSON, Habit Fomation. Almost all of the mice used wee between six and eight weeks old at the beginning of thei taining. The exact age of each, togethe with its numbe, is stated in table. This table shows also the geneal classification of ou expeiments. They natually fall into thee sets. These ae designated by the oman numeals TABLE. Age in days, at the beginning of taining, of each mouse, with a statement of the conditions of taining. Condition of discimination. stength of stimulus. Numbe. MALES. ige in days. Numbe. FEMALES. ige in days. Medium Weak f 8 ' ' ' Set Medium A '9 '9 '9' '9 Medium Stong m+ '' 9 '' Geat ' '9 8 '8 8 9 Set 9 Easy '89 '9 ' ' 8 ' 9 8 Slight Set ' '9 Difficult,, and in the table, and will thoughout the pape be efeed to as the expeiments of set, set and set. As is suggested by the heading "condition of discimination," at the top of the fist vetical column of table, these sets of expeiments diffe fom one anothe fist of all as to condition of visual discimination o, moe explicitly stated, in the amount by which the two electic

6 Jounal of Compaative Neuology and Psychology. boxes diffeed fom one anothe in bightness. Fo set this diffeence was medium, in compaison with late conditions, and discimination was theefoe of medium difficultness. Fo set the diffeence was geat, and discimination was easy. Fo set the diffeence was slight, and discimination was difficult. t is clea, then, that the seies of wods, medium, geat, slight, in the table efes to the amount by which the electic boxes diffeed in bightness, and the seies medium, easy, difficult, to the demand made upon the visual disciminating ability of the mice. Fo the sake of obtaining esults in this investigation which should be diectly compaable with those of expeiments on the modifiability of behavio in the dance which have been conducted duing the past thee yeas, it was necessay fo us to use the same geneal method of contolling the visual conditions of the expeiment that had peviously been used. This we decided to do, notwithstanding the fact that we had befoe us methods which wee vastly supeio to the old one with espect to the descibability of conditions and the accuacy and ease of thei contol. To any expeimente who wishes to epeat this investigation with othe animals we should ecommend that, befoe ecouse is had to the use of cadboads fo the pupose of endeing the boxes distinguishable, thoough tests be made of the ability of the animal to disciminate when the boxes ae endeed diffeent in bightness by the use of a sceen which excludes a measuable amount of light fom one of them. We have discoveed that the simplest and best method of aanging the conditions fo such expeiments with the dance as ae now to be descibed is to use two electic boxes which ae alike in all espects and to contol the amount of light which entes one of them fom the top. t is easy to obtain satisfactoy sceens and to measue thei tansmitting capacity. We eget that the fist use which we wished to make of ou esults in this investigation foced us to employ conditions which ae elatively complicated and difficult to descibe. Fo the sake of the scientific completeness of ou pape, howeve, and not because we wish to encouage anyone to make use of the same conditions, we shall now descibe as accuately as we may the conditions of visual discimination in the seveal sets of expeiments. The cads at the entances to the electic boxes wee the same in all of the expeiments. Each cad (the black and the white)

7 YERKES AND DODSON, Habit Fomation. was. cm. in height and. cm. in width, with a hole. by. cm. in the middle of its lowe edge as is shown in fig.. These entance cads wee held in place by small metal caies at the edges of the electic boxes. The aea of white suface exposed to the view of a mouse as it appoached the entances to the electic boxes was 9.8 sq. cm. and the same amount of black suface was exposed. The white cadboad eflected. times as much light as the black cadboad. Special conditions of set. The inside length of each electic box was 8. cm. the width cm. and the depth. cm. The inside cads extended fom the inne edge of the font of each box a distance of. cm. towad the back of the box. Consequently thee was exposed to the view of the mouse a suface. cm. by. cm. (the depth of the box and of the cadboad as well) on each side of the box. The section of cadboad at the top measued. cm. in length by. cm. in width. The total aea of the white (o black) cadboad exposed on the inside of an electic box was theefoe. x. x (the sides) +. x. (the top) = 98. sq. cm. f to this we add the aea of the entance cad we obtain 8. sq. cm. as the amount of suface of cadboad caied by each electic box. But anothe condition, in connection with the amount of cadboad pesent, detemined the diffeence in the bightness of the boxes, namely, the amount of open space between the end of the inne cadboads and the end of the expeiment box. The lage this opening the moe light enteed each box. n the case of the expeiments of set this uncoveed potion of each electic box was cm. long by cm. wide; its aea, theefoe, was sq. cm. Special conditions of set. Both the oute and the inne cadboads wee pecisely the same in fom and aangement as in the case of set, but in ode that discimination might be endeed easie, and the time equied fo the acquisition of the habit thus shotened, a hole 8. cm. long by.9 cm. wide was cut in the middle o top section of the white cadboad. This geatly inceased the amount of light in the white electic box. The diffeence in the bightness of the boxes was still futhe inceased by a eduction of the space between the end of the cadboad and the end of the box fom cm. to cm. o, in tems of aea, fom sq. cm. to sq. cm. This was accomplished by cutting cm. fom the ea end of the expeiment box. Fo the expeiments of set

8 Jounal of Compaative Neuology and Psychology. the black box was much dake than it was fo those of set, wheeas the white box was not makedly diffeent in appeaance. Special conditions of set. The expeiments of this set wee conducted with the visual conditions the same as in set, except that thee was no hole in the white cadboad ove the electic box. This endeed the white box much dake than it was in the expeiments of set, consequently the two boxes diffeed less in bightness than in the case of set, and discimination was much moe difficult than in the expeiments of eithe of the othe sets. n the second column of table the values of the seveal stengths of electical stimuli used in the investigation ae stated. To obtain ou stimulus we used a stoage cell, in connection with gavity batteies, and with the cuent fom this opeated a PORTER inductoium. The induced cuent fom the seconday coil o this appaatus was caied by the wies which constituted an inteupted cicuit on the floo of the electic boxes. Fo the expeiments of set the stengths of the stimuli used wee not accuately detemined, fo we had not at that time discoveed a satisfactoy means of measuing the induced cuent. These expeiments theefoe seved as a peliminay investigation whose chief value lay in the suggestions which it funished fo the planning of late expeiments. The expeiments of sets and wee made with a PORTER inductoium which we had calibated, with the help of D. E. G. MARTN of the Havad Medical School, by a method which he has ecently devised and descibed. On the basis of the calibation measuements which we made by MARTN S method the cuve of fig. was plotted. Fom this cuve it is possible to ead diectly in ilnits of stimulation the value of the induced cuent which is yielded by a pimay cuent of one ampee fo any given position of the seconday coil. With the seconday coil at, fo example, the value of the induced cuent is units; with the seconday at. centimetes on the scale of the inductoium, its value is units; and with the seconday at, its value is units. The value of the induced cuent fo a pimay cuent geate o less than unity is obtained by multiplying the eading fom the calibation cuve by the value MARTN, E. G. A quantitative study of faadic stimulation.. The vaiable factos involved. Ame. Jou. of Physol., col. zz, pp The calibation of the inductoium fo beak shocks. bd., pp..

9 YERKES AND DODSON, Habit Fomation. of the pimay cuent. The pimay cuent used fo the expeiments of sets and measued. ampees, hence the value of the stimulating cuent which was obtained when the seconday coil stood at o was x. = units of stimulation. FG.. Calibation cuve fo PORTER inductoium. The numbes below the base line efe to the position of the seconday coil with efeence to the pimay. The positions ae ead, as on the scale of the inductoium, in centimetes. The numbes in the magin epesent values of the induced cuent in tems of MARTN S unit of stimulation. As conditions fo the expeiments of set, we chose thee stengths of stimuli which we designated as weak, medium, and stong. The weak stimulus was slightly above the theshold of stimulation fo the dances. Compaison of the esults which it yielded with those obtained by the use of ou calibated inductoium enable us to state with a fai degee of cetainty that its value was f units of stimulation. The stong stimulus was decid

10 8 Jounal of Compaative Neuology and Psychology. edly disageeable to the expeimentes and the mice eacted to it vigoously. ts value was subsequently ascetained to be & units. Fo the medium stimulus we tied to select a value which should be about midway between these extemes. n this we succeeded bette than we could have expected to, fo compaison indicated that the value was & units. Fotunately fo the intepetation of this set of esults, the exact value of the stimuli is not impotant. By the use of ou calibated inductoium and the measuement of ou pimay cuent, we wee able to detemine satisfactoily the stimulating values of the seveal cuents which wee used in the expeiments of sets and. The pimay cuent of. ampees, which was employed, seved to actuate the inteupte of the inductoium as well as to povide the stimulating cuent. The inteuptions occued at the ate of f pe second. We discoveed at the outset of the wok that it was not woth while to attempt to tain the dances with a stimulus whose value was much less than units. We theefoe selected this as ou weakest stimulus. At the othe exteme a stimulus of units was as stong as we deemed it safe to employ. Between these two, thee intemediate stengths wee used in the case of set, and two in the case of set. Oiginally it had been ou intention to make use of stimuli which vaied fom one anothe in value by units of stimulation, beginning with and inceasing by steps of though 9,,, to as nealy as possible. t poved to be needless to make tests with all of these. We may now tun to the esults of the expeiments and the intepetation theeof. Befoe the beginning of its taining each mouse was given two seies of tests in which the electic shock was not used and etun to the nestbox though eithe the white o the black box was pemitted. These twenty tests (ten in seies A and ten in seies B) have been temed pefeence tests, fo they seved to eveal whateve initial tendency a dance possessed to choose the white o the black box. On the day following pefeence seies B, the egula daily taining seies wee begun and they wee continued without inteuption until the dance had succeeded in choosing coectly in evey test on thee consecutive days. Results of the expeiments of set. The tests with the weak stimulus of set wee continued fo twenty days, and up to that time only one of the fou individuals in taining (no. 8) had

11 YERKES AND DODSON, Habit Fomation. 9 acquied a pefect habit. On the twentieth day it was evident that the stimulus was too weak to funish an adequate motive fo the avoidance of the black box and the expeiments wee discontinued. A few wods in explanation of the tables ae needed at this point. n all of the tables of detailed esults the method of aangement which is illustated by table was employed. At the top of the table ae the numbes of the mice which wee tained unde Seies. TABLE. The esults ofthe expeiments of set, stimulus weak ( f units). No. 8 MALES. NO.. Aveage. No.. FEMALES. NO.. Aveage. Geneal Aveage. A B..o..o 8 9 ' ' ' 8 '9.o....o....o...o..o m. the conditions of stimulation named in the heading of the table. The fist vetical column gives the seies numbes, beginning with the pefeence seies A and B and continuing fom to the last seies demanded by the expeiment. n additional columns appea the numbe of eos made in each seies of ten tests, day by day, by the seveal subjects of the expeiments; the aveage numbe of eos made by the males in each seies; the aveage numbe of eos made by the females; and, finally, the geneal

12 Youna of Compaative Neuology and Psychology. aveage fo both males and females. n table, fo example, it appeas that male no. 8 chose the black box in pefeence to the white times in seies A, times in seies B, times in seies, times in seies. Afte seies he made no eos duing thee consecutive seies. His taining was completed, theefoe, on the eighteenth day, as the esult of 8 tests. We may say, howeve, that only tests wee necessay fo the establishment of a pefect habit, fo the additional thity tests, given afte the fifteenth seies, seved meely to eveal the fact that he aleady possessed a pefect habit. n view of this consideation, we shall TABLE. The esults of the expeiments of set, stimulus medium ( f units). Seies. MALES. No. 9. No. 9. Aveage. No. 9. FEMALES. NO. 9. Aveage. Geneal Aveage. A B o.o...o.o...o.....o....o so...y... take as a measue of the apidity of leaning in these expeiments the numbe of tests eceived by a mouse up to the point at which eos ceased fo at least thee consecutive seies. Pecisely as the individuals of table had been tained by the use of a weak stimulus, fou othe dances wee tained with a medium stimulus. The esults appea in table. All of the subjects acquied a habit quickly. Compaison of these esults with those obtained with the weak stimulus clealy indicates that the medium stimulus was much moe favoable to the acquiement of the whiteblack visual discimination habit. n its esults the stong stimulus poved to be simila to the weak stimulus. All of the mice in this case leaned moe slowly

13 YERKES AND DODSON, Habit Fomation. than did those which wee tained with the medium stength of stimulus. The geneal esult of this peliminay set of expeiments with thee oughly measued stengths of stimulation was to indicate that neithe a weak no a stong electical stimulus is as favoable to the acquisition of the whiteblack habit as is a medium stimulus. TABLE. The esults of the expeiments of set, stimulus stong ( f units). Seies. No.. MALES. No.. Aveage. FEMALES. No. 9. No.. Aveage. Geneal Aveage. A B..o.o ' ' 8 '9..O...o.O.....o..O..o.....o.o..o.O.o...o.O..O...o..o Contay to ou expectations, this set of expeiments did not pove that the ate of habitfomation inceases with incease in the stength of the electic stimulus up to the point at which the shock becomes positively injuious. nstead an intemediate ange of intensity of stimulation poved to be most favoable to the acquisition of a habit unde the conditions of visual discimination of this set of expeiments.

14 Jounal of Compaative Neuology and Psychology. n the light of these peliminay esults we wee able to plan a moe exact and thooughgoing examination of the elation of stength of stimulus to apidity of leaning. nasmuch as the taining unde the conditions of set equied a geatdealof time, we decided to shoten the necessay peiod of taining by making the two electic boxes vey diffeent in bightness, and the discimination coespondingly easy. This we did, as has aleady been explained, by deceasing the amountof light which enteed the black box, while leaving the white box about the same. The influence of this change on the time of leaning was vey maked indeed. With each of the five stengths of stimuli which wee used in set two pais of mice wee tained, as in the case of set. The detailed esults of these five goups of expeiments ae pesented in tables to. Casual examination of these tables eveals the fact that in geneal the apidity of leaning in this set of expeiments inceased as the stength of the stimulus inceased. The weakest stimulus ( units) gave the slowest ate of leaning; the stongest stimulus ( units), the most apid. TABLE. The esults of the expeiments of set, stimulus units. Seies. No. 8. No.. Aveage. No.. FEMALES. No. 9. Aveage. Geneal Aveage. A B 8 9 ' ' ' ' o. 8..o.....O...O ,

15 ~~~ ~ YERKES AND DOUSON, Habit Fomation. Seies. MALES. No.. No. 8. Aveage. FEMALES. No. a. No.. Aveage. Geneal Aveage. A. B..o a.o. a a.. a..o o...a a.. a.o...w... o.as.... TABLE 8. The esults ofthe expeiments of set, stimulus ~ $ units. Seies. No.. MALES. No.. No.~9. FEMALES. N.. Aveage. Geneal Aveage. i. 9 ' o.as......

16 ~~ Jounal of Compaative Neuology and Psychology. A B 8 9 Seies. No. 9. TABLE 9. The esults of the expeiments of set, stimulus units. MALES. No. 98. FEMALES. Aveage. No. 89. No. 9. Aveage.....o Geneal. Aveage **S... TABLE. The esults of the expeiments of set, stimulus units. No. 8. ; MALES. No.. Aveage. No.9. No. 8. FEMALES. Geneal. Avcagc. Aveage o The esults of the second set of expeiments contadict those of the fist set. What does this mean? t occued to us that the appaent contadiction might be due to the fact that discimination was much easie in the expeiments of set than in those of set. To test this matte we planned to use inou thid sec of expeiments a condition of visual discimination which should be extemely difficult fo the mice. The eade will bea in mind that fo set

17 YERKES AND DODSON, Habit Fomation. the diffeence in bightness of the electic boxes was geat; that fo set it was slight; and fo set, intemediate o medium. Fo the expeiments of set only one pai of dances was tained with any given stength of stimulus. The esults, howeve, ae not less conclusive than those of the othe sets of expeiments because of the smalle numbe of individuals used. The data of tables to pove conclusively that ou supposition was coect. The vaying esults of the thee sets of expeiments ae explicable in tems of the conditions of visual discimination. n ll The esults of the expeiments of set, The esults of the expeiments of set TABLE. stimulus units. TABLE., stimulus 9 units.

18 Jounal of Compaative Neuology and Psychology. TABLE. The esults of the expeiments of set, stimulus units. TABLE. The esults of the expeiments of set, stimulus units. Seies. MALE. FEMALE. ~ No. 8. No. 8. Aveage. MALE. FEMALE. No. 8. No. 8. Aveage. A. B...o..O.o.. 8.o o. ' :j ' o....o.o....o.o

19 YERKES AND DODSON, Habit Fomation. ERRORS FG.. Cuves of leaning. Odinates epesent seies of ten tests each, and abscissae epesent the aveage numbe of eos fo fou mice in each seies. W, designates the eo cuve fo the individuals which wee tained unde the condition of weak electical stimulation; M, designates the coesponding cuve fo the medium stength of stimulation; and 8, that fo the stong stimulus.

20 8 Jounal of Compaative Neuology and Psychology. eation and cetainty. Fig. exhibits the chaacteistic diffeences in the cuves of leaning yielded by weak, medium, and stong stimuli. These thee cuves wee plotted on the basis of the aveage numbe of eos fo the mice which wee tained in the expeiments of set. Cuve Pis based upon the data of the last column of table, cuve M, upon the data in the last column of table ; and cuve S upon the data of the last column of table. n addition to exhibiting the fact that the medium stimulus yielded a pefect habit much moe quickly than did eithe of the othe stimuli, fig. shows a notewothy diffeence in the foms of the cuves fo the weak and the stong stimuli. Cuve P (weak stimulus) is highe thoughout its couse than is cuve S (stong stimulus). This means that fewe eos ae made fom the stat unde the condition of stong stimulation than unde the condition of weak stimulation. Although by actual measuement we have demonstated maked diffeence in sensitiveness to the electic shock among ou mice, we ae convinced that these diffeences do not invalidate the conclusions which we ae about to fomulate in. the light of the esults that have been pesented. Detemination of the theshold electic stimulus fo twenty male and twenty female dances poved that the males espond to a stimulus which is about pe cent less than the smallest stimulus to which the females espond. Table contains the condensed esults of ou expeiments. t gives, fo each visual condition and stength of stimulus, the numbe of tests equied by the vaious individuals fo the acquisition of a pefect habit; the aveage numbe of tests equied by the males, fo any given visual and electical conditions; the same fo the females; and the geneal aveages. Although the numbes of the mice ae not inseted in the table they may eadily be leaned if anyone wishes to identify a paticula individual, by efeing to the tables of detailed esults. Unde set, weak stimulus, fo example, table gives as the ecods of the two males used and + tests. By efeing to table, we discove that male no. 8 acquied his habit as a esult of tests, wheeas male no. was impefect at the end of tests. To indicate the latte fact the plus sign is added in table. Of pimay impotance fo the solution of the poblem which we set out to study ae the geneal aveages in the last column of the table. Fom this seies of aveages we have constucted the cuves of fig.. This figue

21 Te 8 im. FG.. A gaphic epesentation of the elation of stength of electical stimulus to condition of visual discimination and apidity of leaning. Odinates epesent value of electic stimulus in units of stimulation; abscissae epesent the numbe of tests given. Cuve epesents the esults of the expeiments of Set. Each dot indicates a value of stimulus which was used in the expeiments. Fo example, the fist dot to the left in cuve signifies that the stimulus whose value was units gave a pefect habit, in the case of the fou individuals tained, with 8 tests; the second dot, that fo the stimulus value of units 8 tests wee necessay; and the thid that fo the stimulus value of, tests. Cuves and similaly epesent the esults of the expeiments of sets and, espectively.

22 8 Jounal of Compaative Neuology and Psychology. vey clealy and biefly pesents the chiefly significant esults of ou investigation of the elation of stength of electical stimulus to ate of habitfomation, and it offes pefectly definite answes to the questions which wee poposed fo solution. n this figue the odinates epesent stimulus values, and the abscissae numbe of tests. The oman numeals,,, desig nate, espectively, the cuves fo the esults of set, set, and set. Dots on the cuves indicate the stengths of stimuli which wee employed. Cuve fo example, shows that a stength of stimulus of units unde the visual conditions of set, yielded a pefect habit with 8 tests. TABLE. The numbe of tests equied by the mice fo the acquisition of a pefect habit of discimination i Stong Weak Medium 8 ' { ; weage '+ 8 ' zoo+ 8 zoo+ 8 iveage :en. Av Fom the data of the vaious tables we daw the following conclusions :. n the case of the paticula habit which we have studied, the apidity of leaning inceases as the amount of diffeence in the bightness of the electic boxes between which the mouse is equied to disciminate is inceased. The limits within which this statement holds have not been detemined. The highe the cuves of fig. stand fom the base line, the lage the numbe of tests epesented by them. Cuve is lowest, cuve comes next, and cuve is highest. t is to be noted that this is the ode of inceasing difficultness of discimination in the thee sets of expeiments.

23 YERKES AND DODSON, Habit Fomation. 8. The elation of the stength of electical stimulus to apidity of leaning o habitfomation depends upon the difficultness of the habit, o, in the case of ou expeiments, upon the conditions of visual discimination.. When the boxeswhichae to be disciminated between diffe vey geatly in bightness, and discimination is easy, the apidity of leaning inceases as the stength of the electical stimulus is inceased fom the theshold of stimulation to the point of hamful intensity. This is indicated by cuve. Ou esults do not epesent, in this instance, the point at which the apidity of leaning begins to decease, fo we did not cae to subject ou animals to injuious stimulation. We theefoe pesent this conclusion tentatively, subject to coection in the light of futue eseach. Of its coectness we feel confident because of the esults which the othe sets of expeiments gave. The iegulaity of cuve, in that it ises slightly fo the stength, is due, doubtless, to the small numbes of animals used in the expeiments. Had we tained ten mice with each stength of stimulus instead of fou the cuve pobably would have fallen egulaly.. When the boxes diffe only slightly in bightness and discimination is extemely difficult the apidity of leaning at fist apidly inceases as the stength of the stimulus is inceased fom the theshold, but, beyond an intensity of stimulation which is soon eached, it begins to decease. Both weak stimuli and stong stimuli esult in slow habitfomation. A stimulus whose stength is neae to the theshold than to the point of hamful stimulation is most favoable to the acquisition of a habit. Cuve T veifies these statements. t shows that when discimination was extemely difficult a stimulus of 9 units was moe favoable than the weake o the stonge stimuli which wee used in this set of expeiments.. As the difficultness of discimination is inceased the stength of that stimulus which is most favoable to habitfomation appoaches the theshold. Cuve, cuve, cuve is the ode of inceasing difficultness of discimination fo ou esults, fo it will be emembeed that the expeiments of set wee given unde difficult conditions of discimination; those of set unde medium conditions; and those of set unde easy conditions. As thus aanged the most favoable stimuli, so fa as we may judge fom ou esults, ae,., and 9. This leads us to infe that an easily acquied habit, that is one which does not

24 8 Jounal of Compaative Neuology and Psychology. demand difficult sense disciminations o complex associations, may eadily be fomed unde stong stimulation, wheeas a difficult habit may be acquied eadily only unde elatively weak stimulation. That this fact is of geat impotance to students of animal behavio and animal psychology is obvious. Attention should be called to the fact that since only thee stengths of stimulus wee used fo the expeiments of set, it is possible that the most favoable stength of stimulation was not discoveed. We feely admit this possibility, and we futhemoe wish to emphasize the fact that ou fifth conclusion is weakened slightly by this uncetainty. But it is only fai to add that pevious expeience with many conditions of discimination and of stimulation, in connection with which moe than two hunded dances wee tained, togethe with the esults of compaison of this set of expeiments with the othe two sets, convinces us that the dances would not be likely to lean much moe apidly unde any othe condition of stimulation than they did with a stength of f units of stimulation. Natually we do not popose to est the conclusions which have just been fomulated upon ou study of the mouse alone. We shall now epeat ou expeiments, in the light of the expeience which has been gained, with othe animals.