HANS WALLACH, GARY S. YABLICK, and ANN SMITHSwarthmore College, Swarthmore, Pennsylvania 19081

Target distance and adaptation in distanceperception in the constancy of

visual direction"

Hay and Sawyer recently demonstrated that the constancy of visual direction(CVD) also operates for near targets. A luminous spot in the dark, 40 em fromthe eyes, was perceived as stationary when S nodded his head. This implies thatCVD takes target distance, as well as head rotation, into account as a stationaryenvironment is perceived during head movements. Distance is a variable in CVDbecause, during a turning or nodding of the head, the eyes become displacedrelative to the main target direction, the line between the target and the rotationaxis of the head. This displacement of the eyes during head rotationcauses an additional change in the target direction, i.e., a total angular changegreater than the angle of the head rotation. The extent of this additional angulardisplacement is greater the nearer the target. We demonstrated that the naturalcombination of accommodation and convergence can supply the informationneeded by the nervous system to compensate for this additional targetdisplacement. We also found that wearing glasses that alter the relation betweenthese oculomotor adjustments and target distance produces an adaptation inCVD. An adaptation period of 1.5 h produced a large adaptation effect. Thiseffect was not entirely accounted for by an adaptation in distance perception.Measurements of the alteration between oculomotor cues and registered distancewith two kinda of tests for distance perception yielded effects significantlysmaller than the effect measured with the CVD test. We concluded that thewearing of the glaaaes had also produced an adaptation within CVD.

counterclockwise direction by thesame angle. This main displacement ofthe target normally leads to theperception of a stationary target, whilea displacement relative to the headthat is not produced by an objectivelystationary target, i.e., a relativedisplacement whose angle does notmatch the rotation angle of the headbut is larger or smaller by a fewpercent, leads to the perception of atarget that is seen to move during thehead movement.P When the target isnear and the displacement of the eyesin relation to the target's maindirection matters, a clockwise rotationof the head through an angle, 0<, causesa change in the target direction fromthe vantage point of the eyesamounting to 0< + 13, where Il is theadditional target displacement causedby the displacement of the eyesrelative to the target's main direction.The relation of fj to 0< depends on thetarget distance (see Fig. 1). Thesmaller this distance, the larger fj willbe for a given head rotation Qt. Suchdisplacement ratios can be calculatedfrom the following equation:

where a is, as stated, the angle of thehead rotation, fl is the angle of thesecondary target displacement (andhence fl/a the ratio of the secondarytarget displacement), d T the distanceof the target from the rotation axis ofthe head, and dE the distance of theeyes from the rotation axis. 3 Becauseof this dependence of the secondarytarget displacement on targetdistanc~-with this dispalcement ratioinversely proportional to the diatanceof the target from the eyes(d T - dE r-a proceu that compensatesalso for the secondary targetdisplacement must take this distanceinto account, and thus depentW oncues for the target m.tance. Such aproceu would be rather complex: as inCVD of distant targets, it takes intoaccount information about the anileof the head rotation, and, in addition,it takes into account informationabout target distance.

EXPERIMENT 1MEASURING THE CONSTANCY

OF VISUAL DIRECTIONFOR NEAR TARGETS

Our first experiment resembled thatof Hay and Sawyer in many ways.These authors measured CVD for threetarJet distances, two of which weresimulated by uainI artificial conditionsthat caused the eye to converge for thesimulated distances. The values ofCVD measured under these artificialconditions could then be comparedwith the measurement of CVD for thetrue target distance (which was

target point and the eyes attainsappreciable amounts. Wallach andKravitz (1965), who were the first tomeasure the constancy of visualdirection (CVD), avoided dealing withthe secondary displacements by usinglarge target distances. 1 Hay andSawyer measured CVD for a target40 cm distant, using the methodintroduced by Wallach and Kravitz,and found complete compensation forthe additional secondary displacement.They a~o tried to ahow thatconvergence of the eyes was the maincue mediating distance for theconstancy of visual direction of neartargets, but the results did not confirmthis. The purpose of our work was(1) to show that the naturalcombination of accommodation andconvertence employed .. the solem.tance cues would yield good CVDfor near targets (Experiment 1), and(2) to find out whether CVD could bemodified by ..aptinC s. to spectaclesthat alter these oculomotoradjustments (Experiment 2).

The previous work of the seniorauthor and his co-workers on CVD wasconcerned with the compensatingproceaa that causes the visual field andindividual distant tareets to appearstationary durinl a head rotation. If,for example, the head ia turnedclockwise throulh an antle, Qt, adistant target becomes displaced inr e la tion to the head in a

Hay and Sawyer (1969) reportedthe discovery that the distance of theobserved object is a factor in theconstancy of visual direction. Thelatter is the compensating processwhich causes the apparent immobilityof objects during head movements. Itcompensates for the angulardisplacement between a target pointand the eyes cauaed by some rotatinghead movement. Such a displacementhas two components: The maindisplacement is caused by a rotation ofthe head .. a whole in relation to thestationary enYironment; every target inthe environment will change itsdirection relative to the head by theanile by which the head is beingrotated. The eecondary displacement isrelated to the fact that the eyes arelocated forward of the various rotationaxes of the head. TbII cause- the eyesto be diaplMled relative to • target'smain direction, which is the line thatconnects a tupt point with theparticular uia of the head rotation.This displacement of the eyes baa littleeffect on distant taraets, but, fortarget distances of leu than 2 m, thesecondary m.placement between a

-ThIs .0* .u suppol1ed by Gnnt11019 from the Natioaal I..ututes ofMenW Health se SwartluDore CoDece. II-.Wallach, pdndpal iIlvesUf,awr. We are mcmarateful to Walter C. Gop]. for nluableadvice on the presentation of tbill work.

Perception & Psychophysics, 1972, Vol. 12 (2A) Copyright 1972, Psychonomic SocietY,Inc., Austin, Texas 139

simulate a target at infinite distance,for which the displacement of the eyesrelative to the main target directiondoes not matter. If the nervoua systemwere partially compensating for thesecondary target displacement, astationary near target would, during ahead rotation, still appear to moveagaifUlt the head, but leu so, and asmaller objective target displacementwith the head would suffice to makethe target appear stationary. Thisobjective target displacement wouldtherefore measure the degree to whichthe nervous system fails to compensatefor the secondary target displacement,Le., it would measure the lag incompensation for the secondary targetdisplacement. If, for example, a target40 cm from the eyes, for which thesecondary target displacementamounts to 25% of the angle of thehead rotation, were to require adisplacement in the direction with thehead amounting to 10% of the headrotation angle (PIa = .1) in order toappear stationary, we could say that acompensation oCCU1'8 for 15% of the25% of secondary target displacement.Such a lag, or an opposite error inCVD, can have two reasons: (1) S doesnot compensate well for secondarytarget displacement. (2) The cuesgiving information about the distanceof the target are inadequate.

Measurements of this sort, while Swears one or the other of thementioned spectacles, permit anassessment of the roles ofaccommodation and convergence inour experimental conditions. To thedegree to which our measurementstaken with glasses in place confinn thetheoretical predictions given below,accommodation and convergencealone were responsible for thedifference in the results obtainedunder our three experimentalconditions.

The theoretical predictions for themeasurements with glasses are derivedin the following manner. When thenear glasses are worn, and theoscilloscope screen is 40 cm from S'seyes, the oculomotor adjustment ofthe eyes is for a distance of 25 em. f)

This is the target's equivalentdistance. 6 But the secondarydisplacement of the target is still 25%in the direction against the headmovement, because the actual distanceof the spot on the screen remains40 cm and its secondary displacementdepends on the actual distance. Since,theoretically, a target at an actualdistance of 25 em has to undergo asecondary displacement of 40% of thehead rotation in order to appearstationary, the spot at 40 em distancemust be made to displace during everyhead movement in the directionagainst it at the rate of 15% in order to

c

account for the better results weobtained.

What are the results to be expectedunder the assumption that CVDcompletely compensates for thesecondary displacement? When thespot is seen without glasses, it wouldbe perceived as stationary when itactually was stationary. For astationary target spot 40 em from theeyes, the secondary displacementamounts to 25% of the headdisplacement, in the direction againstthe head movement, and at thedistances of 25 and 100 em from theeyes, the distances simulated by thenear and the far glasses, the angles ofthe secondary displacement of astationary target are 40% and 10%,respectively, of the angle of the headrotation. These displacement ratioswere calculated from Eq. 1 byselecting for the distance between theeyes and the rotation axis the value of10 cm, which is approximately correctfor most adult Ss. All our calculationswill be based on this value.

If the nervous sytem did notcompensate for the secondary targetdisplacement pIa at all, but stillcompensated for the displacement ofthe main target direction relative tothe head, S would perceive astationary near target as moving in thedirection against the head rotation. Inorder that he see such a target asstationary, it would be necessary togive the target an objectivedisplacement in the direction with thehead rotation in the amount of PIa.Such an objective displacement wouldcause the near target to stay in themain target direction, i.e., it would

~~:::;;;-'---J;~---------"TCD

40 em), and the effects of thealterations of convergence on CVDcould thereby be assessed. Thoughstatistically very significant, theseeffects were small-18% and 16% ofthe effects predicted for the simulateddistances. In our experiment, the truetarget distance was also 40 em,measured from the eyes, and thesimulated distances were produced byhaving S see the same target throughspectacles that caused him to viewobjects with accommodation increasedby 1.5 lens diopters and convergenceincreased by 5 prism diopters for eacheye (near glasses) or through spectacleshaving the opposite effect (farglasses)." The near glasses caused thetarget at the 40-cm distance to be seenwith an accommodation andconvergence for a distance of 25 em,and the simulated distance in the caseof the far glasses was 100 cm.

As did Hay and Sawyer, we had ourS use a nodding motion, a headrotation about a horizontal axis, andhad his head connected to apotentiometer whose outputcontrolled the vertical position of thespot on the screen of a cathode rayoscilloscope. Our setup differed in thatS saw the oscilloscope screen directlyand in that we varied the extent anddirection of the motion of the spot inrelation to the head movement bymeans of a variable transmission.Instead of the biteboard arrangementused by Hay and Sawyer to transmitthe head movement to thepotentiometer, we had S wear aheadgear that could be attached to thevariable transmission. But thesetechnical differences probably cannot

Fig. 1. A is the rotation axis of the head and T is a target. AT is, therefore, themain target direction. Initially, the head position is such that AT is straightahead. Hence, the eyes (E1 ) fall on AT; the main target direction and the truedirection of the target, ita direction from the vantage point of the eyes, coincide.After the head has been rotated clockwise through an angle, a, AC is the newdirection straight ahead, and AT, the main target direction, will have beendisplaced relative to the head by a. E2 is the position of the eyes after the headrotation, and E2 T. is the new true target direction, which now differs by theangle P from the main target direction, which is represented by E2 T. . (E 2 T, isparallel to AT.) The angle CE 2 T,is the total displacement of theootrue targetdirection relative to the head caused by the head rotation. It consists of a,representing the main target displacement caused by the head rotation, and of P,the secondary target displacement brought about by the displacement of theeyes relative to the main target direction. AT equals dT and AE equals dE.

A~-~----!~":"-_----=-

140 Perception &: Psychophysics, 1972, Vol. 12 (2A)

appear stationary. because this bringsits displacement ratio up to 40%. 8incethe far glaaes produce an equivalentdistance of 100 em, and since thesecondary displacement ratio at thatdistance is 10% in the direction againstthe head movement, the spot on thescreen must be given a displacementratio of 15% in the direction with thehead to diminilh its 25% displacementratio normal for its objective distanceof 40 em to one of 10%.

EquipmentThe 8 wore a welder's headgear with

a mounting extending on the right aidedown to the level of his upper neck.There, an aluminum rod was attachedin a poaition perpendicular to themedian plane of 8's head. Themounting bad to be adjusted for each8 in such a way that the rod formedthe extension of the rotation axis ofthe nodding movement 8 had toperform.T The rod could be attachedto the input shaft of a variabletranamisaion, which w. mounted on aheavy stand on the rilbt of S's seat.With this uranpment, S's head wuripdly coupled to the tranaDliAion sothat his noddint mcnementa turnedthe input abaft. 'Ihia attachment.rved aIao to podtion S's head so thathia eyea wen at the level of the~ semen and 40 em from it.The heilht of S', ..t w. IIdjuated tobrinl hia beIId to the proper level. Thetnn....-ion wu of the ball and diaktype. By means of a larp knobmounted in front and to the right of Sat the level of his lower chest and ofan arrangement of Bhalta, geam, andangle geam, S could tum the controlabaft of the tranamiaion and changeita ratio and the direction of rotationof the output abaft in relation to thatof the input shaft. A Veeder counteralao attached to the control shaftpermitted E to read the tranamillaionsetting. The output shaft of thetranamisaionwu connected with theshaft ot a potentiometer used u avoltage divider of a 25-V dc source. Apointer fixed to the potentiometershaft permitted E to read thepotentiometer ,etting whenevernecesaary. The output of thepotentiometer controlled the verticalposition of the spot on theo-cilla.cope screen. Initially, E alsocould polIition the spot by uaing thecontrols on the OICillo-cope. A specialtube with minimal afterglow wu used.To eliminate the faint 110w of the faceof the tube when it wu in operation,the intensity of the target spot wu!WIde low. AlIo, the tube wu coveredwith bW:k cardboard, except for avertical gap through which the path otthe target spot w. visible, and testingwas done in total darkneu. Work withCVD requires absence of lines or

boundaries which could serve as avisual framework for the targetdisplacements to be presented.

When his head was attached, 8could, by nodding it up and down,cause the spot to shift up and down,or down and up, depending on thetransmission setting which hecontrolled. From a center po-ition atwhich the rate of transmiasion waszero and the spot remained at rest, 8could gradually increae the ratio ofthe motion of the spot in relation tohis head movement, and, depending onthe direction in which he turned theknob from this center position, thespot would move with or against thedirection in which the bead movementdisplaced the eyes. The resulting ratioof the angular displacement of thespot'Y (meaured from a point behindthe eyes in the vertical plane of theinput shaft) and of the angle of therotation of the input shaft(representing the angle of rotation ofthe head a:) wa the 'Yla-diaplacementratio, the unit of meuurement thatour equipment yielded. Once the gaincontrol on the oscilloscope w.adjusted, these displacement ratioswere in a fixed relation to the readinpon the Veeder counter, and thatrelation was empirically determined.

The displacement ratio that isme88ured by our device is not thesame • the displacement ratio tJ/a:disc:uaaed earlier. The latter me.ureathe secondary target displacement, thedisplacement of the direction betweena stationary target and the eyes causedby a head rotation; this change in thetarget dinction is due to the fact thatthe head rotation displacea the eyesrelative to the main target direction.The displacement ratio 'Y/a: measuredby our device is concerned with theobjective target displacementproduced by that device, and isgeometrically diffennt from the st«ratio. The angular displacement 'Y ofthe target spot is mellllured from therotation axis of the head and is, thus,geometrically the same as the angle ofthe head rotation a, wherea the apexof the angle tJ is at S's eyes. Whereasour equipment meuurea the objectivetarget displacement needed tocompensate for the stationary target'sapparent displacement in 'Y la ratios,the theoretical predictions for theeffects of the glauea on CVD weremade in terms of pIa ratia.. Thefollowing equation can be used totranaform a "'fla ratio into a pIa: ratio:

where, a before, dT ia the distancefrom the rotation axis to the targetand dE the distance from the rotationaxis to the eyes, Le., 10 em. Ourresults will be transformed in this

fashion and presented as pIa ratios. Tocorrespond to the units ofmeasurement used in earlier papers onCVD, these ratios will be multiplied by100 and given as percentl3-displacement ratios, abbreviated%{3-DR.

ProcedureOur tests were designed to

determine the limits of the range ofdisplacement ratios that did not causeS to report apparent targetdisplacements during his headmovements (no-motion range). But wedid not employ the proceduredeveloped by Wallach and Kravitz(1965, 1968), because work byWallach and Frey (1969) and byWallach and Floor (1970) haddemonstrated very rapid adaptation inCVD. To keep to a minimum expoaureof 8 during the teat to targetdisplacements that might cause smalladaptation effects, we let 8 changediaplacement ratios, and determinedonly one upper and one lower limit ofthe no-motion range. The awrase ofthe two limits waa recorded 88 S'sno-motion point. E made a -ttinI ofthe tranamiaion that produced am.allapparent target displacements in oneof the two vertical directions and toldS in which IeDle to tum the controlknob to diminiah the target motioncaused by hia head movements. S wasinstructed to stop chaDIiDI thetranamiaion letting u soon • thetarget no longer seemed to move. If 8thought he bad overshot the limit, Ereaet the tranamiaaion to cause mildtarget motion in the former directionand asked him to make his chan..more slowly. When S had found onelimit, he closed his eyes and E read thetransmission aetting from the Veedercounter with a small flashlight. E thenset the transmission to produce targetmotion in the opposite direction andasked S to find the other limit. Withhalf the Sa, the limit for apparenttarget motion in the direction with thehead rotation wu found first, and forthe other half, the limit of targetdisplacement against the headmowment was first determined. Forall SII, the no-motion point wu firstmeuured without glasses and thenwith the near and the far glauea, inthat order. No instructions were givento fixate the target spot, because wewanted S to behave as naturally as wucompatible with the teat conditions.Art. in our previous publications onCVD, the question of whether targetdisplacements relative to the head areregistered· by the eyes through imatedisplacement or through taking pursuitmovements into account is not raisedhere. Research designed to answer thisquestion is in preparation.

Thirty-one paid undergraduate Sa

Perception I: Psychophysics, 1972, Vol. 12 (2A) 141

Perception & Psychophysics, 1972, Vol. 12 (2A)

participated. Of these, 20 Sacompleted the experiment. Eight Sswere eliminated because theirno-motion point for the directlyviewed target differed from objectiveimmobility by a displacement largerthan 2% of the head rotation in one orthe other direction. For three other Sa,the upper limit of the no-motion rangetaken with the far glaues could not bedetermined, because these Sa tried touse a displacement ratio that wasbeyond the range of the tranamiasion.

ResultaThe mean midpoint of the

no-motion range for all 31 S.amounted to .67± .89%p-DR in thedirection with the head movement forthe directly viewed target spot 40 emfrom the eyes. That is, for randomlyselected Ss, the target spot had to begiven a displacement with the headmovement of .67% of the headrotation to appear stationary.' Thisdeviation of the mean no-motion pointfrom objective immobility was, ofcourse, smaller for the 20 selected Sa,namely, .19±.42%p-DR. When the nearg1assea are worn and the head ismoved, a stationary target spot willappear to move in the direction withthe head movement, and an objectivetarget diaplacement in the directionagaimt the head movement is neededto compensate for this apparent targetdiaplacement. The mean no-motionpoint measured with the near glassesamounted to a target displacement of6.90%:1: 1.58% of the head rotation inthe direction agaimt the headmovement. As explained above, atarget displacement in the directionagaind the head movement of15%p-DR was expected to compensatefor the effect of the near glasses on atareet objectively 40 em away. Ourresult ahowa that even with seJectedSa, compensation for the secondarytarget diaplacement is incomplete for adiatance as small as 25 em. Thisinterpretation of the result is justifiedby the quite different result for the farglasses. To appear stationary, thetarget at an objective distance of40 em had to undergo a mean pdisplacement of 15.76% ± 2.52% inthe direction with the head movementwhen the far glasses caused anequivalent distance of 100 em fromthe eyes. This result agrees well withthe difference between the secondarydisplacementa of targets at objectivedistances of 40 em and of 100 em,which amounts to 15%p-DR and showsthat accommodation and convergencesuffice as distance cues when CVDcompensates for secondary targetdisplacements.

DiscussionThe experiment just reported shows

that, in the case of near targets, the

142

constancy of visual directioncompensates not only for the maintarget displacement brought about bya head rotation as such but also for thesecondary target displacement. CVD isvirtually complete in the case of thetarget distances of 100 em and 40 em,whereas compensation for thesecondary t8rlCet displacement wasincomplete in the ease of the 25-cmdistance. There are two possiblereasons why, in the case of simulateddiatances, our results differed 80 muehfrom those obtained by Hay andSawyer. These authon manipulatedonly convergence, whereas our glaaeaaltered convergence andaccommodation in correspondJqfashion. It seems unlikelf, however,that this alone could account for 80large a difference. Hay and Sawyerprobably failed to keep the surroundof the target spot completely dark.Visual landmarks cause an objectivetarget displacement to be noticed thatotherwise would not be perceived(Wallach & Kravitz, 1965).

Since the amount of secondarytarget displacement becomes larger astarget distance decreases,compensating for it means takiqtarget distance into account. Themeasurementa of CVD with the farglauea show that accommodation andconvergence, in combination, can hereserve as potent distance cues. Wetherefore adapted Sa to our neargIasaea and tested for changes in CVDin the same manner in which we hadtested for the effect of wearing thenear and the far glasses inExperiment 1. If we succeeded inobtaining a cban,e in CVD on thebasis of adaptation to near glMseS, wemight thereby demonstrate stillanother manifestation of adaptation indistance perception' and contribute tothe investigation of the role thatregistered distance playa in theoperation of CVD.

EXPERIMENT 2ADAPTATION TO NEAR GLASSES

There are two ways in which suchan adaptation might operate. Theglasses may alter the relation betweenthese oculomotor adjustments and thedistance they denote in perception,i.e., the registered distance, as theywere shown to do by Wallach and Frey(1972) and by Wallach, Frey, andBode (1972). After such an alterationof registered distance, CVD should bealtered also. Wallach, Frey, and Bodefound that, when such an adaptationhad taken effect and was tested underconditions where only oculomotorcues operated, a change in registereddistance manifested itself when Srepresented the perceived distance of atarget by pointing to it, as well as bychanges in perceived size and

stereoscopic depth. If such a change ofregistered distance altera CVD also,complete adaptation to near glasseswould eliminate the apparent targetmotion in the direction with the headmovement observed when these glassesare worn. Inasmuch as adaptation tothe near glasses consists in increasedregistered distance, its effect on CVDwhen no glasses are worn would be thesame as that of wearing our far glasses.Such adaptation would cause astationary target nearby to beregistered as farther than it actuallywas and make it appear to move in thedirection agairut the head movement,because the target'. actual secondarydisplacement is larger than itasecondary displacement at theregistered distance would be. Anobjective displacement of the taqet inthe direction with the head would beneeded to make it appear stationaryand would, thus, measure the effect ofadaptation on CVD.

Another fonn that adaptation toour gl ...es might take is amodification of CVD itself. When thenear glasses are worn, objecta at alldistance. are given with oculomotorcues that place them nearer to S thanthey objectively are. The secondarydisplacementa the objects actuallyundergo during a head rotation areamalIer than would be warranted bythe registered distances that theseoculomotor cues normally produce.This discrepancy between' distancecue. and the given objectdisplacements during headmovementamay cause an adaptation such thatapparent target immobility will resultfrom a different combination ofsecondary displacements andoculomotor adjuatmenta than thenormal one. An adaptation producedby this discrepancy may take one oftwo forma: It may co.w.t in amodification in distance perceptionsuch that oculomotor cues willproduce registered distances more inagreement with the given objectdisplacements caused by headmovements. This would be the samekind of adaptation we discussed above;it would, however, be caused by fielddisplacements during head movementa.The adaptation may, however, alsoconsist in a modification within CVDsuch that the process that compensatesfor the object displacements relative tothe eyes caused by head movementa isaltered. This is the same kind ofmodification that results fromexposure to objective targetdisplacements or from optical fielddisplacement during head movements,produced, for instance, by wearingmagnifying or minifying glasses(Wallach & Kravitz, 1968). Since thenear glasses cause registered targetdistances smaller than the truedistances, the secondary displacements

of any target due to head movementswould be smaller than that shorteneddistance would warrant, and apparenttarget motion inthe direction with thehead movement should result. Whenthe target is a luminous spot in anotherwise dark field, this motion isobserved; our first experiment is basedon this fact. But when the whole fieldis visible, it does not appear to movewhen S nods or turns his head and 8experiences no apparent fielddisplacements during the adaptationperiod.1 0 Nevertheless, S is exposed toa discrepancy between the actuallygiven field displacements and distancecues that would warrant larger fielddisplacements againat the headmovement than those actually given. Ifthis were to cause an adaptationwithin CVD, it would be an adaptationto field displacement in the directionwith the head movement and wouldcause an objective target displacementwith the head movement to appearstationary. In the end, this is, ofcourse, the same overt effect that amodification in distance perceptionresulting from wearing near glasseswould produce in a CVD test; only theunderlying processes are different.

How, then, can we distinguishbetween the two possible forma thatadaptation to near glasses maytake-an adaptation process withinCVD and an adaptation in distanceperception? One could preventmodification in CVD by keeping 8'shead stationary throughout theadaptation period, but this is difficultto do. We made it possible todistinguish between the two forma ofadaptation by also testing foradaptation in distance perception inother ways, using two of the testsWallach, Frey, and Bode hademployed. Sa were given a pointingtest and size estimation tests inaddition to being tested for amodification in CVD. If the amount ofadaptation measured by theM testacorresponded to the chan,e inregiatered distance implied in the CVDteat resulta, adaptation would conaistonly in a moclillcation in w.tanceperception. If the cbanIe in CVD werelarpr than the chan... in reptereddistance implied by the two other testawould warrant, it would seem likelythat adaptation withiQ. CVD alsooccurs. Adaptation within CVD wouldaccount for that part of CVDadaptation that C8IlIlot be ascribed tothe chan. in recistered w.tance asmeuured by the other testa.

ProcedureBecause we planned to use a I8t 9f

five testa, two testa for CVD, apointing test, and two size estimationtests, we made the adaptation periodlonger than it had ever been in

adaptation to our near glasses, and inorder to gain information about thelong-range growth of the adaptationeffect, we interrupted the adaptationperiod for a set of tests. After thepreadaptation tests, S wore the nearglasses for 1.5 h, was given a set oftests, and immediately resumedwearing the glasses. After this secondadaptation period, which lasted 3 h,another set of tests was given.Afterwards, S sat quietly with eyesclosed for 15 min and then was givenstill another pointing test and the twosize estimation tests. This retesting wasdone to learn at what rate theadaptation effects would weaken withmere time lapse between exposure tothe adaptation conditions and thetests. This information was neededbecause the three kinds of test werealways given in the same order. Theretesting after a time lapse wouldallow us to estimate the loss inadaptation effect due to the durationof the tests. We always gave the CVDtest first, because our primary interestwas in the effect of adaptation to ourglasses on CVD, and because webelieved that the test did not provideconditions tending to reestablishnormal distance perception. Thencame the pointing test that took verylittle time; it was taken with 8 wearingthe glasses and therefore could notprovide "unlearnine" conditionseither. Two size estimation testsfollowed, which had previously beenused by Wallach, Frey, and Bode(1972). Wallach and Frey (1972) hadfound that the effect of adaptationdoes not decline during a sequence offour size estimation tests. "

Because such long adaptation peri­ods had never been used in connectionwith glasses that alter oculomotor ad­justment, we had to envisage the possi­bility that distance adaptation might becomplete. We, therefore, employedonly two of the conditions of testingCVD we had previously used, namely,viewing a target 40 em from the eyeseither directly or through the nearglaaIes. We omitted testing with the farg1Maes, because they cause the tareetat 40 em to be I88n with anoculomotor adjustment equivalent to1 lens diopter. Complete adaptation tothe near g1Maes, on the other hand,would mean an increase in reeiatereddistance equivalent to 1.6 lens diopteRand could not become manit.t whenthe test object was I88n withoculomotor adjustmentscorresponding to 1 diopter ofaccommodation.

On .u three OCCMions, the CVD testwith direct viewing preceded the onewith gI.auea. The test procedure wasthe aame u before, except that eachtest started with a I8tting of thetransmission that produced clearly

visible target displacement with thehead rotation. This is the targetdisplacement associated with wearingthe near glasses. When such adisplacement occurs during the test, itcannot possibly weaken an establishedadaptation. After, under theSefavorable conditions, one limit-andthereby the approximate value of theno-motion range-was found, the otherlimit could be found by presenting 8initially with a weak apparent targetdisplacement against the headmovement, avoiding a larger apparentdisplacement that might tend todiminish the adaptation.

The procedure for the pointing testand for the size estimation tests was asdescribed by Wallach, Frey and Bode.In the latter tests, half of the Sa werealways tested first with the test objectat the distance of 33.3 cm and thenwith the test object at 66.7 em; for theother half, these tests were given in thereverse order. The object sizes to beestimated were so transposed that theyproduced identical retinal images.During the two adaptation periods, Sa,paid undergraduates, spent their timeas they pleased, accompanied by aguide. They were, however, asked towalk about for the last 20 min beforereturning for a set of tests. Our 10 Sawere selected on the buis of two CVDtests, one without and the other withthe near glasses, given on the previousday. To participate, an 8's no-motionpoint meuured without glasses couldnot deviate from rest by more than.5~-DR in the direction with thehead movement. Also, with the nearglasses in place, the tareetdisplacement against the headmovement needed to produceapparent rest had to amount to at least7%~-DR.

ResultsThe first two columns of Table 1

give mean no-motion points for theCVD tests. The results for thepreadaptation tests, which are againgiven in ~-DR, differ from thosereported earlier, because we employeda highly selected troup of Sa. Whereaspreviously the mean no-motion pointamounted to 6.9~-DR against thehead rotation direction when the nearglaues were worn, the present troupgave a mean of 10.08~-DR, whichwas considerably closer to the value of15% expected for the. near ldIlII8Sunder the assumption thatcompensation for the secondarydisplacement of a target 26 em distantis complete. The mean of .61~-DRfound with direct viewing of the tareetpoint was in the direction againat thehead mowment and therefore doesnot represent a lag in compensation; itsimply retlecta the smallneu of oursample, as do the high confidence

Perception & Psychophysics, 1972, Vol. 12 (2A) 143

Tllble 1Mean Effect of Adaptation to Ne. G~a

Poin~Dtatance Size EstimationTeated With CVD Teat In 'llIP-DR in em iDem

40 em 33.3 emTeat Distance 40 em WUh Neu Glauea With Ne. GJaaea 33.3 em 66.7 em

PreadaPtation .61 ± 1.49 ApiDat 10.08 ± 3.30 ApiDat 29.7 ± 3.10 6.36:1: 0.95 9.44 ± 1.52

Aft. 1.5 h 10.44 ± 6.59 With 2.09 ± 3.11 AlaiDR 36.4 ± 2.20 7.84:l: 1.l~3 12.03 ± 3.21adaptation

ChaD&efrom 11.04 7.99 6.7 1.48 2.59preadaptation

After 3 h more 8.65 :l: 3.55 With 1.13 :I: 2.21 ApiDat 36.0:1: 1.81 7.67± 1.50 11.91 ± 8.18Mlaptation

ChaD&e from 9.26 8.95 6.8 1.31 2.47pnmaptation

Aft.U miD 87.0:1: 2.62 7.84± 1.78 12.15:1: 3.84diaApation

Effect ratio for .89 .44 .471.5 h adaptation

limits of all means in the table.In the case of teats without glaues,

the means of 10.44%11-DR and8.65%p-DR measured after 1.5 and 3 hof adaptation, respectively, amount tochanges of the no-motion point of1l.04%11-DR and 9.26~-DR in thedirection with the head movement.Theile changes represent more thanhalf of complete adaptation, sincecomplete adaptation would havemeant a change of the no-motionpoint by 15%tl-DR in the withdirection. The effect of completeadaptation to near glasses is the sameas that of wearing the far glasses, sincethe far glasses diminishaccommodation and convergence inthe aame amount as the near glassesincrease it, Le., by the equivalent of1.5 lena diopten. We had found in ourfint experiment that the effect of farglaues on a target 40 em distant wasnot different from the theoreticalvalue of 159F>tl-DR, namely,15.76~-DR. To find out whetheradaptation to the near glasses wascomplete, we compared this value withthe mean no-motion point of10.44%I3-DR and 8. 65 '1"p-DR,measured after the adaptation periods.For the 10.44 value, because of highvariance, the difference from the valuefor full adaptation failed to besignificant, but for the meanno-motion point obtained at the endof the period of 3 h, this differencewas significant (p < .01).

In the case of the test with the nearglasses in place, adaptation consistedin a decrease in the targetdisplacement needed prior toadaptation to compensate for theeffects of the glasses; as reported, thatcompensating displacement amountedto 10.089F>tl-DR in the direction againstthe head movement. After theadaptation periods, the meanno-motion points were different by

7.99%I3-DR and 8.95~-DR,

respectively. A comparison of theseeffect. with those obtained whentesting was without glasses shows thatthe effects measured under the twotest conditions were approximatelythe same. Further, a comparison of theadaptation effects measured after thefirst adaptation period (1l.049b/3-DRand 7.999F>tl-DR) with the effectsmeasured after the second period(9.26~-DR and 8.959F>tl-DR) gives noevidence of larger effects after thesecond and longer adaptation period.

Corresponding results were obtainedwith the two other tests; they aregiven in the last three columna ofTable 1. Here, too, the effects ofadaptation measured after the secondadaptation period were not larger thanafter the lint. Another striking resultis the small adaptation effectsmeasured with the pointing test andthe two size estimation tests. They areabout half as large as the effectobtained by Wallach, Frey, and Bode,in their Experiments 3 and 1, after20 min of walking with the nearglasses in place. Whether this is due tothe fact that we did not select our Safor good performance on these tests,as the previous authon had done,11 oris the result of the much longeradaptation periods cannot beascertained. Experiments are beingplanned to explore the course ofadaptation to near glasses overextended periods.

To compare the magnitude of theadaptation effects measured with ourthree kinds of tests, we determined thedegree to which each effect differedfrom complete adaptation. This wasdone by computing for each kind oftest an effect ratio, the proportion ofthe adaptation achieved over thechange representing completeadaptation. In the case of CVD,complete adaptation to our glasses

means, of course, that S, with glassesin place, accepts the same targetdisplacement as stationary which priorto adaptation appeared to himstationary without the glasses. Thus,for the mean of 10.089b/3-DR measuredprior to adaptation with glasses, thepreadaptation mean of .619F>tl-DRobtained without glaue8 repl'elentscomplete adaptation, and thedifference of 9.479F>tl-DR betweentheBe meana is the empiricallydetermined effect of completeadaptation in the case of the test withglauea. This value can be comparedwith the actual adaptation effectmeaaured with the glasses, whichamounted to 7.99~·DR. The effectratio, therefore, is approximately7.99/9.47. We computed such effectratiOlJ for individual Sa and uaed themto compare the result of the CVD teststatistically with our other twoadaptation measures. The mean effectratio for 1.5·h CVD adaptationmeaaured with the near glaues in placeand computed in this manner was .89.

The effect ratio for the sizeestimation test was computed incorresponding fashion. This waspossible because the distances of thetwo test objects from S were 33.3 and66.7 em, and these distances differedby the equivalent of 1.5 lens diopten.Complete adaptation to the nearglasses means that the registereddistance that corresponds to a givenoculomotor adjustment is greater bythe equivalent of 1.5 lens diopters.Thus, in the case of a test object33.3 cm distant, the registereddistance corresponding to completeadaptation is 66.7 em. Because thesizes of the two test objects were sochosen that they produced identicalretinal images, a size estimate of theobject at 66.7 em, given prior toadaptation, represents thepostadaptation size estimate of the

144 Perception & Psychophysics, 1972, Vol. 12 (2A)

object at 33.3 cm under theassumption of complete adaptation.The difference in the size estiamtes ofthe two test objects given prior toadaptation therefore represents theeffect of complete adaptationmeasured at the 33.3-cm test distance.This difference, thus, becomes thedenominator in the effect ratio, withthe difference between- the post- andpreadaptation size estimates of theobject 33.3 em distant becomes thenumerator. The mean of the effectratios computed for individual Ssamounted to .47.

Because it had previously beenfound that pointing distances areapproximately linearly related toobject distances, effect ratios couldhere be computed in a differentmanner. The pre- and thepostadaptation pointing distances ofindividual Sa were changed into theirdiopter equivalents, and the ratio ofthe difference between these diopterequivalents over 1.5, the diopter valueof complete adaptation, wascomputed. The mean effect ratioamounted to .44, and was thus quitesimilar to that computed for the sizeestimation test.

Both theae mean effect ratios weresmaller than the effect ratio for theCVD test. In spite of the small sample,the difference between the effectratios for the pointing test and theCVD test was significant at better thanthe .01 level, and for thecorresponding difference between thesize estimation teat and the CVD test,we found p < .025. These differencescannot be ascribed to the test order.As the fifth line in Table 1. shows,retesting with pointing and sizeestimations 15 min after the secondpostadaptation tests revealed nodissipation of the adaptation effect asa result of the preceding tests and ofthe time la})lle. All means were thesame (or insignificantly higher) in thisretest as in the two preceding tests.

DISCUSSIONAbove, we have stated that an

adaptation effect meaaured with CVDmay represent two differentadaptation proce.es,. an adaptationwithin CVD and an adaptation indiatance perception, the lattermeaaurable aIIo by other tests foradaptation in diatance perception. Ourfinding that adaptation meaa~dwiththe CVD teat waa aignificantly greaterthan the effeeta measured with thepointing teat and the size estimationtest shows that part of the effectmeasured with the CVD test resultedfrom adaptive change within CVD. If

this inference is correct, we haveobtained a modification of CVD bymeans of an adaptation period inwhich S was not aware of fielddisplacements during head movements.Only the conditions of atimulationnecessary to bring about such anadaptive modification were given:Object displacements relative to thehead as normally produced by headmovements (and normally leading to astationary experienced field) werepaired with oculomotor adjustmentsthat corresponded to object distancesnormally associated with larger objectdisplacements. When a single target isobserved in the dark, displacementsare experienced that are warranted bythe oculomotor adjustments. But nosuch object displacements wereexperienced during our adaptationperiods. It seems that consciousexperience of object displacementsduring head movements is notnecessary for adaptation and that onlythe conditions of stimulation-thediscrepancy between the given objectdisplacements and the givenoculomotor adjustments-rnatter.

REFERENCESHAY. J. C.... SAWYER. S. Position

constancy and binocular conveqence.Perception '" Psychophysics. 1969. 5.Slo-S12.

LEIBOWITZ, H.. .. MOORE. D. Role ofchan.elI in accommodation andconveqence in the perception of size.Journal of the Optical Society ofAmerica. 1966. 56, H2o-H2S.

WALLACH. R., .. FLOOR. L. On therelation of adaptation to fielddisplacement during head movements tothe constancY of visual direction.Perception .. PsychophyRcll, 1970. 8.95-98.

WALLACH. H.... FREY, K. J. Adaptationin the constanCy ot visual directionmeasured by a one-tJial method.Perception ,. Psychophysics. 1969, O.249-252.

WALLACH, H.... FREY. x, J. Adaptationin distance perception based onoculomotor cues. Perception ..PsYchophysics, 1972, 11. 31-34.

WALLACH. H•• FREY. K. J.... BODE. x,A. The natme of adaptation in distanceperception bued on oculomotor cues.Perception .. Psychophyalcs. 1972. 11.Uo-U6.

WALLACH. H., FREY, It. J••• ROMNEY.G. Adaptation to fielcl disp1a<lementduring head movement umeJated to theconstancY of visual direction. Perception• PsYchophysics, 1969. 5. 258-256.·

WALLACH. H.. .. XllAVITz.. J. H~ Themeumement of the coDatancy of visWlldirection and of Its adaptation.PsYchonomic Science. 1965. 2.217-218.

WALLACH, H., .. KRAVITZ. J. H.Adaptation in the conatancy of visualdirection tested by meMWina theconstancy of auditory direction.

Perception &: Psychophysics 1968 4299-30S. •• ,

NOTES1. Wallacb and Kravitz (1965) obtained a

small but significant deviation from idealCVD and ascribed it to this secondarydisplacement of the eyes. but the seniorauthor did not anticipate Hay and Sawyer'sdiscovery.

2. Excellent compensation for the maintarget displacement due to head movementswas measured by Wallacb and Kravitz (1965and 1968). The article of 1968 contains themore detalled description of CVD and itsmeasurement. Measurements of CVDconcerned with nodding of the head werereported bY Wallach and Floor (1970).These three articles, as well as those byWallach and Frey (1969) and Wallach, Frey.and Romney (1969). also report work onadaptation in CVD to conditions ofabnormal field displacements during headmovements.

3. This equation is an approximationderived from the follOwing relation:

tanp- sin a- dT/dE - cos a .

The approximation is based on theassumption that head movements will notexceed 10 de. to either side of the targetdirection. Under this assumptiOD tan P andsin a may be replaced by the respectiveangles and cos a glven the value one.

4. For a description of thew~ andthe waY they operate, see Wallach and FleY(1972).:

5. A distance of 40 cm from the eYIl8 isequivalent to 2.5 lena diopteD. The I1a8escaQS& the eyes to inczease accommodationby 1.5 lena diopteu (and cauee acorresponding c:banIJe in conveqence) to atotal of .. lens dioptera. and that ve1uecorrespondll to a viewiD& distance of 25 em.

6. The tena. "equivalent cIiAaDCe" wuintroduced by Leibowitz and Moole (1966)'for the distance for which the eYIl8 areadjusted behind devi~ ~t alteraccommOdation and convergence incorrespondinc amounts.

7. A description of the head8ear may befound in Wallach and Floor (19'70).

8. This result represents a lac incom p e nsation for the secondarydl8placement of a stationary target. Adisplacement of the tuaet spot in thedirection with the head movementcompensates objectively for an apparenttaraet dl8placement tJIIaiMt the headmovement. and when an S sees that. hisnervous system fails to compensate for thesecondary tuaet dl8placement.

9. Wallach and Fley (1972) and WlI1Iach.Frey. and Bode (1972) measured adaptationin dl8tance perception in three ways: bYhavina S indicate apparent distance bYpointin&. and by the effect that a cbanlJe indistance perception bas on perceived sizeand on perceived stereoscoPic depth. Theyalso demonstrated the existence of anintervenina vadable, caned ~distance. that results from cues for distance.whose relationship to these cues can bee1ieftd by adaptation, and which, in turn,manifests itself in size and in depthperception.

10. This is probably due to the tact thatveridical distance cues are available whenthe whole field is liven, in addition to theoculomotor cues., which the .-- modify.

11. Our Ss wele selected for .GOdperformance on the CVD test only.

(AcceptJed for publication April 14, 1912.)

Perception &; Psychophysics, 1972, Vol. 12 (2A) 145