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Doesthetiltafter-effectoccurintheobliquemeridian?

ARTICLEinVISIONRESEARCH·FEBRUARY1976

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Vision Res. Vol. 16, pp.609 to 613. Pergamon Press 1976. Printed in Great

DOtrS THE, TILT AFTER.EFFECT OCCI.]R IN THEOBLIQIJE MERIDIAN?

DoNnrD E. MrucuELL and DnnwrN W. MurRlPsychology Department, Dalhousie University, Halifax, N.S., Canada

(Receiued 23 September 1975)

Abstract-In contrast to a recent report,be similar in magnitude and direction to

tilt after-effects induced on obliquethose observed on stimuli that were

stimuli were found tovertical or hortzontal.

In 1933 J. J. Gibson observed that prolonged inspec-tion of a curved line or a bent line segment causeda subsequently viewed straight line to appear brieflyeither slightly curved or bent, respectively. Shortlyafterward it was shown that it was also possible toalter the apparent orientation of a straight line byfirst having the subject look for a few min at a linehaving a slightly different orientation (Gibson, 1934;Vernon, 1934). This tilt after-effect, like the distur-bance of rectilinearity that follows adaptation tocurved or bent lines is usually negative (i.e. the testline is apparently rotated in the opposite directionto the adapting stimulus) when the angle between theadapting and test lines is small. However, with largerangles (exceeding 45") between the test and adaptingstimuli an indirect effect is observed; under these con-ditions the test line is apparently tilted in the samedirection as the adapting stimulus (Gibson, 1934;Gibson and Radner, 1937)"

In the course of a recent re-examination of the tiltafter-effect using gratings as stimuli, Campbell andMaffei (1971) observed that the tilt after-effect onlyoccurred with stimuli close to either vertical or hori-zontal. They were totally unable to change the appabent orientation of an oblique test grating by prioradaptation to gratings at nearby angles. Since thisfinding is at variance with both the results of earliermeasurements made with single lines as stimuli(Kohler and Wallach, I94I) and later measurementswith grating stimuli (Taylor and Parnes, 1972), wedecided to investigate the phenomenon once more inan attempt to resolve this apparent conflict.

METHOD

The apparatus (Fig. 1) was a modification of that de-scribed in detail by Mitchell and Ware (L97 4). The testand adapting gratings, G1 and Gr, had a sinusoidalluminance profile and were generated on the faces of twoTektronix 360 series oscilloscopes. The gratings had a spa-tial frequency of 7'5 cf deg, a luminance of 3 cdfmz, anda contrast of 0'6. They were circular and subtended 2"at the subject's eye. The vertical test grating, Gr, wasviewed by the subject through the half-silvered mirror, M,from a distance of 17l cm. The adapting gratinE, Gz,, whoseorientation relative to the test grating was altered mechani-

1 Present address: Department of Psychology, Queen'sUniversity, Kingston, Ontario, Canada.

cally,, was seen after reflection from this same semi-silveredmirror.

The subject indicated the apparent orientation of thetest grating before and after adaptation by rotating aluminous line, L, so that it appeared parellel to the barsof the grating. The line subtended 2" in height and wassituated 3' to the right of the closest edge of the test grat-ing. The subject was instructed to carefully maintain fixa-tion upon the test grating while making these parallel set-tings so that the image of the luminous line would alwaysfall on a peripheral part of the retin a that had not beenpreviously exposed to the adapting grating. The exper-iments were performed in a darkened room so that onlythe gratings and the line L were visible. The stimuli wereviewed through a Dove prism, P, located immediately infront of the subject's eye, which enabled the orientationof the test grating to be altered for measurement of themagnitude of the after-effects in various retinal meridians.Shutters 51 and 52 permitted either grating to be separ-ately exposed.

Procedure

At the beginning of each session the test grating andline were presented simultaneously for 2 sec every 7 secand the subject was instructed to rotate the line duringthe brief period it was exposed until it appeared parallelto the test grating. The subject was permitted as manyexposures as he wanted to make these pre-adaptation set-tings. Upon completion of 10 of these settings, the subjectviewed the adapting grating for 3 min. Following thisadaptation period, 2-sec presentations of the test gratingwere alternated with 10-sec presentations of the adaptinggrating. As before, the subject was instructed to make atotal of 10 parallel settings during the 2-sec exposures ofthe test grating. The subjects were specifically instructed

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to avoid the temptation to look at the luminous line whenmaking either the pre- or post-adaptation settings. The dif-ference between the means of these two gror"rps of settingswas used as the measure of the magnitude of the aftereffect. In order to be certain that the after-effect had completelydissipated, a period of at least 2 hr was allowed to elapse

between sessions

Sub.jects

There were four sutrjects in all, including the twoauthors. Apart from DEM, who had a moderate meridi-onal amblyopia for horizontal stimuli (Mitchell and Wil-kinson, 1974) all ol the subjects had normal visualacuity. Two of these (JH and AC) were emmetropic, whilethe third (DWM) was a well corrected myope.

RESULTS

Vertical and horizontal test gratings

The magnitude of the tilt after-effect was firstmeasured in all subjects as a function of the orien-tation of the adapting grating relative to a test gratingthat was vertical. Figure 2 shows the size of the after-eff'ect in all four subjects following adaptation to grat-ings oriented at angles from 90" clockwise to 90'anticlockwise (50" in the case of DEM) from vertical.Each point plots the difference between the means

of the 10 parallel settings made before and after adap-

tation; the S.E,.M. of these differences was typically20' . The size of the after-effect increased rapidly inall subjects as the angle between the test and adaptinggrating was increased from 0" to 10'. The largest

after-effect, which occurred with adapting angles ofeither B" or 10o, was between 4' and 6". With largeradapting angles the size of the after-effect slowly de-

clined. Three of the subjects, most notably DWM andDEM, showed evidence of an indirect effect (Gibsonand Radner, 1937) after adaptation to gratings closeto horizontal. With large adapting angles like these,

the apparent orientation of the test grating wasshifted toward the adapting grating. For example, fol-lowing adaptation to a grating rotated 80" clockwisefrom vertical the test grating was apparently tiltedclockwise. In the two subjects who showed the mostpronounced indirect effect, its magnitude was aboutone half of the largest direct effect observed with anadapting grating either 8' or 10' from vertical.

The basic measurement of the size of the after-effectas a function of the orientation of the adapting grat-ing was repeated on DEM with the test grating hori-zontal. Although this subject manifests a mild amb-lyopia in the horizontal meridian (Mitchell and Wil-kinson, 197 4), the general form of the results, shownin Fig. 3, were very similar to those obtained witha vertical test grating.

Oblique test gratings

The experiments were next repeated with the testgrating at an oblique angle (135'). In striking contrastto the very obvious subjective changes in the apparentorientation of vertical or horizontal test gratings fol-lowing adaptation, none of the subjects reported anyobvious alteration in the apparent orientation of an

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after-effect measured on four subjects as a function of the orientationto a vertical test grating. Stimuli rotated clockwise from vertical are

rotated anticlockwise are negative. Gratings 7'5 cldeg; contrast 0'6;mean luminance 3 cdlmz.

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orientotion of odcpting groting relotive to test groting, degFig. 3. The magnitude of the tilt after-effect measured on DEM as a furrction of the orientation ofan adapting grating relative tb a horizontal test grating. The bars through each point indicate + 1

S.E.M. Stimuli as in Fig. 2.

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oblique test grating following inspection of adaptinggratings at any angle. Nevertheless the changes in theparallel settings made by each subject were just asgreat as those seen with vertical and horizontal testgratings. rhe results of these measurements on threeof the subjects are shown in Fig. 4. As in Figs. 2and 3 the points represent the difference between themeans of two sets of 10 parallel settings made beforeand after adaptation. The vertical bars indicate

Does the tilt after-effect occur in the obiique meridian? 6n

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Fig. 4. The magnitude of the tilt after-efffect measured onthree subjects as a function of the orientation of the adapt-ing grating relative to an oblique (at 135") test grating.The bars through each point indicate + I S.E.M. Stimuli

as in Figs. 2 and 3.

1 S.E.M. on either side of this value. In thecase of DEM the size of the after-effect was measuredfollowing adaptation to gratings oriented from 30"anticlockwise to 90" clockwise from the test grating.The range of adapting angles was expanded to covera full 180" for the other two subjects.

The basic form of the results were very similar tothose shown for vertical and horizontal test gratingsin Figs. 2 and 3 respectively. Except for large adaptingangles which sometimes resulted in indirect efI'ects,the apparent orientation of the test grating wasaltered following adaptation so that it appeared tobe rotated away from the adapting grating. with theexception of one subject (JH) the largest after-effectswere observed after adaptation to gratings rotatedfrom the test grating by a very similar amount (8"to 10') to the adapting gratings which produced themaximum after-effects with vertical or horizontal testgratings. In the case of JH the largest after-effectsoccurred with adapting gratings tilted at slightlylarger angles, between 12" and 16o, from the test grat-ing.

The only prominent difference between thesemeasurements and those obtained with vertical orhorizontal test gratings was the larger variability inthe parallel settings when the test grating was obllque.under these conditions the standard errors of themean of the difference between the parallel settingsmade before and after adaptation were typically 30'in the case of DWM and 40' for the other two sub-jects.

DISCUSSION

The results of Fig. 4 indicate that tilt after-effectsdo occur in the oblique meridian and that the basicshape of the function that relates the magnitude ofthe effect to the orientation of the adapting stimulusis comparable to that obtained in either the verticalor horizontal meridians. while this is in accord withearlier reports (Kohler and wallach, r94l; Taylor andParnes, 1972; Gilinsky and cohen, rg72) it conflictswith the more recent finding of campbeil and Maffei(r9l1) who observed no change in the orientation of

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trig. 5. A demonstration of a tilt after-effect in the obliquemeridian. See text for explanation.

an oblique test grating following adaptation to grat'ings oriented at nearby angles. The most probablereason for the discrepancy between the results is the

increased variability associated with parallel settings

in the oblique meridian. None of our subjects

reported observing any apparent ctrange in the orien-tation of the oblique test gratingz after adaptation,yet their parallel settings of the test line (L) indicatedthat its apparent orientation did change. Unless alarge number of settings are made the gre atet variabi-lity associated with parallel settings in the obliquemeridian (Gibson and Radner , 1937; Howard and

Templeton, 1966; Campbell and Maffei, l97l) mayswamp the effect. Figure 5 has been prepared in orderto demonstrate to the reader the existence of the effect

in the oblique meridian. The reader should fixateupon the short hortzontal line between the two grat'ings on the left (A) for about 1 min and then viewthe corresponding line between the two oblique gtat-ings on rhe right (B). It will be noted that the tworight-hand gratings, which physically have the same

orientation (135'), will now appear to be tilted inopposite directions. The upper grating on the left is

rotated 10" clockwise with respect to the two test

gratings on the right while the lower adaptation gtat-ing is tilted 10' anticlockwise. As a result tilt after-effects in opposite directions are produced on the twoidentically oriented test gratings following adaptation.

One of the most intriguing aspects of the tilt after-effect is the indirect effect, the reversed after-effectthat is sometimes observed following adaptation tostimuli oriented at angles exceeding about 60' fromthe test stimulus. Since the first reports of thisphenomenon (Gibson and Radner, 1937) there have

been a number of failures to replicate it (Prentice andBeardslee, 1950; Kohler and Wallach, 1,944; Taylorand Parnes, 1972). A possible explanation for these

negative findings may lie in Muir and Over's (1970)

observation that the indirect effect appears to be con-fined to central vision. Both Prentice and Beardslee

(1950) and Taylor and Parnes (1972) had their sub-jects adapt to stimuli located approx 2o extrafoveally.

2 The reader may verify this apparent negative findingby observing the demonstration of the tilt after-effect pre-

pared by Campbell and Maffei (1971) (Fig. 1) with the

page rotated so that the gratings are oblique.

DoNar-p E. MTTcHELL and DnRwIN W. Mutn

On the other hand, when the adaptation stimuli are

viewed in central vision, indirect effects are usuallyobserved (Gibson and Radner , 1937; Morant and

Mistovtch, 1960; Morant and Harris, 1965; Muir andOver, I9l0; Campbell and Maffe| l9l l; Mitchell and

Ware, 1974). Kohler and Wallach (1944) employedcentral adaptation but failed to observe an indirecteffect if a number of vertical and horizontal contourswere present in the visual field; if however, these

landmarks were excluded from view, indirect effects

were obtained (p. 312).The results of Fig. 3 indicate that with the excep-

tion of AC all of our subjects showed evidence ofindirect effects with vertical test gratings when the

adapting gratings were more than 60" from vertical.The data of Fig. 4 indicate that indirect effects also

occur with oblique test gratings under similar condi-tions.

In recent years there have been a number ofattempts to explain the tilt after-effect as being the

result of either prolonged excitation (for example,

Coltheart, l97I; Over , I9l l) or inhibition (for

example, Tolhurst and Thompson, I9l5) between

orientation-specific neurones in the visual system. Iftilt after-effects do not occur in the oblique meridian,it must be assumed that the neurones subservingoblique orientations (or the interactions between

them) must differ in some way from those that opti-mally respond to either vertical or horrzontal con-tours. However, our results (Fig. 4) indicate that the

tilt after-effects in the oblique meridian are no differ-ent from those induced in either the vertical or hori-zontal meridians. Thus on this basis there is no faa.-

son to suppose that the neurones that are optimallystimulated by either vertical or horizontal contoursare qualitatively any diftbrent from those that respondbest to contours of other orientations.

Acknowledgemenfs--We would like to thank C. Ware forhis assistance in some of these experiments and Drs. F.

W. Campbell and L. Maffei for their comments on an ear-

lier draft of this paper. This work was supported by a

grant from the National Research Council of Canada(APA-7660).

REFERENCES

campbell F. w. and Maffei L. (1971) The tilt after-effect:

a fresh look. Vision Res. 11' 833-840.Coltheart M. (1.911) Visual feature-analyzers and after-ef-

fects of tilt and curvature. Psychol. Reu. 780 IL -|2LGibson J. J. (1933) Adaptation, after-effect and contrast

in the perception of curved lines. J. exp. Psychol. 16,r-31.

Gibson J. J. (1934) Vertical and horrzontal orientation invisual perception. Psychol. BuIl. 31,, 739-740.

Gibson J. J. and Radner M. (1937) Adaptation, after-effectand contrast in the perception of tilted lines. I Quantita-tive studies. J. exp. Psychol. 20, 453-461 .

Gilinsky A. S. and Cohen H. H. (1972) Reaction time tochange in visual orientation. Percept. Psychophys. 11,

129-134.Howard I. P. and Templeton W. B. (1966) Human Spatial

Orientation. Wrley, New York.Kohler W. and Wallach H. (1944) Figural after-effects: an

investigation of visual processes. Pro c. Am, phil. Soc. 880

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Mitchell D. E. and ware c. (1974) Interocular transferof a visual after-effect in normal and stereoblind humans.J. Physiol,, Lond. 236, 707-72L.

Mitchell D. E. and wilkinson F. (1974) The effect of earlyastigmatism on the visual resolution of gratings . J. phy-siol., Lond. 243, 739-756.

Morant R. B. and Mistovich M. (1960) Tilt after-effectsbetween the vertical and horizontal axes. percept. Mot.Skills 10, 75-8 l.

Morant R. B. and Harris J. R.(1965) Two different after-effects of exposure to visual tilts. Am. J. psychol. 78,218-226.

Muir D. and over R. (1970) Tilt after-effects in centraland peripheral vision. J. exp. PsychoL B5o 165-170.

over R. (1971) comparison of normalization theory anclneural enhancement explanation gf negative after-effects.Psychol. Bull. 75, 225-243.

Prentice w. c. H. and Beardslee D. c. (1950) visual "nor-malization" near the vertical and horrzontal. J. exp. psy-chol. 40, 355-364.

Taylor M. M. and Parnes R. A. (1972) Mapping rhe riltafter-effect. D.C.I.E.M. Research Paper 72-Rp-818,Downsview, Ontario, Canada.

Tolhurst D. J. and Thompson P. G. (1975) orientationillusions and after-effects: inhibition between channels.Vision Res. 15, 967-972.

vernon M. D. (1934) The perception of inclined lines. Br.J. Psychol. 25, 18G196.