RESEARCH NOTE ON lMEASURING INFEROCULAR
TRANSFER
STEPHEXW. LEHMIGHLEand ROBERT Fox Department of Psychology. Vanderbilt University. Nashville. TN 57240. Li.S.A. (Receiced
17
.4ugusr 1975: ih recked
Interocuiar transfer is a well known and widely used technique for inferentially partitioning the visual system. The usual experimental design consists of stimulating one eye with an adapting stimulus and then testing the other eye for the effects of adaptation. At issue in this paper is the influence of the method used to block stimulation of one eye during adaptation and during test. The most convenient method for blocking stimulation is an opaque occluder. but its use involves a number of potentially und~irab~e effects. For example. during the adaptatton phase the occluded eye’s dark view could interact with the processing of the adapting stimulus. Occlusion alters pupil diameter in the open eye and may influence accommodation and eye movements in that eye. Occlusion brings about dark adaptation. which produces a sensitivity to light that is different from the sensitivity of the open eve. Finally. the occiuder provides the observer with information about which eye is under test. Verhoeff (1935) has argued eloquently that these kinds of differences make it impossible ever to obtain true monocular vision. Although it may be logically impossible to obtain true monocular vision. it is possible to provide the eyes with equivalent stimulation. With a haploscopic presentation system. one eye can be stimulated by the adapting stimulus while the other eye views a homog~~us fiefd of the same mean luminance as the adapting stimulus. Convergence. pupii size. and accommodation are the same for both eves. Under these conditions observers report that it ‘is difficult to discern which eye is being stimulated, an observation consistent with the literature on utrocular discrimination (see. for example. Smith. 1945. and Enoch, Goldmann and Sunga, 1969). We shall call this method of stimulation equivalence occlusion. in contrast to the more convenient method of achieving monocularity through the use of an opaque occluder. In the experiment reported here we compared the effects of opaque occlusion and of equivalence occlusion on the magnitude of the motion aftereffect and on the amount of interocutar transfer of the aftereffect. We found that the method of occlusion does not influence the magnitude of the aftereffect but does substantially alter the amount of transfer. with less transfer produced under conditions of opaque occlusion than under equivalence occlusion. METHOD .-Lpparaftrs
The basic apparatus was a large haploscope constructed from optic bench components enclosed in a lightproof
jbrm
18 Septernhrr
1975)
housing. The viewing port of the housing uas fitted with a head rest. two eye apertures ?j-lcm dia. and electronic shutters that could be used to occlude each eye independently. The pivot points of the haploscope arms were located under the pivot points of the observer’s et.cs. Rotation of the haploscope arms and rotation of mirrors in the two optic paths permitted adjustment of convergence angle for each subject. .A cathode ray tube (CRT) was mountetl in an enclosed housing at the end of each haploscope arm at a distance of 1 m from the eye apertures. Each CRT displayed either a vertically oriented sine-5vav.e grating (spatial frequency 3.0 c/de& mean luminance 6% cd,m’. contrast 0.30) or a homogeneous raster of 6.85 cd,‘m’ mean luminance. These dispiavs were produced bv standard methods. A XWkHz sinus&daf voltaee aoolieh to the vertical axis of the oscilloscope generated a homegeneous raster. .A l.J-kHz sinusoidal voltage applied to the Z-axis modulated the homogeneous raster to produce a sine-wave grating. A svnchroresolver. which varied the phase relation between the sinusoidal voltages present at the external trigger and at the Z-axis. was used to set the grating in linear motion. When used as the aftereffect-inducing stimulus. the velocity of the grating was I deg, see: when used as the test stimulus. the grating was stationary. Each CRT display was viewed through a 1’ square apcrture in the center of a plexiglas sheet transiIluminated b) an array of small incandescent lamps at a mean luminance level of 10.X cd;m’. Vertical black contours 7.7’ wide separated by __ “.i’_ were located on the left and right flanks of both piexiglas apertures to facilitate fusion of the NO displays. A set of timing and logic units automatically controlIed the events associated with each trial. Subjecrs
All six observers possessed well corrected vision; acuity and phoria were within normat limits according to tests on the modified Orthorater. Three observers were not aware of the purpose of the experiment. All received practice in observing the motion aftereffect and in reporting upon its duration. Procerirtre
The experiment consisted of four conditions. monocular vs interocular transfer under both methods of occlusion. For the monocular~opaque occlusion condition, the right eye was physically occiuded by a shutter while the left eye viewed both the inducing and the test stimuli. In the monocular/equivalence occlusion condition. the right eye viewed the homogeneous raster while the left eye viewed the inducing and test stimuli. In the interocuiar;opaque occlusion condition. the left eye was physically occluded while the right eye viewed the mducing stimulus: then the right eye was physically occluded and the left eye viewed the test stimulus. In the interoculariequivalence occlusion condition, the left eye viewed the homogeneous raster while the ri_ghteye viewed the inducing stimulus: then the right eye viewed the raster and the left eye viewed the
Research Note test stimulus. Under each of these conditions the observer reported upon the duration of the aftereffect by depressing a switch, which activated a clock. as soon as aftereffect motion started and releasing the switch when motion ceased. For each of the four conditions 15 trials were run. Each trial consisted of a 45set induction period followed by presentation of the stationary test p&tern. Trials were obtained in three I-hr daily sessions; each session consisted of 20 trials. 5 trials for each of the four conditions. The order of presentation of each condition was quasirandom. After every four trials the direction of the inducing movement was reversed to preclude any long-term growth of the aftereffect. To allow for dissipation of the aftereffect. a rest period of 90 set occurred after every trial. Considerable care was taken to insure that each subject could achieve and maintain binocular fusion of the displays throughout the experiment. RESCLTS
The per cent of transfer [(interocular aftereffect duration/monocular aftereffect duration) x 100-j was calculated for opaque occlusion and for equivalence occlusion. The per cent of transfer obtained with opaque occlusion (M = 527;. S.E. = 49,) is consistent with earlier research (see Holland. 1965). The per cent of transfer obtained with equivalence occlusion (M = 760;. SE. = 1.7%) was significantly greater, I (5) = 7.70. P c 0001. Motion aftereffect duration is plotted as a function of condition in Fig. 1. There were significant effects for both the type of occlusion, F (1,s) = 2057, P < 0.01, and the type of aftereffect, F (IS) = 34.91. P < 0005. There was a significant interaction between conditions, F (1.5) = 25.4s. P c 0405. Further analyses showed that there was no effect for the type of occlusion for the monocular aftereffects. t (5) = IGO, P > @50, but a significant effect for the type of occlusion for the interocular aftereffects, t (2) = 501. P < 0.01. This interaction and the significant difference between amount of transfer under the two occlusion conditions are both due to the increased duration under the interocular/equit;alence occlusion condition. The pattern of results revealed
t
L
I
I
OPAO”C
EQVIvALLNtC
OCCLUSION
Fig. I. The duration of the motion aftereffect plotted for opaque and equivalence occlusion and for monocular and interocular transfer conditions. Each data point represents the mean duration for six observers. The confidence intervals represent I S.E.
429
by this group analysis was also present in the results of individual observers. DISCUSSION
The interpretation of our results is straightforward. Duration of the aftereffect under monocular conditions was the same regardless of occlusion method. This result would seem to imply that the darkened view of the occluded eye does not interact with the adaptation process. But the duration of the aftereffect under interocular transfer conditions is substantially altered by the occlusion method. The duration obtained under opaque occlusion is significantly less than that obtained under equivalence occlusion. One possible explanation is that aftereffect measurement is confounded by changes in light adaptation in the previously occluded eye. Further. it is conceivable that sudden onset of physical occlusion in the adapted eye produces an interruptive masking effect. Since these potentially confounding factors cannot operate during equicalence occlusion. we feel that a more nearly adequate estimate of transfer is provided by this method. Whether or not these differences between opaque and equivalence occlusion are important depends upon purpose. If the purpose is only to test for the presence of transfer, then the more conventional opaque occlusion method may be sufficient. But when quantitative estimates of transfer are of interest, equivalence occlusion may be essential. For instance. the model for binocular interaction of aftereffects that we have recently described (Lehmkuhle and Fox. 1975) makes quantitative predictions about the magnitude of aftereffects obtained under binocular, monocular. interocular. and rivalry viewing conditions for observers with normal binocular vision and for observers with anomalous binocular vision. One prediction of the model is that the amount of interocular transfer is given by the equation I = ZM - B, where I = interocular transfer, M = monocular aftereffect, and B = binocular aftereffect. In one early test of that prediction we used opaque occlusion so that we could quickly obtain data from a large number of subjects, and we found that the amount of transfer predicted for each observer was consistently greater than that obtained. Repeating the experiment with the equivalence occlusion method yielded quite close agreement between predicted and obtained transfer for each subject (correlation r = 0.90 for 15 subjects). With respect to prior research on the interocular transfer of various kinds of visual aftereffects, it is likely that those studies that used opaque occlusion underestimated the amount of transfer. It remains an open question whether or not the method of equivalence occlusion would reveal interocular transfer in those instances where transfer has not been found. It is interesting that the transferred aftereffect obtained in this experiment was always less than the aftereffect yielded by monocular viewing. even under the more optimal conditions provided by equivalence occlusion. We expect that this inequality will always hold. It is a relationship predicted by our model and is consistent with other theoretical formulations and other lines of evidence.
430
Research SOW
.~c~ito~lrdgernmt-This work was supported by a grant from the National Institutes of Health (EYOOXW REFERENCES Enoch J., Cioldmann H. and Sunga R. (1969) The ability to distinguish which eye was stimulated by tight. Inrescre Ophch. 8, 317-331. Holland H. C. (1965) The Spiral Ajtereficr. Pergamon Press. Oxford.
Lehmkuhle S. W. and Fox R. (1975) Binocular interaction of the motion aftereffect: a simple linear model. Paper presented to the Association for Research in Vision and bphthalmology. Smith S. (1915) Utrocular or “which eve” discrimination. J. rxp. &chol. 35. 1-1-t. Verhoeff F. H. (1935) A new theory of binocular vision. .bchs Ophrhal. 13. 151-175.
TRANSFER
STEPHEXW. LEHMIGHLEand ROBERT Fox Department of Psychology. Vanderbilt University. Nashville. TN 57240. Li.S.A. (Receiced
17
.4ugusr 1975: ih recked
Interocuiar transfer is a well known and widely used technique for inferentially partitioning the visual system. The usual experimental design consists of stimulating one eye with an adapting stimulus and then testing the other eye for the effects of adaptation. At issue in this paper is the influence of the method used to block stimulation of one eye during adaptation and during test. The most convenient method for blocking stimulation is an opaque occluder. but its use involves a number of potentially und~irab~e effects. For example. during the adaptatton phase the occluded eye’s dark view could interact with the processing of the adapting stimulus. Occlusion alters pupil diameter in the open eye and may influence accommodation and eye movements in that eye. Occlusion brings about dark adaptation. which produces a sensitivity to light that is different from the sensitivity of the open eve. Finally. the occiuder provides the observer with information about which eye is under test. Verhoeff (1935) has argued eloquently that these kinds of differences make it impossible ever to obtain true monocular vision. Although it may be logically impossible to obtain true monocular vision. it is possible to provide the eyes with equivalent stimulation. With a haploscopic presentation system. one eye can be stimulated by the adapting stimulus while the other eye views a homog~~us fiefd of the same mean luminance as the adapting stimulus. Convergence. pupii size. and accommodation are the same for both eves. Under these conditions observers report that it ‘is difficult to discern which eye is being stimulated, an observation consistent with the literature on utrocular discrimination (see. for example. Smith. 1945. and Enoch, Goldmann and Sunga, 1969). We shall call this method of stimulation equivalence occlusion. in contrast to the more convenient method of achieving monocularity through the use of an opaque occluder. In the experiment reported here we compared the effects of opaque occlusion and of equivalence occlusion on the magnitude of the motion aftereffect and on the amount of interocutar transfer of the aftereffect. We found that the method of occlusion does not influence the magnitude of the aftereffect but does substantially alter the amount of transfer. with less transfer produced under conditions of opaque occlusion than under equivalence occlusion. METHOD .-Lpparaftrs
The basic apparatus was a large haploscope constructed from optic bench components enclosed in a lightproof
jbrm
18 Septernhrr
1975)
housing. The viewing port of the housing uas fitted with a head rest. two eye apertures ?j-lcm dia. and electronic shutters that could be used to occlude each eye independently. The pivot points of the haploscope arms were located under the pivot points of the observer’s et.cs. Rotation of the haploscope arms and rotation of mirrors in the two optic paths permitted adjustment of convergence angle for each subject. .A cathode ray tube (CRT) was mountetl in an enclosed housing at the end of each haploscope arm at a distance of 1 m from the eye apertures. Each CRT displayed either a vertically oriented sine-5vav.e grating (spatial frequency 3.0 c/de& mean luminance 6% cd,m’. contrast 0.30) or a homogeneous raster of 6.85 cd,‘m’ mean luminance. These dispiavs were produced bv standard methods. A XWkHz sinus&daf voltaee aoolieh to the vertical axis of the oscilloscope generated a homegeneous raster. .A l.J-kHz sinusoidal voltage applied to the Z-axis modulated the homogeneous raster to produce a sine-wave grating. A svnchroresolver. which varied the phase relation between the sinusoidal voltages present at the external trigger and at the Z-axis. was used to set the grating in linear motion. When used as the aftereffect-inducing stimulus. the velocity of the grating was I deg, see: when used as the test stimulus. the grating was stationary. Each CRT display was viewed through a 1’ square apcrture in the center of a plexiglas sheet transiIluminated b) an array of small incandescent lamps at a mean luminance level of 10.X cd;m’. Vertical black contours 7.7’ wide separated by __ “.i’_ were located on the left and right flanks of both piexiglas apertures to facilitate fusion of the NO displays. A set of timing and logic units automatically controlIed the events associated with each trial. Subjecrs
All six observers possessed well corrected vision; acuity and phoria were within normat limits according to tests on the modified Orthorater. Three observers were not aware of the purpose of the experiment. All received practice in observing the motion aftereffect and in reporting upon its duration. Procerirtre
The experiment consisted of four conditions. monocular vs interocular transfer under both methods of occlusion. For the monocular~opaque occlusion condition, the right eye was physically occiuded by a shutter while the left eye viewed both the inducing and the test stimuli. In the monocular/equivalence occlusion condition. the right eye viewed the homogeneous raster while the left eye viewed the inducing and test stimuli. In the interocuiar;opaque occlusion condition. the left eye was physically occluded while the right eye viewed the mducing stimulus: then the right eye was physically occluded and the left eye viewed the test stimulus. In the interoculariequivalence occlusion condition, the left eye viewed the homogeneous raster while the ri_ghteye viewed the inducing stimulus: then the right eye viewed the raster and the left eye viewed the
Research Note test stimulus. Under each of these conditions the observer reported upon the duration of the aftereffect by depressing a switch, which activated a clock. as soon as aftereffect motion started and releasing the switch when motion ceased. For each of the four conditions 15 trials were run. Each trial consisted of a 45set induction period followed by presentation of the stationary test p&tern. Trials were obtained in three I-hr daily sessions; each session consisted of 20 trials. 5 trials for each of the four conditions. The order of presentation of each condition was quasirandom. After every four trials the direction of the inducing movement was reversed to preclude any long-term growth of the aftereffect. To allow for dissipation of the aftereffect. a rest period of 90 set occurred after every trial. Considerable care was taken to insure that each subject could achieve and maintain binocular fusion of the displays throughout the experiment. RESCLTS
The per cent of transfer [(interocular aftereffect duration/monocular aftereffect duration) x 100-j was calculated for opaque occlusion and for equivalence occlusion. The per cent of transfer obtained with opaque occlusion (M = 527;. S.E. = 49,) is consistent with earlier research (see Holland. 1965). The per cent of transfer obtained with equivalence occlusion (M = 760;. SE. = 1.7%) was significantly greater, I (5) = 7.70. P c 0001. Motion aftereffect duration is plotted as a function of condition in Fig. 1. There were significant effects for both the type of occlusion, F (1,s) = 2057, P < 0.01, and the type of aftereffect, F (IS) = 34.91. P < 0005. There was a significant interaction between conditions, F (1.5) = 25.4s. P c 0405. Further analyses showed that there was no effect for the type of occlusion for the monocular aftereffects. t (5) = IGO, P > @50, but a significant effect for the type of occlusion for the interocular aftereffects, t (2) = 501. P < 0.01. This interaction and the significant difference between amount of transfer under the two occlusion conditions are both due to the increased duration under the interocular/equit;alence occlusion condition. The pattern of results revealed
t
L
I
I
OPAO”C
EQVIvALLNtC
OCCLUSION
Fig. I. The duration of the motion aftereffect plotted for opaque and equivalence occlusion and for monocular and interocular transfer conditions. Each data point represents the mean duration for six observers. The confidence intervals represent I S.E.
429
by this group analysis was also present in the results of individual observers. DISCUSSION
The interpretation of our results is straightforward. Duration of the aftereffect under monocular conditions was the same regardless of occlusion method. This result would seem to imply that the darkened view of the occluded eye does not interact with the adaptation process. But the duration of the aftereffect under interocular transfer conditions is substantially altered by the occlusion method. The duration obtained under opaque occlusion is significantly less than that obtained under equivalence occlusion. One possible explanation is that aftereffect measurement is confounded by changes in light adaptation in the previously occluded eye. Further. it is conceivable that sudden onset of physical occlusion in the adapted eye produces an interruptive masking effect. Since these potentially confounding factors cannot operate during equicalence occlusion. we feel that a more nearly adequate estimate of transfer is provided by this method. Whether or not these differences between opaque and equivalence occlusion are important depends upon purpose. If the purpose is only to test for the presence of transfer, then the more conventional opaque occlusion method may be sufficient. But when quantitative estimates of transfer are of interest, equivalence occlusion may be essential. For instance. the model for binocular interaction of aftereffects that we have recently described (Lehmkuhle and Fox. 1975) makes quantitative predictions about the magnitude of aftereffects obtained under binocular, monocular. interocular. and rivalry viewing conditions for observers with normal binocular vision and for observers with anomalous binocular vision. One prediction of the model is that the amount of interocular transfer is given by the equation I = ZM - B, where I = interocular transfer, M = monocular aftereffect, and B = binocular aftereffect. In one early test of that prediction we used opaque occlusion so that we could quickly obtain data from a large number of subjects, and we found that the amount of transfer predicted for each observer was consistently greater than that obtained. Repeating the experiment with the equivalence occlusion method yielded quite close agreement between predicted and obtained transfer for each subject (correlation r = 0.90 for 15 subjects). With respect to prior research on the interocular transfer of various kinds of visual aftereffects, it is likely that those studies that used opaque occlusion underestimated the amount of transfer. It remains an open question whether or not the method of equivalence occlusion would reveal interocular transfer in those instances where transfer has not been found. It is interesting that the transferred aftereffect obtained in this experiment was always less than the aftereffect yielded by monocular viewing. even under the more optimal conditions provided by equivalence occlusion. We expect that this inequality will always hold. It is a relationship predicted by our model and is consistent with other theoretical formulations and other lines of evidence.
430
Research SOW
.~c~ito~lrdgernmt-This work was supported by a grant from the National Institutes of Health (EYOOXW REFERENCES Enoch J., Cioldmann H. and Sunga R. (1969) The ability to distinguish which eye was stimulated by tight. Inrescre Ophch. 8, 317-331. Holland H. C. (1965) The Spiral Ajtereficr. Pergamon Press. Oxford.
Lehmkuhle S. W. and Fox R. (1975) Binocular interaction of the motion aftereffect: a simple linear model. Paper presented to the Association for Research in Vision and bphthalmology. Smith S. (1915) Utrocular or “which eve” discrimination. J. rxp. &chol. 35. 1-1-t. Verhoeff F. H. (1935) A new theory of binocular vision. .bchs Ophrhal. 13. 151-175.
