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Stereoscopic contours and optokinetic nystagmus in normal and stereoblind subjects
0042-6!989/87 S3.00 + 0.00 Copyright Q 1987 Pergamon Journals Ltd
Vitim Ress. Vol. 27, No. 5. pp. 841-844, 1987 Printed in Great Britain. All rights reserved
RESEARCH NOTE STEREOSCOPIC CONTOURS AND OPTOKINETIC NYSTAGMUS IN NORMAL AND STEREOBLIND SUBJECTS STEVENM. ARCHER, KATHLEENK. MILLER and EUGENEM. HELVESTON Department of Ophthalmology, Indiana University School of Medicine, 702 Rotary Circle, Indianapolis, IN 46223, U.S.A. (Received 27 January 1986; in revtied form 19 August 1986) Abstract-Moving stereoscopic contours in a dynamic random-dot stereogram have been previously shown to induce optokinetic nystagmus in subjects with normal stereopsis. For this to be validated as an objective test of stereopsis, stereoblind subjects must also be shown not to develop OKN, especially since it has been shown that the optomotor system of stereoblind individuals retains sensitivity to some cyclopean stimuli. In this report we verify that stereoblind subjects do not have an optomotor response to stereoscopic contours-regardless of the alignment angle at which the stereo image pair is presented. Binocular vision
Cyclopean contours
Optokinetic nystagmus
INTRODUCTION
Objective tests of stereopsis are of considerable interest in the evaluation of strabismus patients and are particularly useful when the patient is a preverbal infant for whom the usual subjective clinical tests cannot be used. Fox et al. (1978) suggested that an objective test of stereopsis could be devised based upon optokinetic nystagmus (OKN) elicited by moving stereoscopic (“cyclopean”) contours in a dynamic randomdot stereogram. They showed that such a stimulus does produce OKN in normal subjects, but tests showing that stereoblind subjects would necessarily fail to show OKN were not reported. On the surface, this later point seems too obvious to require verification. However, Wolfe et al. (1981) have shown that the oculomotor system of stereoblind individuals retains the ability to respond to at least one type of cyclopean stimulus; this raises the interesting possibility that stereoblind subjects might develop OKN in response to moving stereoscopic cyclopean contours, even though they are subjectively unable to perceive those contours. The purpose of this paper is to directly test this possibility.
METHODS
A horizontally drifting grating composed of cyclopean contours was generated in a manner
Random-dot stereogram Stereopsis
similar to that described by Fox et al. (1978). A dynamic random-dot stereogram generator (Shetty et OZ., 1979) produced a stereo image pair of pseudorandom dots which was replaced every 16.4 msec. Each dynamic random-dot image was displayed on a monochrome monitor (modified Panasonic TR-5110T). These two monitors were mounted on the arms of a mirror haploscope (American Optical Wottring Troposcope) which allowed horizontal, vertical and cyclotorsional adjustment of image alignment. The stereogram generator introduced 30’ horizontal disparity at positions specified by the signal from an RCA TClOO5 television camera. This camera was focused on the display produced on an RCA TC1212 monochrome monitor by an Apple IIe computer with a Micromint E-Z Color graphics board. The system was programmed to produce a stereogram consisting of a vertical grating of stereoscopic bars 1.3” wide, with 2.5” separation. This grating could be made to drift either right or left across the screen at 9.3”/sec. The entire field of view for each eye was 18.5”. Each dot subtended 8’ of arc and the dot matrix had a density of 5 dots/deg horizontally and 10 dots/deg vertically. A monocularly visible stimulus consisting of vertical bars of the same dimensions portrayed in random dots on a black background was used as a control. During testing, the subject’s horizontal eye movements were recorded electro-oculographic841
Research Note
842
CONTRbL
STEREO
1 2 SEC
Fig. I. Typical electro-oculograms of responses to horizontally moving monocular control and stereoscopic contours from a stereonormal (top) and a stereoblind (bottom) subject. Upward deflection is Ievoversion. Arrows indicate direction of grating movement.
ally from silver/silver chloride surface electrodes at the lateral canthi with a common electrode centered on the forehead. A Tracoustics RV-275 saccadic velocity recorder was used to amplify the signal and record it on a chart recorder at 5 mm/see. Two groups of 5 subjects each were tested. The first group consisted of subjects who had no history of ocular motility abnormality and normal stereopsis as determined with the Titmus and TN0 tests. The second group were judged to be completely stereoblind by the same tests. There was invariably a history of infantile strabismus in this latter group. All subjects had equal vision in the two eyes and had a corrected acuity of at least 20130. Appropriate optical correction was worn during testing. Normal subjects were tested with the haploscope arms set to zero vergence. Strabismic subjects were tested at zero vergence, at their subjective angle of deviation, and at their objective angle of deviation when this was different from their subjective angle. The haploscope arms were also slowly and continuously moved through a wide range of angles to assure that any alignment angle at which the subject might have a response would not be missed. All subjects except one of the stereoblind subjects were naive as to the purpose of the experiment. All subjects were tested with both crossed and uncrossed disparities. The test runs for each presentation condition lasted approximately 45 set during which the direction of grating movement was reversed once. All records were examined in a masked fashion for evidence of
OKN. Ad~tionally, an index of OKN activity was computed for each record. This index was computed as the difference of the total amplitudes for all saccades opposite to the direction of grating movement versus all saccades in the same direction as the grating movement. This difference was normalized by dividing by the total amplitudes of all saccades in either direction. This index is, in effect, the fraction of saccadic activity appropriate for an OKN response to the grating movement, over and above that which would be expected from random eye movements. This OKN index has a value of 1.Owhen the subject is engaged in OKN throughout the entire record; the value approaches zero when only random eye movements are present (the value from 5 normal subjects viewing the dynamic random-dot display without an embedded stereogram ranged from -0.14 to 0.17). RESULTS
In all cases, the dete~ination of the presence or absence of OKN by masked inspection was unambiguous (Fig. 1) and consistent with the OKN index computed for each record. All subjects developed OKN in response to the monocularly visible control stimulus (OKN index range (X72-0.97 for normals, O-46-0.88 for stereoblind subjects). All normal subjects had OKN in response to the stereoscopic stimulus with both crossed and uncrossed disparity (OKN index range 0.82-0.99). No stereoblind subject exhibited any OKN with the stereo-
a43
Research Note
scopic stimulus under any condition (OKN index range -0.14-0.11). For the normal subjects, the characteristics of the OKN in response to the mon~ularly visible control stimulus was compared to that with the stereoscopic stimulus. There was no consistent difference in frequency of nystagmus cycles, saccade amplitudes or duration of the pursuit phase. However, the mean velocity of the pursuit phase was slower for the stereoscopic condition by an average factor of 0.81 (range 0.72-0.94). DISCUSSION
Our results confirm the report of Fox et ai. (1978) in showing that cyclopean contours produced by binocular disparity are capable of eliciting OKN in stereonormal individuals. We were not able to confirm the longer duration of the pursuit phase and lower nystagmus frequency with cyclopean stimuli which they reported (perhaps due to differences in grating velocities employed). However, the implication which they drew from this combination of findings was that the pursuit lags behind the stimulus movement to a greater extent with cyclopean stimuli (lower gain). This overall conclusion was confirmed by direct measurement of the average velocity of the pursuit phases in our data. This is not surprising since other optomotor responses (refixation saccades) are also known to exhibit longer latencies with cyclopean targets (Archer et al., 1986). We have extended the results of Fox et al. (1978) by showing that stereoscopic contours do not produce OKN in individuals who are stereoblind due to infantile strabismus-regardless of the alignment angle at which the stereo pair is presented. This finding is not in conflict with that of Wolfe et al. (1981) because the stimuli employed are not comparable. It must be emphasized that in this ex~~ment, the dots themselves do not move across the screen. A stimulus capable of eliciting OKN is not physically present in either of the monocular images; the horizontal movement which is the requisite for OKN exists only in the context of stereoscopic combination of the two images, making stereopsis a precondition for eliciting OKN. In contrast, Wolfe et al. (1981) used moving dot stimuli which elicit OKN under monocular conditions and have shown an interaction when presented binocularly in or out of phase. While the binocular phase information in their experi-
ment fits the definition of cyclopean in the strong sense (Julesz, 1971), there is no reason to suppose that it has any relationship to global stereopsis. Consistent with this, Wolfe et al. (1981) postulate two different cyclopean processes, the process demonstrated in their experiment being less susceptible to disruption by the anomalous sensory experience of strabismus than is stereopsis. Our results show that in stereoblind subjects, the Wolfe et al. (1981) finding of a cyclopean influence on the mechanism responsible for OKN cannot be extended to include other types of cyclopean stimuli-specifically not those involving global stereopsis. Furthermore, the types of cyclopean information available to the OKN mechanism have not been shown to differ from those which can be detected subjectively. For moving dot fields under stroboscopic illumination, a change in binocular illumination phase causes both a change in the percentage of time spent in OKN (Wolfe et al., 1981) and a change in the associated vection sensation (Wolfe and Held, 1980) in both normal and stereoblind subjects. Our results also show a correlation of subjective responses and objective OKN responses in that subjects who are able to perceive the randomdot stereogram target also give OKN responses to moving stereoscopic contours while those who cannot perceive such targets (stereoblind by definition) do not give an OKN response. We conclude that optomotor responses to moving stereoscopic contours in a dynamic random-dot stereogram remain a valid means for distinguishing stereonormal from stereoblind individuals. Ackmowiedg~~ts-his resarcb was supported in part by an NIH grant EYO4392 (EMH). S. Archer was supported by Heed&x&tee and H~~napp feilowshi~.
REFERENCES Archer S. M., Hdveston E. M., Miller K. K. and Ellis F. D. (19%) Stereopsis in normal infants and infants with congenital esotropia. Am. J. Ophthul. 101, 591-596. Fox R., Lehmkuhk S. and Leguire L. E. (1978) Stereoscopic contours induce optokinetic nystagmus. Vision Res. 18, 11894192. Julesz 8. (1971) Forut&rfionsofCycfopeau Perception. Univ. of Chicago Press, Chicago. Shetty S. S., Broderson A. J. and Fox R. (1979) A system for generating dynamic random-element stereograms. Behav. Res. Meth. Insman. 11,485490. Wolfe J. M. and Held R. (1980) Cyclopean stimulation can
844
Research Note
influence sensations of self-motion in normal and stereoblind subjects. Percept. Psychophys. 28, 139-142. Wolfe J. M., Held R. and Bauer J. A. (1981) A binocular
contribution to the production of optokinetic nystagmus in normal and stereoblind subjects. Vision Res. 21, 587-590.
Vitim Ress. Vol. 27, No. 5. pp. 841-844, 1987 Printed in Great Britain. All rights reserved
RESEARCH NOTE STEREOSCOPIC CONTOURS AND OPTOKINETIC NYSTAGMUS IN NORMAL AND STEREOBLIND SUBJECTS STEVENM. ARCHER, KATHLEENK. MILLER and EUGENEM. HELVESTON Department of Ophthalmology, Indiana University School of Medicine, 702 Rotary Circle, Indianapolis, IN 46223, U.S.A. (Received 27 January 1986; in revtied form 19 August 1986) Abstract-Moving stereoscopic contours in a dynamic random-dot stereogram have been previously shown to induce optokinetic nystagmus in subjects with normal stereopsis. For this to be validated as an objective test of stereopsis, stereoblind subjects must also be shown not to develop OKN, especially since it has been shown that the optomotor system of stereoblind individuals retains sensitivity to some cyclopean stimuli. In this report we verify that stereoblind subjects do not have an optomotor response to stereoscopic contours-regardless of the alignment angle at which the stereo image pair is presented. Binocular vision
Cyclopean contours
Optokinetic nystagmus
INTRODUCTION
Objective tests of stereopsis are of considerable interest in the evaluation of strabismus patients and are particularly useful when the patient is a preverbal infant for whom the usual subjective clinical tests cannot be used. Fox et al. (1978) suggested that an objective test of stereopsis could be devised based upon optokinetic nystagmus (OKN) elicited by moving stereoscopic (“cyclopean”) contours in a dynamic randomdot stereogram. They showed that such a stimulus does produce OKN in normal subjects, but tests showing that stereoblind subjects would necessarily fail to show OKN were not reported. On the surface, this later point seems too obvious to require verification. However, Wolfe et al. (1981) have shown that the oculomotor system of stereoblind individuals retains the ability to respond to at least one type of cyclopean stimulus; this raises the interesting possibility that stereoblind subjects might develop OKN in response to moving stereoscopic cyclopean contours, even though they are subjectively unable to perceive those contours. The purpose of this paper is to directly test this possibility.
METHODS
A horizontally drifting grating composed of cyclopean contours was generated in a manner
Random-dot stereogram Stereopsis
similar to that described by Fox et al. (1978). A dynamic random-dot stereogram generator (Shetty et OZ., 1979) produced a stereo image pair of pseudorandom dots which was replaced every 16.4 msec. Each dynamic random-dot image was displayed on a monochrome monitor (modified Panasonic TR-5110T). These two monitors were mounted on the arms of a mirror haploscope (American Optical Wottring Troposcope) which allowed horizontal, vertical and cyclotorsional adjustment of image alignment. The stereogram generator introduced 30’ horizontal disparity at positions specified by the signal from an RCA TClOO5 television camera. This camera was focused on the display produced on an RCA TC1212 monochrome monitor by an Apple IIe computer with a Micromint E-Z Color graphics board. The system was programmed to produce a stereogram consisting of a vertical grating of stereoscopic bars 1.3” wide, with 2.5” separation. This grating could be made to drift either right or left across the screen at 9.3”/sec. The entire field of view for each eye was 18.5”. Each dot subtended 8’ of arc and the dot matrix had a density of 5 dots/deg horizontally and 10 dots/deg vertically. A monocularly visible stimulus consisting of vertical bars of the same dimensions portrayed in random dots on a black background was used as a control. During testing, the subject’s horizontal eye movements were recorded electro-oculographic841
Research Note
842
CONTRbL
STEREO
1 2 SEC
Fig. I. Typical electro-oculograms of responses to horizontally moving monocular control and stereoscopic contours from a stereonormal (top) and a stereoblind (bottom) subject. Upward deflection is Ievoversion. Arrows indicate direction of grating movement.
ally from silver/silver chloride surface electrodes at the lateral canthi with a common electrode centered on the forehead. A Tracoustics RV-275 saccadic velocity recorder was used to amplify the signal and record it on a chart recorder at 5 mm/see. Two groups of 5 subjects each were tested. The first group consisted of subjects who had no history of ocular motility abnormality and normal stereopsis as determined with the Titmus and TN0 tests. The second group were judged to be completely stereoblind by the same tests. There was invariably a history of infantile strabismus in this latter group. All subjects had equal vision in the two eyes and had a corrected acuity of at least 20130. Appropriate optical correction was worn during testing. Normal subjects were tested with the haploscope arms set to zero vergence. Strabismic subjects were tested at zero vergence, at their subjective angle of deviation, and at their objective angle of deviation when this was different from their subjective angle. The haploscope arms were also slowly and continuously moved through a wide range of angles to assure that any alignment angle at which the subject might have a response would not be missed. All subjects except one of the stereoblind subjects were naive as to the purpose of the experiment. All subjects were tested with both crossed and uncrossed disparities. The test runs for each presentation condition lasted approximately 45 set during which the direction of grating movement was reversed once. All records were examined in a masked fashion for evidence of
OKN. Ad~tionally, an index of OKN activity was computed for each record. This index was computed as the difference of the total amplitudes for all saccades opposite to the direction of grating movement versus all saccades in the same direction as the grating movement. This difference was normalized by dividing by the total amplitudes of all saccades in either direction. This index is, in effect, the fraction of saccadic activity appropriate for an OKN response to the grating movement, over and above that which would be expected from random eye movements. This OKN index has a value of 1.Owhen the subject is engaged in OKN throughout the entire record; the value approaches zero when only random eye movements are present (the value from 5 normal subjects viewing the dynamic random-dot display without an embedded stereogram ranged from -0.14 to 0.17). RESULTS
In all cases, the dete~ination of the presence or absence of OKN by masked inspection was unambiguous (Fig. 1) and consistent with the OKN index computed for each record. All subjects developed OKN in response to the monocularly visible control stimulus (OKN index range (X72-0.97 for normals, O-46-0.88 for stereoblind subjects). All normal subjects had OKN in response to the stereoscopic stimulus with both crossed and uncrossed disparity (OKN index range 0.82-0.99). No stereoblind subject exhibited any OKN with the stereo-
a43
Research Note
scopic stimulus under any condition (OKN index range -0.14-0.11). For the normal subjects, the characteristics of the OKN in response to the mon~ularly visible control stimulus was compared to that with the stereoscopic stimulus. There was no consistent difference in frequency of nystagmus cycles, saccade amplitudes or duration of the pursuit phase. However, the mean velocity of the pursuit phase was slower for the stereoscopic condition by an average factor of 0.81 (range 0.72-0.94). DISCUSSION
Our results confirm the report of Fox et ai. (1978) in showing that cyclopean contours produced by binocular disparity are capable of eliciting OKN in stereonormal individuals. We were not able to confirm the longer duration of the pursuit phase and lower nystagmus frequency with cyclopean stimuli which they reported (perhaps due to differences in grating velocities employed). However, the implication which they drew from this combination of findings was that the pursuit lags behind the stimulus movement to a greater extent with cyclopean stimuli (lower gain). This overall conclusion was confirmed by direct measurement of the average velocity of the pursuit phases in our data. This is not surprising since other optomotor responses (refixation saccades) are also known to exhibit longer latencies with cyclopean targets (Archer et al., 1986). We have extended the results of Fox et al. (1978) by showing that stereoscopic contours do not produce OKN in individuals who are stereoblind due to infantile strabismus-regardless of the alignment angle at which the stereo pair is presented. This finding is not in conflict with that of Wolfe et al. (1981) because the stimuli employed are not comparable. It must be emphasized that in this ex~~ment, the dots themselves do not move across the screen. A stimulus capable of eliciting OKN is not physically present in either of the monocular images; the horizontal movement which is the requisite for OKN exists only in the context of stereoscopic combination of the two images, making stereopsis a precondition for eliciting OKN. In contrast, Wolfe et al. (1981) used moving dot stimuli which elicit OKN under monocular conditions and have shown an interaction when presented binocularly in or out of phase. While the binocular phase information in their experi-
ment fits the definition of cyclopean in the strong sense (Julesz, 1971), there is no reason to suppose that it has any relationship to global stereopsis. Consistent with this, Wolfe et al. (1981) postulate two different cyclopean processes, the process demonstrated in their experiment being less susceptible to disruption by the anomalous sensory experience of strabismus than is stereopsis. Our results show that in stereoblind subjects, the Wolfe et al. (1981) finding of a cyclopean influence on the mechanism responsible for OKN cannot be extended to include other types of cyclopean stimuli-specifically not those involving global stereopsis. Furthermore, the types of cyclopean information available to the OKN mechanism have not been shown to differ from those which can be detected subjectively. For moving dot fields under stroboscopic illumination, a change in binocular illumination phase causes both a change in the percentage of time spent in OKN (Wolfe et al., 1981) and a change in the associated vection sensation (Wolfe and Held, 1980) in both normal and stereoblind subjects. Our results also show a correlation of subjective responses and objective OKN responses in that subjects who are able to perceive the randomdot stereogram target also give OKN responses to moving stereoscopic contours while those who cannot perceive such targets (stereoblind by definition) do not give an OKN response. We conclude that optomotor responses to moving stereoscopic contours in a dynamic random-dot stereogram remain a valid means for distinguishing stereonormal from stereoblind individuals. Ackmowiedg~~ts-his resarcb was supported in part by an NIH grant EYO4392 (EMH). S. Archer was supported by Heed&x&tee and H~~napp feilowshi~.
REFERENCES Archer S. M., Hdveston E. M., Miller K. K. and Ellis F. D. (19%) Stereopsis in normal infants and infants with congenital esotropia. Am. J. Ophthul. 101, 591-596. Fox R., Lehmkuhk S. and Leguire L. E. (1978) Stereoscopic contours induce optokinetic nystagmus. Vision Res. 18, 11894192. Julesz 8. (1971) Forut&rfionsofCycfopeau Perception. Univ. of Chicago Press, Chicago. Shetty S. S., Broderson A. J. and Fox R. (1979) A system for generating dynamic random-element stereograms. Behav. Res. Meth. Insman. 11,485490. Wolfe J. M. and Held R. (1980) Cyclopean stimulation can
844
Research Note
influence sensations of self-motion in normal and stereoblind subjects. Percept. Psychophys. 28, 139-142. Wolfe J. M., Held R. and Bauer J. A. (1981) A binocular
contribution to the production of optokinetic nystagmus in normal and stereoblind subjects. Vision Res. 21, 587-590.