Spatial Characteristics of Static and Dynamic Stereoacuity in

Clifton Schor, Bruce Bridgeman, and Christopher W. Tyler*

The spatial and temporal organization of stereoscopic depth were compared in normal and strabismic observers. The minimum and maximum disparities for stimulating static and dynamic in strabismus were examined as a function of spatial separation of disparate stimuli. Disparities and their spacing were produced by spatial modulation of two vertical lines viewed haploscopically. Most strabismics had normal upper disparity limits but elevated static and dynamic stereothresholds. Moderate stereothreshold elevations (100 arc sec) were constant for spatial separations greater than 15 arc min. Two new types of spatial crowding effects upon stereopsis were observed. The first type resulted from the constant elevation of the disparity threshold. The second type consisted of a reduced maximum disparity limit for stereopsis. In both cases, the constriction of the range of perceivable depth produced a reduction in the spatial and temporal frequency limits for . Clinical tests of stereoacuity that crowd stimuli closer than 0.25 degree underestimated the strabismic patients' potential stereoacuities by a factor of 2 to 4. Similarly, tests of dynamic stereopsis that use temporal frequencies greater than 1 Hz will underestimate optimal dynamic stereoacuity. Invest Ophthalmol Vis Sci 24:1572-1579, 1983

Both static1 and dynamic stereoacuity2'3 can be re- form constant disparities subtended by large targets, duced in the central visual field of patients with stra- yield stereoacuity inversely proportional to target size. bismus and other anomalies of . Recent In this study, we have measured both the upper dis- psychophysical4"8 and electrophysiologic studies9 in- parity limit and lower disparity threshold for static dicate that separate mechanisms underlie static and stereopsis in strabismic observers with spatially varying dynamic stereopsis. Furthermore, static disparities are targets. We observed a constant elevation of the stereo- believed to be processed optimally by spatial channels threshold over a range of coarse target sizes and an whose receptive field sizes are proportional to disparity increase in the minimum target size for any depth magnitude.10 This model is supported by a correlation perception, although the spatial limit for fusion, as between stimulus size and amplitude of static stereo- shown previously, remained normal.17 scopic threshold.11"13 Disturbances of stereopsis as- Dynamic stereoacuity usually is measured with ver- sociated with binocular disorders may involve some tical straight lines oscillating in depth.18"21 The straight or all of these mechanisms that support binocular depth lines subtend a constant disparity and yield dynamic perception. stereothresholds that decrease slightly as temporal fre- Spatial properties of clinical tests for both static and quency increases up to 1 Hz and then remains constant dynamic stereoacuity vary markedly. Some tests use up to 5 Hz.1819-21 We have examined dynamic stereo- small figures (Keystone visual skills series),14 whereas acuity in observers with normal and abnormal bin- others use large objects that subtend a constant dis- ocular vision using simultaneous spatial and temporal parity (Wirt test and Verhoeff Stereopter).15-16 Tyler11-12 modulation of disparity. The purpose of this study was observed for normal subjects that the abrupt changes to define in greater detail the types of stereoscopic loss in disparity that occur with small targets and the uni- that can occur with complex disparity modulation similar to that found in a normal environment. From the University of California at Berkeley and The Smith- Kettlewell Institute for Visual Sciences,* Medical Research Institute, Materials and Methods San Francisco, California. Supported by NEI Grant No. EY 03532, NSF Grant BNS 79- We measured static stereopsis for a full range of 06858, and NIH Grant EY 03884. stimulus patterns of the form introduced by Tyler.11>12 Experimental work was conducted while B.B. was on sabbatical Spatial variations of disparity were produced on two leave from the University of California, Santa Cruz. Submitted for publication: December 1, 1982. CRTs with dichoptic vertical sinusoidal lines, one be- Reprint requests: Clifton Schor, University of California, School fore each eye, (8° long and 1.5 min thick) having a 2 of Optometry, Berkeley, CA 94720. luminance of 15 cd/m and a binocular spatial phase

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CS

Fig. 1. Complete region of depth perception as a func- tion of spatial disparity mod- ulation frequency of vertical sinusoidal disparity varia- tions for two observers with normal stereopsis. (Inset: Upper portion—left and right eye views of a sample anti-phase stimulus; lower portion—depiction of stim- ulus as seen by the observer.)

0.1 0.2 0.5 1 2 5 0.1 0.2 0.5 1 SPATIAL FREQUENCY OF DISPARITY MODULATION (cyc/deg)

difference of 180° (Fig. 1, inset). When these vertical space between the two horizontal nonius lines and sinusoidal lines (one on each CRT) were fused hap- adjust a ten turn pot that controlled the amplitude of loscopically, observers perceived a single line curved sinusoidal disparity modulation, until the sinusoidal sinusoidally in depth, with the plane of curvature pass- line appeared to be just non-uniform in depth, and ing through the midline between the eye (Fig. 1, inset). then until it appeared to be uniform in depth. Since The spatial frequency of the vertical sinusoidal lines depth judgments involved simultaneous comparison determined the proximity or spatial crowding of si- of crossed and uncrossed disparities, amplitude of dis- nusoidal peaks in depth (spatial disparity modulation) parity equalled the difference between uniocular peak- and it also provided a means of denning the gradient to-peak amplitude of sinusoidal modulation. The mean or rate of change of horizontal disparity variations of six settings constituted a threshold measurement. along the vertical meridian. Varying the spatial fre- Stereoscopic thresholds were obtained in the same quency of the disparity modulation allowed exami- manner for square wave variations in disparity as an nation of both upper disparity limits and lower dis- additional condition to more closely approximate the parity thresholds across a full range of disparity pat- disparity profile of standard tests of stereopsis. terns. The maximum sinusoidal disparity that could elicit Horizontal and vertical vergence errors were con- a static depth modulation (upper disparity limit) also trolled by adjusting the mirrors of a haploscope to the was determined by the method of adjustment. Depth observer's angle of strabismus or to an angle where usually was perceived beyond Panum's fusional limit, the targets could be fused easily. The boundaries of ie, where diplopia was present.121722 Subjects were in- the 8 X 13 degree rectangular test field and a small structed to maintain the criterion of absence of depth central fixation spot served as sensory fusion locks. difference, although diplopia was clearly present in the Horizontal nonius lines were placed to the left and display. Both strabismic and normal observers reported right of the vertical sinusoidal line and the small fix- that depth was clearly present for a limited range of ation spot (1.5 arc min). The observer could detect disparities beyond the diplopia threshold and that be- vertical vergence errors by observing a misalignment yond the upper depth limit they perceived two sinu- of the horizontal nonius lines. The targets were viewed soidal lines lying flat in the fixation plane. Subjects at a distance of 57 cm on a background screen with verified their depth perception by correctly identifying a luminance of 50 cd/m2. portions of the sine curve that appeared proximal and Thresholds were obtained by method of adjustment. distal from the fixation point. Comparison of the upper During tests of static stereopsis the observer was in- and lower thresholds for stereopsis provided a measure structed to fixate the small spot in the center of the of the range of disparities as a function of spatial dis-

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CG BB 3 50 Fig. 2. Complete region of depth perception as a func- tion of spatial disparity mod- ulation frequency of vertical sinusoidal disparity varia- tions for three strabismic ob- servers who had moderate elevations of their lower ste- reothresholds.

0.1 0.2 0.5 2 5 0.1 0.2 0.5 1 2 5 0.1 SPATIAL FREQUENCY OF DISPARITY MODULATION (cyc/deg)

parity modulation that could be perceived as depth stereoscopic depth. All acuities were at least 20/20 in differences. The points for maximum spatial frequency each eye with refractive corrections in place as assessed were obtained by adjusting the frequency for a fixed by a multiple E chart which controls for crowding amplitude rather than the reverse. effects.23 Static stereoacuity was assessed using the Dynamic stereopsis was investigated by modulating clinical Wirt test,15 which consists of a series of rings the amplitude of sine images seen by each eye with which subtend disparities ranging from 40 to 800 arc an AM function generator (using methods described sec at a 40-cm viewing distance. Stereoacuities of the previously17). This counterphase stimulus provided a seven clinical observers ranged from 100 to 600 arc sinusoidal disparity modulation in both time and space. sec, whereas the three normal observers had stereo- Observers fixated the stationary spot and perceived a acuities of 15, 18, and 20 arc sec. In the cases of con- vertical line curved sinusoidally in depth. The peaks stant strabismus, stereoacuity was assessed with the of the curved line appeared near and the troughs far angle of strabismus corrected by a prism. away. The peaks and troughs continuously reversed in depth with sinusoidal amplitude modulation. As in Results the investigation of static stereopsis, the observers ad- justed the amplitude of disparity by turning a ten turn Static Stereopsis pot until the dynamic depth variations just appeared Graphs are shown for four of the seven strabismic and then disappeared. Dynamic stereothresholds were observers. The limits of static stereoscopic resolution obtained for combinations of spatial and temporal fre- are plotted on log-log coordinates in Figure 1 for two quencies ranging from 0.05 to 3 cycles/degree, and of the normal observers. The results of three observers from 0.1 to 3 Hz, respectively. with intermittent exotropia who had raised stereo- thresholds (100 to 600 arc sec clinical stereoacuity) are Observers plotted in Figure 2. A reduced upper disparity limit Three normal and seven strabismic observers par- is evident in one exotropic observer (Fig. 3). Circles ticipated in the study. All were experienced in psy- and squares represent thresholds for sine and square chophysical observations. During childhood, six of the wave stimuli, respectively. The standard errors of the clinical observers had had congenital nonaccommo- mean were less than 5%, usually falling within the dative esotropia and the seventh had congenital exo- width of the plotted symbols. tropia (Table 1). The angle of strabismus during child- Normal observers: In Figure 1, the "camel's nose" hood was cosmetically noticeable for all cases, ex- region of stereoscopic sensitivity labeled "DEPTH" ceeding 10 degrees, and was reduced surgically or by replicates that reported by Tyler.12 There is an ap- orthoptic exercises and optical prescription such that proximately inverse linear relationship between the three of the observers became intermittent exotropes, upper disparity limit and the spatial modulation fre- one a constant exotrope, and three became intermittent quency of disparity over a large range. This represents esotropes. Age of onset of strabismus for all observers a disparity scaling effect since the ratio of the upper was before the age of 3 years. No observer had am- disparity limit to the period of the sinusoidal disparity blyopia, and all but two post-surgical exotropes had stimulus remains constant. Thus, the magnitude of normal binocular correspondence. The angle of the upper depth limit increases with the spacing be- anomaly was small (0.5-1.0 degrees) in the two patients tween depth contours. with anomalous correspondence and did not prevent The lower stereothreshold does not show a scaling these observers from making reliable judgements of effect except at the lowest spatial frequencies tested.

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Stereosensitivity is greatest at approximately 1 cycle/ 100r degree, and it is reduced at higher spatial frequencies probably as a result of a crowding effect24 which is an elevation of the stereothreshold resulting from sur- D.M. rounding contours within about 10 arc min of the test target. Some observers also showed reduced sensitivity at lower spatial frequencies, perhaps as a result of a disparity gradient limit at threshold."12'25 Similar dis- parity gradient limits have been shown for binocular fusion.11121725 Since the disparity gradient in square wave stimuli is independent of spatial frequency, the reduction of a stereosensitivity at low fundamental spatial frequencies is not found with the square wave stimulus. Strabismic observers: New observations of the spatial limits and the disparity range of static stereopsis for strabismics with raised clinical stereothresholds are shown in Figure 2. The upper disparity limits of these three observers are indistinguishable from those of the normal observers shown in Figure 1. The difference 0.1 0.2 0.5 1 2 5 SPATIAL FREQUENCY OF DISPARITY MODULATION (cyc/deg) from the normal function is in the elevation of the disparity threshold at all spatial frequencies. At higher Fig. 3. Complete region of depth perception as a function of spatial frequencies these cases exhibited a lower spatial disparity modulation frequency of vertical sinusoidal disparity variations for a strabismic observer who had a threefold reduction threshold for static stereopsis that was elevated above of the upper disparity limit for stereopsis, although the reduced limit the normal upper disparity limit for depth. As a con- still undergoes disparity scaling. sequence, strabismic observers were unable to perceive depth with higher spatial frequencies and their spatial frequency limit for perceived depth was reduced from two-dimensional function of temporal and spatial dis- a mean of 3 cycles/degree for normal observers to a parity modulation frequency. The influence of spatial mean of 1.5 cycles/degree. The higher sensitivity to crowding upon dynamic stereopsis is revealed by com- square wave over sine wave disparity modulation at paring temporal frequency responses at low and at low spatial frequencies indicates that, as with normal high spatial frequencies. The upper left edge of the observers, strabismic observers also have a disparity graphs (A-B) is comparable to the temporal frequency gradient limit for maximum stereoscopic depth. response of dynamic stereothreshold measured without Figure 3 shows a second type of stereoscopic loss spatial crowding by Tyler19 and by Regan and Bev- in a constant exotrope with a clinical stereoacuity of erley.21 Dynamic stereothreshold remained fairly con- 400 arc sec.15 Unlike the other strabismics shown in stant as a function of temporal frequency when tested Figure 2, D.M.'s upper disparity limit was reduced to with a low spatial frequency of disparity modulation. one-third of the normal maximum depth limit. The In contrast, thresholds for dynamic stereopsis measured reduction is uniform across spatial frequency of dis- with a higher spatial frequency (0.5 cycles/degree) in- parity modulation such that disparity scaling with spa- crease by an order of magnitude as temporal frequency tial frequency is still obtained. This observer also had increases from 0.1 to 3 Hz. The increase in threshold a slight elevation of his stereothreshold relative to the was even more dramatic at higher spatial frequencies normal observers, but no greater than a factor of two as shown along the lower right edge of the graphs (C- (in the range of spatial frequencies where it is mea- D). Thus, as with static stereopsis, spatial crowding surable). The combined effect of the reduction of the also reduced dynamic stereoacuity. upper disparity limit and the small increase in stereo- Temporal factors influencing dynamic stereopsis are threshold was to produce a dramatic reduction (by a revealed by comparing the spatial frequency responses factor of five) in the spatial frequency limit for ste- at low and at high temporal frequencies. At low tem- reopsis. poral frequencies (lower left edge, A-D) stereothreshold was elevated at low spatial frequencies(<0.5 cycle/de- gree) just as it was for static stereothresholds (Fig. 1). Dynamic Stereopsis At high temporal frequencies (upper right edge, B-C) Normal observers: Dynamic stereothresholds for the the low spatial frequency elevation in threshold is very three normal observers are plotted in Figure 4 as a small and occurred gradually, whereas there was a

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B.B.

Z2.E

Fig. 4. Dynamic stereoscopic thresholds are plotted for three normal observers on a spatio-temporal surface. The lower left-hand edge resembles the variations in static stereothresholds of Figure 1. The lower right-hand edge reveals the steep rise in dynamic stereoscopic thresholds as temporal frequency is increased from 0.1 to 3 Hz. Optimal dynamic stereoacuity occurs at combinations of low temporal (0.3 Hz) and high spatial (0.5-1 cycles/degree) frequencies.

marked and abrupt elevation of threshold at high spa- Fig. 5. Dynamic stereothresholds for three strabismic observers tial frequencies (>1.5 cycle/degree). Comparison of are plotted on a spatio-temporal surface. The spatial frequency range these two spatial frequency responses reveals that the is limited for all three observers by their elevated dynamic stereoscopic main effects of temporal frequency upon stereoacuity thresholds which are above the normal dynamic disparity range at occurred with crowded (high spatial frequency) targets. high spatial frequencies.

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Table 1. Description of strabismic observers

Stereoacuity (arc sec)

Angle of deviation Binocular correspondence Sine Observer number Childhood Current (A = angle of anomaly) Win Static Dynamic

1 Esotrope Intermittent Normal 140 105 40 18* Exotrope 2 Exotrope Intermittent Normal 200 150 90 20* Exotrope 3 Esotrope Intermittent Anomalous A = 0.5 degree 600 420 150 13* Exotrope 4 Esotrope Constant Anomalous A = 1.0 degree 400 120 60 10* Exotrope 5 Esotrope Intermittent Normal 300 140 100 8A Esotrope 6 Esotrope Intermittent Normal 100 50 35 6* Esotrope 7 Esotrope Intermittent Normal 400 300 200 5A Esotrope

Seven strabismic observers are described in terms of their distance angle of utilized in the current investigation. Dynamic stereoacuity was consistently ocular deviation during childhood and in adulthood, the constancy of the higher than static sine stereoacuity and both of these measures were higher deviation, the state of binocular correspondence, stereoacuity measured with than static stereoacuity measured with the Wirt test. the clinical Wirt test, and with the static and dynamic sinusoidal disparities

Increasing temporal frequency reduced stereoacuity whereas decreasing temporal frequency enhanced ste-

reoacuity. 10.0 3 cycles/degree The two main effects (spatial and temporal) upon O-ONormol (N.C.) dynamic stereopsis and their interaction are seen most D-OStrorjismic(D.M.) clearly in Figure 4, top. The tilt of the reponse plane towards the lower left (C-D) shows the main effect of o temporal disparity modulation, where higher temporal frequency yields higher stereothresholds, while the tilt >- 2.0 of the plane to the lower right (A-D) reflects the main at effect of spatial disparity; higher spatial disparity mod- 1.0 DYNAMIC ulation yields lower thresholds. The counterclockwise DEPTH twist of the plane shows the interaction of these effects, UJ where temporal modulation of disparity has a greater °- 0.5 effect on stereothresholds at high spatial frequencies and spatial modulation of disparity has a greater effect < on stereothresholds at lower temporal frequencies. O. Thus, a combination of high spatial and low temporal frequencies yielded the optimal stimulus for dynamic stereopsis.

Strabismic observers: Figure 5 illustrates dynamic 0.3 1.0 3.0 5.0 stereothresholds for three of the four strabismic ob- servers (B.B., L.H., D.M.) whose static functions were TEMPORAL DISPARITY MODULATION (Hz) plotted in Figures 2 and 3. Although they were ab- Fig. 6. Complete region of dynamic depth perception for a normal normally elevated, dynamic stereothresholds were observer (clear region) and strabismic observer (shaded region) is consistently lower than static stereothresholds (Table plotted as a function of sinusoidal temporal frequency modulation 1). The dynamic stereothreshold function for B.B. re- of horizontal disparity whose amplitude varied sinusoidally (3 cycles/ sembles a uniformly elevated version of the normal degree) in the vertical meridian. Dynamic depth for the strabismic functions in Figure 4. Dynamic stereothreshold func- observer is limited by an elevated dynamic stereothreshold that is above the upper depth limit of the normal observer at high temporal tions for L.H. and D.M. also were elevated and they frequencies. This elevated stereoscopic threshold for strabismic ob- both had reduced spatial frequency limits that corre- servers reduces the cut-off temporal frequency limit for dynamic sponded with the spatial frequency limit for their static stereopsis.

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stereothresholds. D.M. also had a reduced temporal threshold above the disparity subtended by the test frequency limit of 1.0 Hz for all spatial frequencies. target. It is also important for tests of normal stereopsis L.H. had a similar reduction of her temporal frequency that small disparities be spaced at the 1 cycle/degree limit for 0.5 cycle/degree, but retained normal temporal optimum for highest stereoacuity. Similarly, tests of frequency limits at lower spatial frequencies. dynamic stereopsis should be limited to a temporal The reduced temporal frequency limit for dynamic frequency of less than 1 Hz in order to obtain optimal stereopsis resulted from an elevation of the lower dis- acuities. parity threshold for dynamic stereopsis above the nor- For the observers with clinical stereoacuities raised mal upper disparity limit at high spatial frequencies. by factors of about 5, 7, 10, 15, 20, 20, and 30 the Figure 6 compares the dynamic depth range limits of static sinusoidal stereothresholds were raised by factors a normal observer with those of a strabismic observer of only about 1, 2, 2.5, 3, 3, 6, and 8, respectively, (D.M.). D.M.'s lowest threshold at 1 Hz coincides with compared with normal sine stereoacuity (Table 1). the upper depth limit for the normal observer at 2 Hz. Thus, data from this small sample of patients suggest As a consequence, D.M. was unable to perceive dy- that the Wirt test15 seriously underestimates the ste- namic depth at higher temporal frequencies where the reoscopic capabilities of stereoanomalous observers. entire normal depth range was below his elevated This may be a result of the crowding effects of having threshold. a number of disparate stimuli presented in close prox- imity (less than 15 arc min).24 Redesign of target size Discussion and spacing in clinical stereotests could provide a more accurate estimate of stereopsis in people undergoing Two types of stereoscopic loss are evident in the treatment for strabismus. Stereoacuity should be tested strabismic observers. In one type of loss, both static at several low spatial and temporal frequencies, because and dynamic stereoacuities were abnormally elevated. it is clear that spatial crowding effects with the close In the second type of loss, the upper disparity limit spacing used in the current clinical tests will not reveal was markedly depressed, although stereoacuity was af- maximum stereoacuity in patients with elevated fected only slightly. Both losses appear to be fairly thresholds. One simple improvement would be to use uniform across spatial frequency. The uniform reduc- large spots instead of annuli and to vary spot size and tion of stereoscopic vision across spatial frequency separation in proportion to the amplitude of the test could result from reduced sensitivity of a single mech- disparity. anism or disparity pool.24 However, this observation does not exclude the possibility of a uniform reduction of sensitivity of multiple spatial channels tuned to dis- Key words: static and dynamic stereopsis, strabismus, dis- parity.10"12 parity modulation, Panum's area, spatial and temporal fre- The mechanisms involved in the reduced stereo- quency, upper depth limit, spatial crowding scopic resolution are separate from those underlying 17 the binocular sensory fusion process. Schor and Tyler References observed normal temporal frequency variations of Panum's fusional area in a stereoblind observer. This 1. Burian HM and von Noorden GK: Binocular vision and ocular observation was extended to spatial variations of Pan- motility. Theory and management of strabismus. St. Louis, The um's area in observer L.H. Although she had no ste- C. V. Mosby Co., 1974, pp 264-269. 2. Dunlop DB, Neill RA, and Dunlop P: Measurement of dynamic reopsis at spatial frequencies above 0.5 cycles/degree, stereoacuity and global stereopsis. Aust J Ophthalmol 8:35, 1980. she had normal Panum's horizontal fusion limits of 3. Sireteanu R: Binocular vision in strabismic humans with alter- 3 and 1.5 arc min at spatial disparity modulation fre- nating fixation. Vision Res 22:889, 1982. quencies of 1 and 2 cycles/degree, respectively.17 These 4. Beverley KI and Regan D: Selective adaptation in stereoscopic observations support the suggestion that stereopsis and depth perception. J Physiol (London) 232:40P, 1973. fusion result from independent spatial and temporal 5. Beverley KI and Regan D: The relation between discrimination 12 and sensitivity in the perception of motion in depth. J Physiol processes. (London) 249:387, 1975. The elevated thresholds for static and for dynamic 6. Regan D, Beverley K, and Cynader M: The stereopsis in strabismus impose spatial and temporal of motion in depth. Sci Am 241:136, 1979. frequency limitations on stereoacuity. In severe ste- 7. Regan D, Beverley KI, and Cynader M: Stereoscopic subsystems for position in depth and for motion in depth. Proc R Soc Lond reoscopic loss these spatial and temporal limits have [Biol] 204:485, 1979. implications for the design of clinical tests of stereopsis. 8. Richards W: Selective stereoblindness. In Spatial Contrast, The spacing between disparate figures must be great Spekreijse H and van der Tweel LH, editors. North-Holland, enough so that crowding will not elevate the stereo- Amsterdam, 1977.

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9. Cynader M and Regan D: Neurons in cat visual cortex tuned 18. Tyler CW: Stereoscopic depth movement: two eyes less sensitive to the direction of motion in depth: effect of positional disparity. than one. Science 174:958, 1971. Vision Res 22:967, 1982. 19. Tyler CW: Characteristics of stereomovement suppression. Per- 10. Marr D and Poggio T: A computational theory of human stereo ception and Psychophysics 17(3):225, 1975. vision. Proc R Soc Lond [Biol] 204:301, 1979. 20. Richards W: Response functions for sine- and square-wave 11. Tyler CW: Stereoscopic vision: cortical limitations and a disparity modulations of disparity. J Opt Soc Am 62:907, 1972. scaling effect. Science 181:276, 1973. 21. Regan D and Beverley KI: Some dynamic features of depth 12. Tyler CW: Spatial organization of binocular disparity sensitivity. perception. Vision 13:2369, 1973. Vision Res 15:583, 1975. 22. Ogle KN: On the limits of stereoscopic vision. J Exp Psychol 13. Richards W and Kaye MG: Local versus global stereopsis: two 44:253, 1952. mechanisms? Vision Res 14:1345, 1974. 23. Flom MC: New concepts on visual acuity. The Optom Weekly 14. Griffin JR: Binocular Anomalies: Procedures For Vision Therapy. 57(28):63, 1966. Chicago, Illinois. Professional Press Inc., 1976, p. 297. 15. Griffin JR: Binocular Anomalies: Procedures For Vision Therapy. 24. Westheimer G and McKee SP: Stereograms design for testing Chicago, Illinois. Professional Press Inc., 1976, p. 57. local stereopsis. Invest Ophthalmol Vis Sci 19:802, 1980. 16. Griffin JR: Binocular Anomalies: Procedures For Vision Therapy. 25. Burt P and Julesz B: A disparity gradient limit for binocular Chicago, Illinois. Professional Press Inc., 1976, p. 51. fusion. Science 208:615, 1980. 17. Schor CM and Tyler CW: Spatio-temporal properties of Panum's 26. Richards W: Stereopsis and stereoblindness. Exp Brain Res fusional area. Vision Res 21:683, 1981. 10:380, 1970.

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