PERCEPTION 1627 



of a floor or ceiling (154). The impression of a floor 

 slanting up is gained when the left and right views 

 are combined as shown; when they are interchanged, 

 one obtains the impression of a downward slant 

 (like a ceiling). The approach to binocular vision 

 based on disparity gradients makes it possible to 

 understand yet another seeming anomaly of depth 

 perception. In the stereoscope, it is possible to obtain 

 some binocular depth with patterns or settings that 

 are too disparate for fusion. This observation has been 

 reconfirmed by von Tschermak-Seysenegg (513) and 

 Ogle (365). 1S 



ACUITY OF BINOCULAR DEPTH PERCEPTION. If binocular 



stereopsis is not a principal source of depth, it never- 

 theless adds precision. The degree of this precision is 

 astonishing. In defining its limits, one needs to 

 determine the minimal difference in depth that cm 

 be mediated by binocular parallax. Thus, given a 

 distance, Z), along the line of regard, what is the 

 minimal additional distance, AD, that can be per- 

 ceived? The question has been investigated since von 

 Hclmholtz (501 ) with his famous three-needle experi- 

 ment and with many variations, including tin' (rather 

 crude) screening device for assessing binocular 

 stereopsis called the Howard-Dolman apparatus. 

 With this device, the observer on repeated trials 

 aligns two movable vertical black rods until they 

 seem equidistant from himself. Tactile-kinesthetic 



1H The approach to binocular depth perception which 

 invokes gradients of disparity makes it understandable that 

 altering the disparity between monocular stimulus patterns in 



a particular dimension should radically alter the appearance 

 of the visual scene. The aniseikonic glasses (meridional size 

 lenses), studied by Ogle (364), Burian 1771 and Ames (6), 

 have such effects in commonplace environments. "Objects 

 not only appear at distances other than those for which tin- 

 eyes are accommodated and converged, but they appear to be 

 of unexpected sizes and shapes which do not correspond to the 

 'geometrical dimensions' of the stimulus patterns and have 

 unexpected apparent motion" (6). A special, and much simpler 

 form of 'false depth' in binocular vision is induced by placing a 

 Biter of neutral density before one eye while viewing a pen- 

 dulum bob which swings in the viewer's frontal plane. The 

 pendulum then seems to describe an elliptical path (with axes 

 parallel to the floor). Pulfrich (390) who discovered the effect 

 attributed it to the greater latency of visual impressions in the 

 less illuminated eye, yielding a 'surplus' binocular disparity 

 [see also Liang & Pieron (322) and Lit (326)]. If the pendulum 

 is made to swing vertically through the viewer's binocular 

 field, the effect disappears (as it should if it were due to a form 

 of binocular parallax), but intermediate trajectories do not 

 yield the depth values predicted by the simple parallax theory, 

 and there are observers who have obtained the Pulfrich effect 

 when each eye had monocular stimulation in succession (253). 



A HORIZONTAL GRADIENT OF BINOCULAR DISPARITY 



A VERTICAL GRADIENT OF BINOCULAR DISPARITY 



1 in 23. Pairs of Stereograms based on gradients of binocular 

 disparity corresponding to fundamental types ol slanting 

 surfaces. [From Gibson (154).] 



information is eliminated In attaching the rods to .111 

 endless loop which the observer iii.inipul.iii-, and 

 accommodation and convergence are minimized by 

 keeping the apparatus at a distance of at least (j m. 

 Under such conditions the minimal disparity utilized 

 (bv the 'best' observers) can l»' as low as 2 sec. of 

 arc. 19 Such values of AD v.nv systematically with 

 illumination and absolute distance of the rods, with 

 their thickness and angular separation, and other 

 factors (166). Values for minimal disparity that are 

 nearly as low (i.e. 5 to 10 sec. of arc I cm lie obtained 

 on stereoscopes by using, for example, the Kevstone 

 series of stereograms where disparity is systematically 

 increased from pair to pair throughout the series of 

 test cards (549). The acuteness of binocular stereopsis 

 can be utilized in detecting slight discrepancies 

 between patterns, the views of a genuine and a 



19 If Gibson's approach to depth perception is correct, it 

 will be understandable that measurements of this type eg 

 in the Howard-Dolman apparatus) are poor predictors of 

 depth perception in real-life situations, such as in piloting a 

 plane. Only frank diplopia may be a handicap there, although 

 it is true that even mild anoxia can convert a latent into a 

 manifest strabismus with resulting binocular diplopia. 



