Perceiving in Depth, Volume 3: Other Mechanisms of Depth Perception

316 Pages·2012·17.024 MB·English

by  Ian P. Howard

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C O N T E N T S OF VOLUME 3
25. Depth from accommodation and vergence 1 34. Reaching and moving in 3-D space 260
26. Depth from perspective 15 35. Auditory distance perception 277
27. Depth from interposition and shading 63 36. Electrolocation and thermal senses 309
28. Depth from morion parallax 84 37. Animal navigation 318
29. Constancies in visual depth perception 122 38. Final word 334
30. Interactions between visual depth cues 147
31. Seeing morion-in-depth 179 References 336
32. Pathology of visual depth perception 216 bides ofc ited journals 382
33. Visual deprh perception in the Portrait index 38S
animal kingdom 233
Subject indes 386
This page intentional/)' le ft blank
25
DEPTH FROM ACCOM M OD ATIO N
AND VERGENCE
25.1 Accommodation as a distance cue / 25.2.2 Verbal estimation of vergence distance a
25.1.1 Accommodation and absolute distance / 25.2.3 Use oi a comparison object 6
25.1.2 Object blur as a cuc to relative depth 2 25.2.4 Vcrgence distance indicated by pointing 7
25.1.3 I)efocus blur as a cuc to relative depth .i 25.2.5 Illusory motion parallax 8
25.1.4 Adapting to unusual depth blur 4 25.2.6 Vergence and apparent size and distance S
25.2 Vergence as a distance cue 5 25.2.7 Perceptual effects of maintained vergence 10
25.2.1 Introduction 5 25.2.8 Vergence and judgment of relative depth /5
25.1 A C C O M M O D A T IO N viewed luminous diskat distances of25,30, and 40 cm with
AS A D IST A N C E CUF, angular size held constant. Errors were less than 1 cm in rhe
range 25 to 40 cm. When accommodation was optically
adjusted to one distance, and vergence to another distance,
25.1.1 A C C O M M O D A T I O N A N D
judgments of distance were a compromise between the two
A B S O L U T E D IS T A N C E
distances but with more weight given to vergence. These
Although Descartes (1664) had no clear idea about the results indicate onlv that accommodation contributes
mechanism of accommodation, he proposed that the act of to perceived absolute distance. They do not provide a quan­
accommodation aids in the perception ot depth. Berkeley titative measure ot that contribution.
(1709) made the same suggestion. Between 1858 and 1862 In the above experiments it was assumed rhat subjects
Wundt performed a series of experiments on the role ot accommodated on the required target, which most people
accommodation in depth judgments. Subjects judged fail to do accurately.
whether a black silk thread seen monocularly through a Fisher and Ciutfrcda (1988) used both a good accom­
tube was at (he same distance in two successive exposures. modative stimulus, consisting of high contrast patterns, and
Subjects could not judge the absolute distance of the a poor accommodative stimulus in the form of a fuzzy disk.
thread but could detect a change in depth of about 8 cm They measured accommodation with an optometer that
at a distance of 100 cm and of 12 cm at a distance of provided no intruding stimuli. Subjects pointed with a
250 cm. Hillebrand (1894) used the edge of a black card hidden hand to monocular targets. With high-contrast
seen monocularly against an illuminated background so as targets, apparent distance decreased linearly with increasing
to remove the depth cue of changes in image size. When the accommodation, but there were large individual differences.
stimulus was moved slowly in depth, subjects were not able Subjects tended to overestimate distances that were less
to detect the motion. However, when the stimulus moved than about 3.2 diopters (31 cm) and underestimate larger
abruptly, subjects could detect a change of between 1 and distances. Each diopter change in accommodation induced
2 diopters. Dixon (1895) and Baird (1903) produced about a 0.25-diopter change in apparent distance. With the
similar results. poor accommodation stimulus, perceived distance did not
'Ihis evidence suggests that people cannot judge the vary with accommodation.
distance of an object on the basis of the static state of Mon-Williams and Tresilian (1999a) asked subjects to
vergence-accommodation but can use changes in accom­ point with unseen hand to single monocularly viewed
modation to judge ditfcrcnccs in depth. However, more targets at distances between 10 and 50 cm. The target was
recent experiments have revealed that people have some placed along the visual axis of one eye so that vergence cues
capacity to judge absolute distance using accommodation. were eliminated. A vergence motion of the closed eye
Swenson (1932) asked subjects to move an unseen may have been evoked by changes in perceived distance,
marker to the perceived distance of a single binocularly but this would not provide independent information
about distance. Target size and distance were varied inde­ square appears near and out of focus because both its tex­
pendently to remove the distance cue of size. Four of six ture and its edges are blurred. In a second experiment,
subjects showed a correlation between pointing distance Marshall et al. used a sidc-by-side bipartite display, which
and target distance, but responses were very variable. avoided the factor of a figure surrounded by a background.
The gradient of optical blur over an inclined or slanted However, the effects were not as clear-cut.
surface increases with decreasing distance of the surface. Blurring a display reduces its contrast and contrast has
Vishwanath and Blaser (2010) produced evidence that a its own effect on perceived relative distance. O’Shea et al.
frontal surface with a steep gradient of artificial blur appears (1997) varied relative blur and relative contrast indepen­
nearer than a surface with a less steep gradient. dently in the two halves of textured bipartite displays.
A more blurred region appeared more distant than a less
blurred region when contrast was the same. A region
25.1.2 O BJECT BLUR AS A CUE TO
of higher contrast appeared nearer than a region of lower
RELATIVE DEPTH
contrast when blur was the same. The effects of the two cues
A visual object may be physically blurred by a filter that were additive over a moderate range ofcontrast.
removes high spatial frequencies. Unlike accommodative Mather and Smith (2002) used the bipartite display
blur of the retinal image, physical blur is not affected by shown in Figure 25.3 (Portrait Figure 25.2). When the
accommodation—it is open-loop blur. Artists create an boundary was sharp, the blurred region appeared more
impression of depth by simulating the out-of-focus appear­ distant than the nonblurred region. Only when the border
ance of objects not in a specified plane. Photographers was very blurred did the blurred region appear near. They
create an impression of depth by using a large aperture to concluded from subsidiary experiments that moderate
reduce the depth of focus so as to have only the object of degrees of border blur are difficult to detect.
interest in focus, leaving objects in other depth planes with
various degrees of blur.
Pentland (1987) discussed the use of gradients of focus 25.1.3 DEFOCUS BLUR AS A CUE TO
in computer vision systems. Simple physical blur in a RELATIVE DEPTH
photograph provides ambiguous information about depth
25.1.3a Static Blur as a Cue to Depth
because the same blur can indicate an object nearer than
the plane of focus or one beyond the plane of focus. The blur of an in-focus image of an object with a given spa-
Furthermore, physical blur can indicate relative depth only tial-frequcncy content depends on the optics of the eye. The
if the true sharpness of the edges is known. These sources of ability of the visual system to detect image blur depends on
ambiguity can be reduced by using a series of static pictures the sensitivity and spatial sampling of the retina and visual
taken with various levels of camera defocus. In this way, cortex. For given values of these optical and neural factors,
it is possible to compute the relative depths of objects by the blur of the image of an object at a given distance varies
physically scanning each of a set photographs (Rajagopalan with the state of accommodation of the eye. The question
ct al. 2000). The visual system removes these ambiguities in addressed in this section is whether people are able to judge
other ways (see Section 9.6.5). the relative depth of two objects on the basis of the relative
A sharply tcxturcd region within a blurred surrounding blur of their retinal images.
region can induce impressions of relative depth (Mather Grant (1942) asked subjects to set a luminous disk to
1996). Marshall ct al. 1996 used the stimuli shown in the same distance as another disk, when cues to distance
Figure 25.1. All subjects reported that the sharp inner other than image blur were removed. The standard error
square appeared in front of the blurred surround in (A) but of settings was about 0.94 cm at a distance of 50 cm, and
beyond the surround in (B). The in-focus edge of the inner 0.8 cm at a distance of 25 cm.
square in (A) is seen as belonging to the in-focus texture Subjects could distinguish between the image a point
within the square. The square is therefore seen as occluding of light nearer than the plane of focus and the image of
the blurred surround region, 'flic out-of-focus edge in (B) is a point beyond the plane of focus (Wilson ct al. 2002).
seen as belonging to the surround and the sharp inner square The stimulus was presented for 100 ms after a 2-minute
is therefore seen as if through a hole in the surround. In training period in which subjects were given knowledge
Figures (C) and (D) the effect is ambiguous. The sharp of results. Performance improved with increasing image
boundary should be seen .us belonging to the in-focus blur and as pupil diameter was increased from 1 mm
surround and therefore as an occluding edge of a nearer to 5 mm.
surrounding region. But some people see the inner square Nguyen et al. (2005) asked subjects to report the rela­
as nearer. This could be due to a general tendency to sec a tive depth of two monocularly viewed vertical test edges on
surrounded region as a foreground figure. Hie blurred tex­ either side of a gap illuminated by tungsten light, as shown
ture of the inner square is then interpreted as intrinsically in Figure 25.4. The left edge was fixed at a distance of 37 cm.
blurred rather than out of focus. In Figure 25. ID the inner The right edge was presented at various random distances
ii.1. Effects of texture and blur on Apparent depth. (A) Hi с inner square appears near because its sharp edge appears to belong to its sharp
contents. (B) Tl»e inner square appears tar because its blurred edge belongs to the surround. (C) The inner square with sharp edges can appear as
an out-of-focus surface beyond the surround or as a nearer blurred square. (D) The inner square with blurred edges appears near and out-of-focus
because both its contents and edges arc blurred. (n«irawn ь«п м>гЛ-.н «al. iws)
nearer than or beyond chc left edge. This edge moved along
25.1.3b Dynamic Accommodation and
the eye’s visual axis so as the keep the image of the gap con­
Relative Depth
stant in width. In addition, the width of the gap was ran­
domly varied slightly from trial to trial. In one condition, The act of changing accommodation between two objects
subjects remained fixated and focused on the left edge. In at different distances may provide information about their
this case, the only information about the relative depth of relative depth. Also, the changing blur associated with
the two edges was that provided by the relative signed blur changing accommodation could be a depth cue. Helmholtz
of the images. In a second condition, subjects changed (1909, Vol. 3, p. 294) found that an illuminated slit with a
accommodation from one edge to the other several times. red filter appeared nearer than a slit with a blue filter. He
Figure 25.5A shows that, in both conditions, the relative explained the effect in terms of the difference in accommo­
depths of the two edges could be discriminated 75% of the dation required to focus the two slits, arising from chro­
cimc when they were about 0.2 D apart in depth. Detection matic aberration.
of the depth order of the edges was severely degraded when Although Mon-Williams and Trcsilian (2000) found
the stimulus was illuminated by monochromatic sodium that subjects could not judge the absolute distance of a
light, as shown in Figure Б. Monochromatic light does not monocular target, there was some indication that they
produce the chromatic aberration that provides a cue to the could judge whether a target was nearer or more distant
sign of accommodation (Section 9.8). than a previous one. The above experiments indicate that,
different distances through the mediation of size constancy.
This issue is discussed in Section 30.7.
25.1.4 ADAPTING TO UNUSUAL
DEPTH BLUR
Sharp edges are perceived as sharp in spite of the fact
that diffraction and optical aberrations spread the image
over several receptors. This question was discussed in
Section 9.6.5.
When we focus on an object we arc not aware of blur in
the images of objects in other depth planes. One reason is
that we do not normally attend to objects out of the plane
of focus. But there is some evidence that we adapt selectively
to the normal levels of blur associated with different dis­
tances from the plane of focus. It is as if we discount defocus
blur so that we can better detect physical blur.
Battaglia et al. (2004) asked whether adaptation to
image blur is related to perceived depth. Subjects fixated a
central target at a distance of 33 cm for 3 minutes, while
2S.2 GeorgeMati>cr, Born in Liverpool in 1955. He obtained а
В.A. in psychology from Sheffield University in 1976 and a Ph.D. two Hanking tcxturcd surfaces moved back and forth
with B. Mouldcn from Reading University in 1979. After postdoctoral between depths of 23 and 32.2 cm at 0.1 Hz. The surfaces
work at York University in Toronto* he joined the Department
became physically more blurred as they approached and less
of Experimental Psychology at the University of Sussex» England»
blurred as they receded or vice versa. Blurring was produced
where is now a professor.
by filtering the texture. Before and after adaptation, subjects
adjusted the physical blur of a surface at 24.6 cm until
at near distances, accommodation has an effect on perceived it appeared the same as the fixed blur of an adjacent surface
distance. Dynamic accommodation and dynamic image at 30.6 cm, or vice versa. After adapting to surfaces that
blur may be more effective when many objects in different became blurred as they approached, less blur was required
depth planes arc presented at the same time. For this pur­ in the far surface to make it appear the same as the blur in
pose one needs an instrument that presents an array of the near surface. The effect was reversed after adaptation to
objects at different accommodative distances but at the surfaces that became blurred as they receded.
same vergence distance. Another approach to the role of Thus, the unusual blur-depth relationships experienced
accommodation in distance judgments is to test whether during adaptation to surfaces moving in depth changed
accommodation affects the perceived size of an object at the relative perceived blur of stationary surfaces in a
21.У FJfect of blur on perceived depth. The di\plav on the left contains a sharp border between the sharply tcxturcd region and the blurred region.
Observers reported that the blurred region appeared far. The display on the right contains a blurred border. Observers reported that the blurred
region appeared near. (From МаЛ<г WSm.tk 200?.PW>n bmiwa.Un«Jc*>
Beam splitter just as a blind man might feel out adistance with two staves,
I Test edges
one in each hand. The haptic judgment of distance is
discussed in Section 34.2.
л х f \ } [ } Fixed ed9e
In his Essay Towards a New Theory of Vision (1709),
7 з~ у I - Movable edge
Berkeley argued that the perceived distance of an isolated
Light source
Electronic objcct from the viewer depends on muscular sensations of
shutters
convergence and, at near distances, on visual blur and eye
Prefixation edges
strain arising from accommodation (Boring 1942). Briicke
(1841) proposed that the three-dimensional structure of a
Opal screen scene is perceived on the basis of vergence eye movements
that occur as different parts of the scene are fixated. But
Dove (1841) showed that stereopsis can occur with expo­
Light source
sures too brief to allow vergence to occur. Thus, vergence
movements are not necessary for depth perception. Early
experiments on vergence as a cue to distance were con­
I'ijturt The affaratut used by Nguyen ctal. (200$). ducted by Hillebrand (1894), Bourdon (1902), Baird
(1903), and Bappert (1923).
The distance, D, of a fixated object in the median
Tungston light
plane as a function of vergence angle, в and interocular
g> Active-looking Maintained-fixation
distance, a is given by:
a
D = (1)
2 tan 02
Far Near Far [ Near The distance, D', of a second object in the median
0 ------,y —//-----------------------// —// -------
2 -1 0.1 0 0.1 1 -1 0.10 0.1 plane, which has a disparity 6 with respect to the fixated
- -
Depth between test edges (diopters) object, I) is given bv:
A
Sodium light
D '=-
ш Active-looking Maintained-fixation (2)
[ 9 - 5
2 tan
{ 2
When the effects of linked changes in vergence and
accommodation arc being investigated, accommodation
distance is made equal to vergence distance. We use the
term accommodation/vcrgcnce distance to refer to the
optical distance of the target determined by both accom­
Depth between test edges {diopters)
modation and vergence. On the other hand, accommoda­
В
tion distance may be varied while vergence is held constant,
ligaic 15.5- Ibe delation of relative depthfrom blur. Subjects judged die or vergence distance may be varied while accommodation is
relative depth of two edges seen against tungsten light, as in (A) or
held constant. There are two ways to vary vergence while
sodium light, as in (B). They looked from one edge to the other or
holding other cues to distance constant or ineffective. The
fixated one edge. Results for three subjects. (aJa^c! i™.. nb uvcn ct .«1. 200S)
first is to present targets in a stereoscope with variable offset
between the images, and the second is to view the target
distance-specific way. This suggests that the visual system
through basc-in or base-out wedge prisms. However,
modulates perceived blur by signals related to relative
constant accommodation and constant size signify that
depth.
distance is not changing and may therefore interfere with
judgments of distance based on vergence.
25.2 V E R G E N C E AS A D IST A N C E C U E This problem can be solved by randomly varying
accommodation and target size so that they are dissociated
from the distance of the target, as specified by convergence.
25.2.1 I N T R O D U C T I O N
A better solution is to eliminate accommodation as a cue by
Descartes, in his La dioptrique (1637), described the eyes as viewing the stimulus through pinholes, which increase the
“feeling our” a distance by a convergence of the visual axes, depth of focus. Size as a cue to distance can be eliminated by
using a point source of light. The luminance of the target
should also be kept constant or varied at random.
The range of distances over which testing is conducted
is a crucial variable because vergence changes very little
beyond 2 m. It is unlikely to serve as a cue beyond that
distance.
Finally, one must select a psychophysical procedure tor
measuring perceived distance. The following procedures
have been used.
25.2.2 V E R B A L E S T IM A T IO N OF
V F. R G E N С E DIS Т А N С F.
Ovama (1974) asked subjects to judge the size and distance
Stimulus distance (cm)
ot the projected stereoscopic image ot a playing card or a
blank card. The size of the stimulus was varied. Vergence
F.JUIC2S.6. Estimateddistaau as afumtion oftlistonce. Subjects judged the
was changed by changing the lateral separation of the distance of л light spot seen in dark surroundings. {SUilnmn tVum Viguier ct jI.
dichoptic images, with accommodation held constant. 2001)
Estimates of distance decreased linearly with increases in
/
image size or convergence angle. The effect of vergence accommodation/vcrgcnce distance. Subjects alternate
angle was much less than the cfFcct of image size, boch between viewing the test object and the comparison object
for the familiar and unfamiliar objects. The stimulus was presented simultaneously, or the stimuli are presented suc­
viewed through an aperture, which would have introduced cessively. The images of the two objects are made the same
a relative disparity cue. size, to neutralize size cues to distance. In a related proce­
Trained subjects made reasonably accurate verbal dure, subjects judge the relative sizes of two objects rather
estimates of the distance of a point source of light viewed than their relative distances. The idea is that perceived rela­
binocularly in dark surroundings at accommodation/ tive size is proportional to perceived relative distance,
vergence distances of between 0.5 and 9 m (Morrison and according to the size-distancc invariance principle (Section
Whiteside 1984). There was some overestimation of near 29.3.2). These two procedures will be referred to as the
distances and overestimation oi far distances. Accuracv was visual-distancc procedure and the visual-sizc procedure,
still good when the target was exposed for only 200 ms, so respectively. Hie procedures do not indicate the accuracy of
that the eyes did not have time to converge on the stimulus. judgments of absolute distance but only whether perceived
This suggests that the disparity of the flashed target was relative distance or relative size is proportional to relative
combined with information about the resting state of vergence.
vergence. Accuracy was not as high when only accommoda­ Alternatively, a test object can be adjusted until it
tion distance was varied as when only vergence distance appears the same size or distance as a comparison object.
was varied. This procedure indicates only the minimum perceived sepa­
Viguier et aL (2001) asked subjects to verbally ration in depth between two objects—theJND for relative
estimate of the distance of a light seen in dark surroundings. depth. One object could be presented with full depth cues
Figure 25.6 shows that distance estimates were good up to and the other with only vergence as a cue, with the two
a distance of 40 cm, beyond which distance was underesti­ objects not visible at the same time. In this case, results indi­
mated. Vergence varies steeply with distance up to 40 cm, cate the accuracy and precision of depth judgments based
which coincides with the limit of reaching distance. on vergence with respect to the accuracy and precision of
Judgments of distance based on vergence were no worse judgments based on some other depth cue.
than judgments of direction based on version, when both In a related procedure, subjects match the perceived size
were expressed in angular terms (Brenner and Smcets of a target presented at various accommodation/vcrgcnce
2000). distances with the length ol a subsequently presented
frontal rod seen with full depth cues. Wallach and
Floor (1971) used this procedure and found that viewing
25.2.3 USE OF A COM PARISON OBJECT
distance was perceived with 95% accuracy for distances
The joint effects of vergence and accommodation on up to 120 cm.
perceived distance can be studied by comparing the appar­ Frank (1930) measured the change in apparent size of
ent distance of an object seen through prisms and lenses so an object as subjects changed fixation from the object to a
that it is at one accommodation/vcrgcnce distance, with mark some distance in front of it. In this procedure, effects
the apparent distance of a comparison object seen at another of changing convergence are contaminated by blur-induced