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S. Anstis
be highly unlikely and even clearly impossible, the second offering checks 'downwards' from stored
knowledge, and filling gaps which may be fictional and false. But van den Enden and Spekreijse
(1989) offer a non-cognitive explanation. A stereoscopic picture of a face offers two kinds of depth
cues; binocular disparity, and monocular texture disparity -- gradients of texture, which are
geometrically more compressed near the left and right edges of a convex face than they are for a hollow
face. They claimed that the real reason that a pseudoscopically viewed face refuses to look concave is
that each monocular view contains texture information that provides a strong cue that the face is
actually convex. They viewed a convex face through a pseudoscope, and projected'neutral'texture,
which gave no monocular cues, on to the face from a projector near the observer's eyes (Georgeson
1979) Result: the face was coffectly perceived as concave. Deutsch and Ramachandran (1990) and
Peli (1990), however, both point out that this projected texture adds rich binocular disparity cues,
3.
The illumila:t
savi:
to be violated b1'Bii
(on Moon Drive in T
be barely worth
described more rece:
the horizon. It is see
moon, else it would
r
which suffice to explain van den Enden and Spekreijse's results. Furthermore, the texture account
does not explain why an actual hollow face, viewed with both eyes, looks convex. However, Deutsch
et al. and Peli concur that the perceived depth depends up'on the cues present in the stimulus, and no
cognitive factors need be involved.
2 a, (
about 11 o'c1o
Figure
b,
Brian Ror,
11 oc
tiltedto
c, The ne\\' m
look higher
2.
ab
Figure
1.
comes from above. Fig. 1 b is simply Fig. 1 a turned upside down. As a result the
picture appears to be reversed in depth, with the bumps becoming hollows and vice versa. In the
absence of explicit lighting cues the visual system assumes that the light comes from the top of the
picture. The assumption that laight comes from above rather than any other direction does not simplify
our perceptions in any way but it does match them bener to our sunlit physical world in which the light
really is more likely to come from above, so we are applying a perceptual constraiht derived from our
knowledge of the physical world. This is a prime example of the likelihood principle.
Light
Ian Howard (personal communication) has noted that if you bend over and look between your legs at
Fig. 1, the light is assumed to come from above in retinal, not gravitational coordinates, as if the light
source were assumed to be stuck to one's forehead. This is not a highly intelligent perceptual
assumption.
There is some evidence that the constraint is learned by experience. Hess (1972)reared chicks in
special cages that were lit from below through the floor, so that the grains they ate were illuminated
from below. He then exposed them to a pair of photographs of grain. In one photograph the
illumination came from above: in the other, from below. The chicks pecked at the grains illuminated
from below. Once learned in early life, however, such a consrraint could arguably become hard-
wired.
ir
In Fig. 2a the lamp is
of vertical at abour '1
Anyone can see thar rl
but the sphere has bee
known to astronomeis
to be higher than the
sun is lower in the sk.,
contravenes one's pel
setting but the moon i
(with hypotheses bas
implicit perceptual
ass
A mathematician, pei
than it really is. This
so as to make the sur
has sunk below the h
predicts an effect in
An astronomer
i:
sugge
assumption is tn:e. -r:
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282
S. Anstis
A distinguished physicist suggested, admittedly toward
the end of the party, that the sun, moon and
observer do not lie in the same plane. This assumption is false, since as his wife (the mathematician)
reminded him, any three objects always lie in the same plane.
H,,
short-range process
by texture, cyclopt
range motion proc
Mather (1989) har.
\
Michael Swanston (personal communication) has convincingly argued that the sun and moon look
about equidistant from the observer, although the sun is really 400 times as far away as the moon.
This misperception of the sun's position, based on a gross underestimate of its distance, Ieads to the
illusion. Probably the old nor the new moon illusions are both due to the misperception of distances
from the observer -- of the moon itself for the old moon illusion and of the sun for the new moon
illusion.
It is plausible to eq
down. Marr and I
4.
suggested that ther
separation and tasi
Light is bright. Again this sounds too obvious to be worth saying. What else could light be
but bright? Actually it is counterfactual, but not meaningless, to suggest that light could be dark. One
could simulate a lamp with a spray can spraying out white paint on to black objects. When the paint
dried it would give a fossilised rendering of illuminated objects. Black paint sprayed on to white
objects does not give a rendering of any physical light source because there is no "black light" in
nature. Of course, in principle one could construct an internal world of black light and white
shadows, not by changing the extemal physical universe but by re-wiring the visual system so that
highly reflective objects produced the sensation that we now label "black", and unlit space would
produce the sensation that we now label "white". But this is simply to restate the old speculation that
although we agree to call a ripe cherry "red" and a leaf "green", it may be that my "green'r sensation
would look like your "red" sensation if they were somehow both fed into the same brain. More to the
point, we can interchange black and white experimentally by viewing the world through a video link
which converts the picture electronically into a photographic negative. This makes it hard to
distinguish bumps from hollows and impossible to recognise faces of celebrities, especially if the
portraits are high-contrast "lith" photo$aphs (Phillips 1972). The brightness reversal disrupts the
shape from shading which permits us to recognise facial features. Patrick Cavanagh was able to
establish, after two years' work, that shadows look like shadows only if they are darker than
heuristic va1ue.
(1989) and Sekuler
by an alert monkel,
instantaneous meas
tasks as motion se
integration cannot
information over ti:
1979) would proba
of the long-range I
match up correspor
total path lengths o:
A parsimonious
n
luminance, illusory
analyse a scene fo
then use 3-D object
illuminated regions (Cavanagh and Leclerc 1989).
Our ability to decode shadows may have a learned component (Hess 1972). Although naive observers
cannot recognise famous faces in negative, there was a time when people who worked in television
newsrooms routinely acquired this unusual skill. In the early days of television, newsreels were shot
on cine film. The negative film stock was developed and a positive print made for broadcasting. It
was soon realised that valuable time could be saved by broadcasting the negative film directly and
reversing the brightnesses electronically. The film editors were called upon to edit the films while
these were still in negative, and they rapidly learned to recognise world leaders even in negative.
Suneeti Kaushal and I are planning to investigate long-term adaptation to a negative visual world.
Subjects will wear a helmet-mounted stereo display in which two small TV cameras on the front of the
helmet feed into two miniature TV screens which the subject views, one with each eye, through
suitable magnifying lenses. The TV pictures will be electronically reversed in brightness.
Apparent motion
As a microcosm of perceptual processes we used ambiguous apparent motion stimuli in which two
shapes abruptly exchange positions. One can perceive the two objects as moving past each other in
opposite directions, or one object can seem to move while the other does not, or each object can
change shape without shifting position. Which of these percepts occurs is strongly influenced by
stimulus and observer variables, and we shall discuss the role of "bottom-up" Iow-level processes
and "top-down" assumptions, expectations, or leamed constraints about the physical world.
Braddick (1974) proposed a distinction between short-range and long-range processes in motion
perception. The short-range process is thought to occur early in the visual system and has been
identified with directionally selective neurons in the visual cortex that operate passively and in parallel
over the whole visual field. It operates over short distances (< 1 5 min in the fovea) and brief durations
(<100 ms), and adaptation of the short-range system underlies the motion aftereffect. Its inputs come
exclusively from stimuli that are defined by luminance. The long-range process, on the other hand, is
thought to occur at later stage of processing with properties more resembling cognitive or interpretative
processes than the responses of single neurons. It operates over longer distances and times than the
Figure
3I
(Anstis and
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284
S. Anstis
High-contrast objects appear to move
Luminance. Fig. 3a,b shows a.black square,and
a_ white square side by side which instantaneously
change places, so that the black square suddenly becomes-white and at the same instant the white
square becomes black. What will one perceive? Does one see two squares flickering in place, or does
!|9yt il" square jump one waJ, or does the black square jump the other way? Ge-orge MatLer and I
(1985)foundthattheanswer.dependsupontheluminanceofthebackground.
Onali[htsurroundthe
is seen as jump.ing (Fig. 3a), and on a dark surround the ;hite square is ieen as jumping
9}.f^qq.ruf.
(Fig. 3b). The square that differed most from the surround, in other words thi square with ttre ni'gner
conEast, was perceived as jumping.
The observers adjusted the luminance of the background until they saw either the two squares move
equally.frequently or both squTg! move at once. This mid-grey indifference point was equivalent to a
P$nt mixed.frqT eqr11lPaTs of black and white paint, so it *ai the arithmetic, not the geoimetric mean
of black and white. This shows that the motion system operates linearly on luminancis, not upon log
luminance. More recently, however, Shioiri, Cavanagh ind Favreau (t9Sg) trave found a sligit nonlinearity - less marked than a logarithmic function -- wtricfr is consistent with the compresson jf retinal
cone responses reported by Boynton and Whitten (1970). Shioiri et al (1989) have also used this
technique to study the lin^earity ofintensity coding along the three cardinal axes of color space. Two
bars of two colors (Cl,
Q2).were alternated to produce apparent motion. The background iolor was a
mixture of Cl and C2. If.the background color was closer to C1, only the C2 baiappeared to jump.
If the color different in a linear cone activation space controls this apparent motion, th'e setting shouid
be midway between Cl and C2, no matter which pair of colors is cliosen for C1 and C2. Noie of the
cardinal axes (achromatic., R-G-cone, or B axes) showed a linear response, but the non-linearity was
less extreme than a logarithmic function.
Figure
a, t\\'o
{
square.
E
sl
b, a staric
The surround can be partitioned into a1_arge background area and a small area like a picture frame
adj199nt to the test squares (Fig. 3c, d). When these areas are pitted against each orher by setting them
to different luminances, the picture frame luminance has a much stronger influence thln the .1-ot"
surround on the perceived direction of motion.
Time 1.
region
have bee
interchan
to
Illusory brig.htness...Fig. 4a shows two pieces of black/grey checkerboard, each moving back and
forth throrgh the diameter of one checkerboard square. fhe two moving checkerbo#ds do not
interact. Now a surround is added -- a large stationary black-white checkerEoard, positioned so that
each small greychec_kerboar{-r9nlaceg alternately some black squares and some white squares of the
surround (Fig..ab). White (1979, 1981: White and White 1985) ihowed that grey squareithatreplace
-btact
white square.s in a checkerboard look appreciably darker than when they replaie
squ-es, ai Fig.
4b shows. (White used gratings, not checkerboards, but the principie is the same). No* ihe t*o
regions.interact strongly., and a single region is seen as jumping back and forth ihrough a dozen
s.quqe-d.iary9ters. The i].lusorJ.induced brightness controli the apparent morion just as efiectively as
the physical luminance did in Fig. 3.
While's phenomenon has attracted considerable interest because it cannot readily be classified as either
simultaneous contrast o1 1q blghlness assimilation (Hamada, 1984: Foley and McCourt, 1985:
Yo.u]4".n and Kingdom, 1989.: Zaidl 1989). The grey squares that replace white squares are bordered
by black squares, and from simultaneous contrasi grbunds would be'predicted to ipp.ur lighter than
49 qey
that replace black sqLrares, the exact opposite of-what is found. Moilden and
Kingdom (1989) attribute White's effect to two mechanisms, one a local concentric spatial filter
operating at the comers ofthe grey test regions,.the other a spatially extensive filter operating along the
long bars of the grating (or presumably along diagonal rows of checkerboard squares).
squa-rgs
r
1c'<
jump
a
rvhole luminanct
bluning or squir:
iuminance than r
neuraily u'ould
psychologists uh
Thus, the respor
explained b1,a
1
compared to trvo
If the two grating
if one grating, sa
contains more mc
Texture segregatt
where there is no
motion is control
squares consisrin
dots (Fig. 5a) an<
surround, the tlr
textures have the
from the surroun(
surround, and tht
salience controls
two squares sudi
texture jumpin_s
depended upon rh
r
very large jump instead of two small jumps seems to violate principles of
simplicity and likelihood. We believe this is a bottom-up-process that accepts oniy lo',i-level
To perceive
a. s-1.!g19-
luminance cues. First, a comparison of the left hand regions of Fig. + at Times 1 ind 2 shows that the
ldges-of the.checkerboard squares reverse their luminance polariiies in the transition from Time 1 to
Time 2. This will militate strongly against seeing the smali local morions (Anstis 1970: Anstis and
Rogers 1975: Anstis and Mather 1985). Second, aOAing the background checkerboard changes the
square, the
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286
S. Anstis
Hidd
was finer than the fine square, the coarsely textured square was perceived in apparent motion. The
square that differed most from the surround was seen in motion.
Figure 6 Ap1
a, When thest
the protruding
the plane of ti
b, When the c
the display lu
depth, along r
Figure
5
Perceptual salience, based on texture discrimination without luminance
cues, can mediate crossover motion.
a, two patches of random-dot texture, one fine and one coarse, exchange places.
Apparent motion is ambiguous.
bf when the stimulus in a is superimposed on a very fine textured background, the
coarse texture is more salient and is seen in apparent motion (arrow).
c, when the stimulus in a is superimposed on a very coarse textured background, the
fine texture is more salient and is seen in apparent motion (arrow).
To summarise, our crossover effects appear to operate at a fairly low level, after the level of luminance
discrimination or of texture segregation. They require no simplicity or likelihood assumptions by the
visual system. Motion percepiion can be based upon stimulus luminance (Figs 3, 4), or at a_ higher
visual level upon textura-based non-Fourier stimuli (Fig. 5), which admittedly proride somewhat less
compelling impressions of movement (Chubb and Sperling 1988). We conclude that stationary
regions can first be defined by any visual cue such as luminance, depth, texture and so on, and then
diiplacements of such regions can then give rise to a long-range motion percept. Cavanagh (1989)
reviews many examples of such inter-attribute apparent motion.
Near objects appear to move
Perspective. Fig. 6 shows a perspective sketch of a protruding square slab next to_a_square_recess.
When the slab and the recess abruptly exchanged places, subjects reported that the slab, not the hole,
moved. When the color of the bottom of the recess was made the same as the top of the slab, the
display now looked like two buttons being pushed in alternation and.subjects now reported motion in
defth, along the line of sight and at right angles to the previous motion. (The stimuli used were these
aciual sketches, not real 3-D objects). These are examples of intelligent processes in long-range motion
perception.
from shading. Fig. 7 shows the familiar Ternus (1926) configuration for apparent motion.
Three ipots jump back and forth between positions a,b,c and time I and positions b', c', d' at time 2.
The pertept depends upon the timing. When there is no interstimulus interval (ISI) the two central
spots are atways visible and look stationary, and subjects report,'element motion' in which_one spot
jumps from en-d to end (Fig. 7a). When there is an ISI the central spots flash gn 1nd gff and-subjects
ieport 'group motion' in which three spots jump back and forth together (Fi-g. 7b)(Pantle and-Picciano
t9i6). Group motion is also seen when the stimuli are presented dichoptically. Braddick and Adlard
Shape
(1978) found that *
was often reported, br
always reported eler
movement depended
moved, but of the elet
by a low level short-r
back and forth, but b'
to a higher leve1, mo:
and Mather (1989).
Now let us give the r
1988). The flat grel
saucers, and the empr
seeing element moric
flipping up and dou'r
However, when thert
observers report a sin
long range motion is
for long range motlol
computed.
Covering and occlLai
They propose that th
solving process. Ti
rectangle back and f<
give good apparenl I
reason why these co
standpoint of proble
appearance and disap
covering and uncove
However, if the recn
anything, so it was n
present dots that $'er
reported apparent roc
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