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Frontiers
The Mind's Eye: Finding Truth in Illusion
June 1996
Roger N. Shepard was always a prankster. As a
boy, he delighted in playing visual tricks -- like the time he surreptitiously
moved all the furniture out of his sister's room.
He went on to a distinguished career at Bell Labs, Harvard, and Stanford, all
the while continuing to play visual tricks on people. By recording and analyzing
their reactions, he discovered astonishing truths about mental processes. Shepard
showed, for example, that when people compare two objects rotated at different
angles, they first reorient the objects in their mind's eye. The astonishing
part: Everyone does this at approximately 60° per second.
In the 1950s, Shepard began to discover universal laws that govern how people
and animals perceive similarities among sensory input like colors, sounds, or
smells. He worked out a computer method for measuring the perceived difference
between stimuli.
When Shepard applied this method to colors, the computer yielded a circle. The
physical wavelengths of visible light fall along a straight line, moving from
red to violet; but the circle more accurately depicts the psychological (as opposed
to physical) truth. This is that people perceive red and violet as more similar
to each other than either color is to an intermediate color such as green. Shepard
also discovered that:
- The distance between stimuli decreases along a particular
curve -- a steep drop, followed by a slow tailing off (in mathematical
terms, an exponential decay curve).
- All stimuli -- simple or complex -- produce the same curve.
- So do all experimental subjects -- whether humans, monkeys,
rats, or pigeons.
Until Shepard began publishing his imagery work in the 1970s, no
one had objectively measured mental imagery. It was thought to be impossible.
The scientific community has since taken notice.
Today, his work influences fields as diverse as psychology, philosophy, computer
science, linguistics, and neuroscience. Shepard says his image orientation work
has been used in a pilot aptitude test. The test accurately identifies trainees
who are most likely to make pilot errors. His imaging methods were also used
in the design of a computer-based diagnostic system for breast and prostate cancers.
Joseph L. Young, the NSF program director who has overseen Shepard's NSF grants
for more than 19 years, anticipates other practical benefits such as better organization
of control rooms and cockpit displays, as well as more effective educational
programs.
In 1995, Shepard was awarded the National Medal of Science. He is noteworthy
for other reasons as well. In an era of specialization, Roger Shepard is not
only a scientist; he is a skilled artist and writer whose publications, while
challenging, have broad appeal.
Shown here are a few drawings from his book Mind Sights, subtitled Original
Visual Illusions, Ambiguities, and Other Anomalies With a Commentary on the Play
of Mind in Perception and Art (W.H. Freeman and Co., 1990). In it, Shepard
explains:
"The drawings ... achieve their effects by means of various visual tricks. But
to call them tricks is not to imply that they are without psychological significance.
The tricks work by taking advantage of fundamental perceptual principles that
have been shaped by natural selection in a three-dimensional world. Our ability
to make pictures, which emerged only recently on an evolutionary time scale,
enables us to present the eyes with visual patterns that systematically depart
from the patterns that we and our ancestors experienced in nature. In considering
the ways pictures can trick the eye, we can gain insight into the nature and
ultimate source of the principles of visual perception."
"OUR PERCEEPTUAL MACHINERY...IS DEEPLY ENTRENCHED"--ROGER N. SHEPARD
"Our perceptual machinery for making use of retinally available information
about the disposition of objects in three-dimensional space is deeply entrenched
in our nervous system and wholly automatic in its operation. Without our bidding
or even our awareness of its existence, this machinery immediately goes to
work on any visual input, including the visual input provided by a two-dimensional
drawing. As a result, we cannot choose to see a drawing merely as what it is
-- a pattern of lines on a flat, two-dimensional surface. To the extent that
the pattern of lines conforms with the rules of linear perspective, for example,
that pattern automatically triggers the circuits in the brain that make the
three-dimensional interpretation appropriate to such a perspective display.
Any consciously adopted intentions to ignore such an interpretation are largely
powerless against the swift deliverances of this underlying machinery. This
should not surprise us. We have inherited this machinery from individuals who,
long before the advent of picturemaking, interpreted -- by virtue of this machinery
-- what was going on in the three-dimensional world around them with sufficient
efficiency to survive and to continue our ancestral line."
GRAVITATIONAL BENDING
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From Mind Sights, by Shepard.
Copyright © 1990 by Roger Shepard.
Used with permission of W.H. Freeman and Company. |
"Of...psychological interest are illusions that are not already objectively present
in the physical pattern of light reaching our eyes but that arise only in the
psychological process of interpreting that pattern of light....[T]he two portions
of the apparently elliptical (or even circular) orbit that overlap the lines
radiating from the central sun are curved only in the eye of the beholder. Objectively,
these two horizontal portions of the 'orbit' are perfectly straight, as can be
verified by means of a ruler."
TERROR SUBTERRA
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From Mind Sights, by Shepard.
Copyright © 1990 by Roger Shepard.
Used with permission of W.H. Freeman and Company. |
"[T]he linear perspective of the subterranean tunnel (along with other depth
cues, such as the relative heights of the projections of the two monsters on
our retinas) supports the automatic perceptual inference that one of the two
monsters is farther back in depth. The two monsters, nevertheless being exactly
the same size in the drawing, subtend the same visual angle at the eye. The visual
system therefore makes the additional inference that in order to subtend the
same visual angle, the monster that is farther back in depth must also be larger."
"In addition to seeing them as different in size, do we also interpret their
identical faces as expressing the different emotions -- such as rage on the part
of the pursuer and fear on the part of the pursued?"
TURNING THE TABLES
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From Mind Sights, by Shepard.
Copyright © 1990 by Roger Shepard.
Used with permission of W.H. Freeman and Company. |
"We remain oblivious to the anomalous character of many illusions until our visual
experience is tested against some other, more objective evidence, such as that
provided by measurement or tracing. There is nothing obviously amiss in Turning
the Tables until one is told that the two seemingly differently shaped table
tops are in fact identical in the picture plane. Clearly, this illusion arises
because our visual system has once again given us depth interpretations of the
two-dimensional drawing."
ARCH REMARK
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From Mind Sights, by Shepard.
Copyright © 1990 by Roger Shepard.
Used with permission of W.H. Freeman and Company. |
"The actual three-dimensional structure of this arch might be strangely twisted,
or simply tipped back -- so that the right-hand pedestal of the arch is suspended
in the air (above the shadow indicated in the foreground). The apparently perfect
alignment of this right-hand pedestal with the rectangular base in the background
would then depend on the unlikely circumstance that we are viewing the scene
from a very special vantage point."
L'EGS-ISTENTIAL QUANDRY
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From Mind Sights, by Shepard.
Copyright © 1990 by Roger Shepard.
Used with permission of W.H. Freeman and Company. |
"The elephant ... belongs to a class of objects that are truly impossible in
that the object itself cannot be globally segregated from the nonobject or background."
TELEPHONE CONFIDENCE
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From Mind Sights, by Shepard.
Copyright © 1990 by Roger Shepard.
Used with permission of W.H. Freeman and Company. |
"[A]n object that is novel and yet similar to an already significant object may
... warrant[s] our close attention. We need to know how far something can depart
from its usual or expected form and still have the consequences that we have
found to follow from its 'natural kind.' In doing so, we also come closer to
an understanding of the critical features of a stimulus that elicits a resonant
response within us and, thus, closer to an understanding of ourselves."
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