visual systems are exquisitely sensitive to information about the shapes
of objects, including depth within an object, that is, which parts are
farther and which closer. Consider a piece of paper lying on a table.
You can easily tell which side is closest to you. How does your brain
know? In other words, what information is available to construct your
percept? This information comes in several forms, which are often called "visual
depth cues." One cue comes from stereoscopic vision. Slight differences
in the two eyes' retinal images are interpreted as depth by the brain.
Another cue comes from an unconscious assumption that the paper is a
rectangle. This cue is a variant of the "linear perspective" cue. (Try
this: trim a thin triangular wedge from one side of a rectangular piece
of blank paper. Put it on a table or surface that doesn't have much texture.
Look at it from a sharp angle [not from overhead] with just one eye.
Looks like a rectangle, doesn't it?) Another cue is height within the
visual field objects that are seen from above (things on tables
and floors, for example) are generally farther from you if they are higher
in the visual field. Now, notice that those last two cues almost always
tell you the correct answer as to what's near and what's far, for real
objects in the real world. They are what vision scientists call "statistically
valid." Because they almost always give correct information, it makes
sense for the brain to pay attention to them and use them during percept
construction. Just as it's statistically valid, but not logically necessary,
to conclude there was a power failure if the alarm clocks, microwave
oven display, and VCR are all blinking when you get home (it's unlikely,
but possible, that a cat jiggled all the plugs instead). Now, what if
a clever artist were to construct, artificially, a peculiar but very
real object in which the depth cues give false testimony? Your brain
brain would be perfectly right to conclude that the most likely object,
the one it should construct in the percept, is not the real object but
the one suggested by the cues.
is what happens in a piece like Six-Ninths Blue by Ron Davis.
The linear perspective cue, and the height-in-visual-field cue, tell
you that the top of the object is much farther away than the bottom
(even though it's not). The perspective is nearly identical to that
produced by a square-footprint object, seen from just above ground
level. In fact, the object has a second perceptual effect: it changes
your idea of where the ground is. It's the fact that we see things
correctly almost all the time that makes illusions of depth so fascinating.
Objects that have invalid depth cues are pretty rare. Unless you're
an artist who creates such objects, and therefore sees them all the
time Davis is not as susceptible to his own illusions as he
used to be. That we fall prey to them is a tribute to our brains' amazing
ability to figure out, for the most part correctly, what cues are reliable
indicators of depth.
Professor of Psychology
University of Pennsylvania