[From Bruce Abbott (2001.05.19.1730 EST)]
Bill Powers (2001.05.19.1142 MDT) --
Bruce Abbott (2001.05.18.1800 EST)
Bill Powers (2001.05.18.1445 MDT)
Sort of like Bruce Abbott and his "precisely tuned color
filters" that he invents and then rejects (in favor of his own view, oddly
enough).
Bill, if you are going to make comments like this, the honorable thing to do
is to support them with some sort of reasoned argument. I trust that one is
coming.
"Argument?" I was just echoing what you said a couple of days ago about
color vision, and what you attributed to me (finely-tuned color filters).
You should go back and re-read that post, if you still have it. I didn't
attribute that model to you at all! I proposed that first-order and
second-order perceptions both might be apprehened consciously (when they
are) as sensations. I suggested that some first-order signals might feed
unchanged into those high-level sensory analyzers whose activities give rise
to conscious perception and yield, not dimensionless intensities, but
particular sensations varying in intensity with the neural current. Others,
I suggested, might go no further than the inputs to second-order functions,
leaving only second-order signals going to the high-level analyzers. To
illustrate the latter, I presented a scheme for color perception where the
outputs of the three primary color receptors are combined in various
proportions to yield all the perceptible colors. I contrasted the
efficiency of this solution to that of an alternative in which every
perceivable color is represented by a separate, narrow-band frequency
sensor. Neither example was intended to be more than an illustration of
principle.
There are many kinds of perceptual input functions that could account for
color vision. One that I particularly like is Edwin Land's proposal, in
which color sensations are functions of long and short wavelength
intensities and an average of intensities over all receptors.
In using the color-vision example to illustrate a second-order perceptual
function, I was not suggesting that this is the actual mechanism of color
perception found in the human brain. I did see it as an example that would
be easy to understand and sensible in view of both the existence of three
detectors broadly tuned to three different freuencies and our experiences
resulting from mixing red, green, and blue lights. After all, only those
three colors are actually present in a color TV's output and yet we perceive
almost the entire rainbow of colors in the screen. It also meets the formal
requirements of a second-order PCT-style perceptual function.
However, I am aware that this simple scheme, elegant though it is,
represents a great simplification of the actual human color-perception
system. Even at the level of the retina, there are bipolar neurons whose
output signals are complex functions of the outputs of the three types of
color receptor. Some of these bipolar cells function very much like the
opponent-cells envisioned by Ewald Hering in the late 19th century, who
based his theory of "opponent process" theory of color perception partly on
observations of various forms of abnormalities in color vision. An example
is the "red +, green -" output: light in the low-frequency region (perceived
as red when present alone) excites the cell's output while light in the
middle-frequency region (perceived as green when present alone) inhibits the
cell's output. (Things get even more complex at the ganglion-cell level,
the next stage in color processing).
I understand that there has been quite a bit of progress recently in working
out the actual physiological mechanisms involved in color processing (and
visual processing in general); I'd really like to become much more familiar
with this literature. If anyone tuned into CSGnet is familiar with these
recent advances, it would be nice get a synopsis of them.
I have a fairly recent, relatively low-level book on visual perception
[Hoffman, Donald D. (1998). _Visual intelligence_. New York: Norton] that
hints at the sophistication of the visual system. As one example, the color
perceived in a swatch of color arranged alongside many others is a function
not only of the frequencies of light being reflected by the swatch, but also
on the frequencies being reflected by other swatches (color contrast) and on
what the visual system "takes" to be the _color-balance of the light shining
on the surface_ (!). (The latter process attempts to separate color as a
property ascribed to the object from color as a property of the illumination
and not of the object.) Another example is the perception of illusory
colored objects, where the brain has painted in a nonexistent object as a
way of explaining certain regularities seen in the objects actually present.
By covering the objects, you can demonstrate for yourself that the illusory
colored object is not really there, as it disappears when the real objects
are removed.
I don't know the details of Land's theory. Does it begin with the three
types of color receptor cells in the retina (as Helmholtz had inferred were
present)? I don't know whether the presence of these three tuned receptors
had been confirmed in Land's day; if not, then it is possible he worked out
a theory that did not start with these three frequencies as primary.
Bruce A.