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Seeking Information About Visual System

Paul Bush paul at phy.ucsf.edu
Thu Jul 11 14:50:51 EST 1996

In article <31E51030.53D5 at thecia.net>, Michael Sean Davis <davis at thecia.net> writes:
|> In perusing intro. level textbooks, I find seemingly agreed 
|> upon theories explaining color vision, but I don't find
|> detailed theories explaining the perception of brightness.
|> One text seems to vaguely suggest that brightness might be 
|> caused by the summed total activity of the photoreceptors 
|> in the retina.
|> Is this theory correct?  If not, what, if any, is the
|> agreed upon physiological theory for explaining the 
|> perception of brightness?  

We don't really perceive absolute brightness, instead we perceive brightness
changes over time and space, called contrast. That's why, when you have adapted 
to them, most environments seem to have about the same level of average 
brightness, even though normal daylight is many times brighter than a room lit
with normal tungsten filament lights.

A contrast signal is obtained by comparing the output of different channels, eg
on and off cells. The brain is more interested in contrast (and
changes/differences in general) because it conveys more information than simple
luminance signals. For instance, high spatial contrast often signals an edge.


How the brain works - http://www.keck.ucsf.edu/~paul/brain.html

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