Clearly, the stimuli we receive
are integral to the mental
pictures of the world our brains
create. Our perception of color is
based on different wavelengths
of light, our perception of tonal
pitch on frequency of sound
vibration, our perception of
brightness on amplitude of light
waves, and so on. The program of
psychophysical investigators of
the late nineteenth century,
who are often credited with
founding scientific psychology,
was precisely one of correlating
subjective sensations with
physical stimuli.

    In visual perception, the
psychophysical approach lost 
its appeal when stimulus
correlates could not be found 
for many perceptual
phenomena. However,
beginning in the 1940s, James J.
Gibson of Cornell and his
associates began to suggest
stimulus correlates for many of
those properties and events that
previously had resisted
psychophysical investigation.