pigmented colorants: dependence on media and time - Cornell ...

example shown in Figure 3.28. Under constant stimulation, the opponent nature
of ganglion receptor fields will produce a weakened response. The off-regions of
the cells are inhibiting the response and we perceive the stimulus as being darker
than it actually is. As we move closer to the edge, more off-region portions of
receptor fields are receiving less light. Therefore more off-region receptor fields
are receiving their desired response, while the on-receptors are still receiving their
desired bright response. Hence, the total amount of excitation is increasing as we
move toward the edge. The maximum response is right at the edge of the light and
dark regions. The result is that we perceive the light area getting slightly brighter
close to the edge. A similar effect occurs for the dark area, as it is perceived as
slightly darker near the edge.
Ratliff was the first to physically measure these differences in intensity and
perceived brightness. He measured the nerve impulses produced by steady il-
lumination of a single receptor in the eye of the horseshoe crab Limulus. The
nerve fibers from the receptor are separated by microdissection and connected to
an electrode from an amplifier and a recorder. However, Ernst Mach explored the
connection between reflected light intensity and the resulting sensation beforehand.
He hypothesized the antagonistic influences in the retina, as his experiments with
intensity and perception of light did not correspond. The pattern in Figure 3.30 is
similar to one of which Mach developed. The apparent lightness within each bar
varies even though the intensity of light reflected is constant. Most people describe
the edges of each bar having slight bands. Hence the term Mach bands, which are
the illusionary spatial gradations in perceived lightness that occurs without corre-
sponding gradations in the actual distribution of light.
Mach bands emphasize the important distinction between intensity and light-
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