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

3.1.6 Overall Response
One can easily quantify a light source’s spectral intensity and the reflectivity of
a material through careful measurements. Through experimentation, the average
human’s perceptual response for each type of photoreceptor can also be determined.
Together, this should determine the overall response of a typical viewer for a given
stimulus. However, this is not the case, as the human brain is very subjective and
does not evaluate a color in isolation. The surrounding environment has a great
deal of influence on a color.
Previously, the opponent nature of human vision was discussed. The human
eye detects differences between neural impulses and magnifies them. This process
takes place after the photoreceptors have elicited a response from the stimulus.
Retinal ganglion cells collect neural impulses from photoreceptors and condenses
them by approximately 80 times [SB02], preserving the important features of the
original signals. Ganglion cells each have a receptor field on the retina, a patch
within which a cell’s activity may be influenced. Light outside of this region has
no effect on the cell. The receptor field roughly takes the shape of two concentric
circles. One region within the field responds to an increase in light, while the
other responds to a decrease in light. We will refer to these regions as ’on’ and
’off’ responses, respectively.
Imagine the center of a receptor field has an on response, while the ring is
an off response, as in Figure 3.28(a). If the cell undergoes uniform illumination,
the cell only gives a weak response. This is because such a stimulus produces
opposite effects in the center and surrounding area–the center is excited, while the
ring inhibits the response. This interaction is called lateral inhibition, which is the
antagonistic neural interaction between adjacent photoreceptors in the retina.
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