Biology Reference
In-Depth Information
elements that could increase sensitivity somewhat. Such interaction,
however, was assumed to take place somewhere in the brain.
The equation found for the extrafoveal test fields was also
assumed to be valid for the central foveal area, where the constant
number of elements for a threshold response (
n
t
) was presumed to be
represented by cone receptors. Since in this case (
n
t
) was assumed to
be very small, the threshold-area equation was reduced to the simple
form:
A
k
×
I
= C
This presumption was supported by available data obtained
within the central fovea.
The mosaic theory offered by Wald may be seen as a successful
attempt to rescue the photochemical theory. Yet, there still remained
a serious challenge to this theory. Thus, it had been found that the
ordinary dark-adaptation curve proceeded faster and further as size
of the test field increased, while the change in concentration of a
photopigment during dark adaptation, on the other hand, would
follow the same course irrespective of test size. Apparently, a neural
factor had to be involved to explain the dark-adaptation process.
Wald (
1958
), however, argued that the change in sensitivity
obtained was just what one would expect provided it was determined
by the synthesizing of a photochemical pigment in a
large
number
of receptors. Thus, one would expect the dark-adaptation curve to
reflect, from moment to moment, the activation of a
sample
of the
most sensitive receptors from a population of hundreds or thousands
of receptors. Different receptors would then be involved in threshold
determination at different times during the dark-adaptation period.
Hence, the larger the population, i.e. the larger the test field, the
further the dark-adaptation curve would be expected to depart from
the adaptation curve of a single rod or cone, yielding a more rapid and
extensive adaptation the larger the field.
