Biology Reference
In-Depth Information
test field. Hence, to explore the influence of area specifically as
area of sensitivity during dark adaptation, it would be necessary to
confine the measurements to retinal areas essentially homogeneous
in sensitivity.
Wald (
1937
/
1938
), therefore, in a follow-up study, measured the
dark adaptation curves with test fields of angular diameters of 1º,
2º, 3º, 4º and 5º at 15º and 25º above the fovea. The results, however,
clearly showed that even in these relatively homogeneous regions,
the sensitivity increased markedly with test area: at 15º a seven-fold
lowering of threshold, at 25º a ten-fold lowering.
Yet, Wald (
1937
/
1938
) held that this increase in sensitivity
with area did not contradict the photochemical theory, since it could
reasonably be accounted for by the simple properties of a mosaic retina
with a population of relatively independent units. His analysis of the
measurements was based on the assumptions that (1) a threshold
response involved the activity of a fixed number of retinal elements,
and (2) throughout all portions of the homogeneous retinal field the
percentage of such elements was the same. Thus, he presumed that
in a series of fields of various sizes the threshold intensity obtained
would always correspond to the activation of a constant number of
elements, and that the number of elements would be directly propor-
tional to the field area.
Based on these assumptions Wald (
1937
/
1938
) arrived at a
relatively simple formula, which could accurately describe the
change in threshold intensity with area:
(
A
−
n
t
)
k
×
I
= C
where
A
= area of test field,
n
t
= the constant number of elements
for a threshold response,
I
= threshold test intensity, and k and
C = constants.
An important feature of this model was the assumption that
the mosaic character of the retina was transferred relatively intact as
far as to the occipital cortex. Nevertheless, he did not preclude the
possibility of some integration of the responses from the individual
