Biology Reference
In-Depth Information
S
=
k
log
R
,
i.e. magnitude of sensation (
S
) is logarithmically related to magnitude
of stimulus (
R
). The transformed signal, then, was thought to be fed
back to the gain box and to summate with the bleaching signal. Finally,
this summed signal was thought to affect log threshold level linearly.
21.4 The adaptation mechanisms explored
by the after-flash technique
Rushton (
1972
) presented an important further development of his
model of light and dark adaptation. Together with Alpern and Torii
he explored the adaptation processes by a new ingenious psychophys-
ical technique. Thus, they adapted the test field area by presenting a
contrast light flash φ and a steady background field θ in a surrounding
annulus area. (Another background field could be presented in the
test area.) The light flash φ was presented shortly
after
a test flash λ
had been exposed.
Both φ and θ were found to increase the threshold level of
the test flash λ and, hence, to reduce the sensitivity of the test
area. This reduced sensitivity was assumed to be determined by
an inhibitory nerve signal
N
. Presupposing that
N
would remain
constant under conditions where the test flash λ was kept fixed at
a certain intensity level and reduced perceptually just to threshold
level by φ and/or θ, they could vary intensity of φ and θ for a
constant inhibitory
N
-signal, and by repeating this procedure for
other fixed intensity levels of the test flash λ, they could establish
the relationship between all the three variables. In this way they
obtained the following formula:
N
= [φ/(φ + σ)] • [θ
D
/(θ
D
+ θ)]
where σ is the so-called semi-saturation constant, i.e. the intensity
of φ that raises
N
to half its maximum, and θ
D
is the intrinsic light of
the eye ('dark light') that may be seen when the eye is in a completely
dark-adapted state.