Page 365 - Color_Atlas_of_Physiology_5th_Ed._-_A._Despopoulos_2003

Adaptation of the Eye to Different glasses decrease the differential threshold in
Light Intensities the latter case.
The mechanisms for adaptation of the eye
The eye is able to perceive a wide range of light are as follows (! C):
intensities, from the extremely low intensity ! Pupil reflex (! C1). Through reflexive re-
of a small star to the extremely strong intensity sponses to light exposure of the retina
of the sun glaring on a glacier. The ability of the (! p. 359), the pupils can adjust the quantity
of light entering the retina by a factor of 16.
eye to process such a wide range of luminance
Central Nervous System and Senses normal daylight into a darkened room, the is to ensure rapid adaptation to sudden
11
(1 : 10 ) by adjusting to the prevailing light in-
Thus, the pupils are larger in darkness than in
tensity is called adaptation. When going from
daylight. The main function of the pupil reflex
room will first appear black because its lumi-
changes in light intensity.
-2
! Chemical stimuli (! C2) help to adjust the
nance value (measured in cd · m ) is lower
sensitivity of photosensors to the prevailing
than the current ocular threshold. As the
light conditions. Large quantities of light lead
stimulus threshold decreases over the next
few minutes, the furniture in the room gradu-
to a prolonged decrease in the receptor’s cyto-
2+
concentration. This in conjunction
ally becomes identifiable. A longer period of
solic Ca
adaptation is required to visualize stars. The
with the activity of recoverin and phosducin
(! p. 348ff.). It therefore decreases the proba-
about 30 min (! A). The minimum light inten-
sity that can just be detected after maximum
bility that a rhodopsin molecule will be struck
dark adaptation is the absolute visual thresh-
by an incoming light ray (photon) or that a
12 maximum level of adaptation is reached in reduces the availability of rhodopsin
old, which is defined as 1 in A and B.
metarhodopsin II molecule will come in con-
The retinal adaptation curve exhibits a tact with a transducin molecule. When the
(Kohlrausch) break at roughly 2000 ! the ab- light intensity is low, large concentrations of
solute threshold (! A, blue curve). This corre- rhodopsin and transducin are available and the
sponds to the point where the excitation photosensors become very light-sensitive.
threshold of the cones is reached (threshold ! Spatial summation (! C3). Variation of reti-
for day vision). The remainder of the curve is nal surface area (number of photosensors) ex-
governed by the somewhat slower adaptation citing an optic nerve fiber causes a form of spa-
of the rods (! A, violet curve). The isolated rod tial summation that increases with darkness
adaptation curve can be recorded in patients and decreases with brightness (! p. 354).
with complete color blindness (rod monochro- ! Temporal summation (! C4). Brief sub-
matism), and the isolated cone adaptation threshold stimuli can be raised above thresh-
curve can be observed in night blindness old by increasing the duration of stimulation
(hemeralopia, ! p. 350). (by staring at an object) long enough to trigger
Differential threshold (or difference limen). an action potential (AP). Thereby, the product
The ability of the eye to distinguish the differ- of stimulus intensity times stimulus duration
ence between two similar photic stimuli is an remains constant.
important prerequisite for proper eyesight. At Successive contrast occurs due to “local
the lowest limit of discriminative sensibility adaptation.” When a subject stares at the cen-
for two light intensities I and I!, the absolute ter of the black-and-white pattern (! D) for
differential threshold (∆ I) is defined as I minus about 20 s and suddenly shifts the focus to the
I!. The relative differential threshold is calcu- white circle, the previously dark areas appear
lated as ∆ I/I, and remains relatively constant to be brighter than the surroundings due to
in the median stimulus range (Weber’s rule). sensitization of the corresponding areas of the
Under optimal lighting conditions (approx. 10 9 retina.
times the absolute threshold; ! B), ∆ I/I is very
small (0.01). The relative differential threshold
rises greatly in dark adaptation, but also rises
352 in response to extremely bright light. Sun-
Despopoulos, Color Atlas of Physiology © 2003 Thieme
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