Biology Reference
In-Depth Information
flash of light after an appreciable latency period caused a discharge
of impulses in the fibre which lasted for several seconds. The total
number of impulses elicited and the discharge frequency were found
to increase with the intensity of the flash. These findings were in
accord with what might be expected. Surprisingly, however, Hartline
found that the magnitude of each impulse discharge was the same
when intensity or duration of the light stimulus was varied. Also,
he found that different test wavelengths could be made to evoke the
same frequency of impulses from a given receptor cell by adjusting
the incident light energy (Graham & Hartline,
1935
). On this
evidence the authors concluded that single receptors were unable to
discriminate between wavelength and intensity, a characteristic of
receptors later termed 'the Principle of Univariance'. This principle
implies that at least two different types of photoreceptor must be
present in the retina in order to distinguish between different colour
qualities of different wavelengths.
With exposure of short duration, Hartline found, in accord with
the reciprocity law of photochemistry (the Bunsen-Roscoe law), that
the response pattern of the nerve fibre did not vary as long as the total
amount of light energy (light intensity multiplied with duration)
remained constant, strongly suggesting that the phototransduction
process was photochemical in nature. Based on this suggestion he
could determine the absorption spectrum of the photosensitive
substance of single receptors in
Limulus
(responsible for excitation
of a single optic nerve fibre) simply by measuring the reciprocal of
the relative light energy of different wavelengths needed to produce a
specific burst of impulses (Graham & Hartline,
1935
).
The impulse pattern obtained from single optic nerve fibres of
Limulus
was also found to be affected by light and dark adaptation.
Thus, during light adaptation the frequency of the discharge of impulses
from single nerve fibres of a light stimulus declined to a steady level,
while it showed a progressive recovery during dark adaptation.
Hartline (
1940
) ascribed the decline and recovery of the frequency
of impulses to depletion and regeneration of the photosensitive
