ON THE STIMULUS OF LIGHT AND RETINAL CURRENTS
Visual impulse: (1) chemical theory; (2) electrical theory--Retinal
currents--Normal response positive--Inorganic response under
stimulus of light--Typical experiment on the electrical effect
induced by light.
The effect of the stimulus of light on the retina is perceived in the
brain as a visual sensation. The process by which the ether-wave
disturbance causes this visual impulse is still very obscure. Two
theories may be advanced in explanation.
#(1) Chemical theory.#--According to the first, or chemical, theory, it is
supposed that certain visual substances in the retina are affected by
light, and that vision originates from the metabolic changes produced in
these visual substances. It is also supposed that the metabolic changes
consist of two phases, the upward, constructive, or anabolic phase, and
the downward, destructive, or katabolic phase. Various visual substances
by their anabolic or katabolic changes are supposed to produce the
variations of sensation of light and colour. This theory, as will be
seen, is very complex, and there are certain obstacles in the way of its
acceptance. It is, for instance, difficult to see how this very quick
visual process could be due to a comparatively slow chemical action,
consisting of the destructive breaking-down of the tissue, followed by
its renovation. Some support was at first given to this chemical theory
by the bleaching action of light on the visual purple present in the
retina, but it has been found that the presence or absence of visual
purple could not be essential to vision, and that its function, when
present, is of only secondary importance. For it is well known that in
the most sensitive portion of the human retina, the _fovea centralis_,
the visual purple is wanting; it is also found to be completely absent
from the retinæ of many animals possessing keen sight.
#(2) Electrical theory.#--The second, or electrical, theory supposes that
the visual impulse is the concomitant of an electrical impulse; that an
electrical current is generated in the retina under the incidence of
light, and that this is transmitted to the brain by the optic nerve.
There is much to be said in favour of this view, for it is an undoubted
fact, that light gives rise to retinal currents, and that, conversely,
an electrical current suitably applied causes the sensation of light.
#Retinal currents.#--Holmgren, Dewar, McKendrick, Kuhne, Steiner, and
others have shown that illumination produces electric variation in a
freshly excised eye. About this general fact of the electrical response
there is a widespread agreement, but there is some difference of opinion
as regards the sign of this response immediately on the application,
cessation, and during the continuance of light. These slight
discrepancies may be partly due to the unsatisfactory nomenclature--as
regards use of terms _positive_ and _negative_--hitherto in vogue and
partly also to the differing states of the excised eyes observed.
Waller, in his excellent and detailed work on the retinal currents of
the frog, has shown how the sign of response is reversed in the moribund
condition of the eye.
As to the confusion arising from our present terminology, we must
remember that the term _positive_ or _negative_ is used with regard to a
current of reference--the so-called current of injury.
[Illustration: FIG. 95. RETINAL RESPONSE TO LIGHT
The current of response is from the nerve to the retina.]
When the two galvanometric contacts are made, one with the cut end of
the nerve, and the other on the uninjured cornea, a current of injury is
found which _in the eye_ is from the nerve to the retina. In the normal
freshly excised eye, the current of response due to the action of light
on the retina is always from the nerve, which is not directly stimulated
by light, to the retina, that is, from the less excited to the more
excited (fig. 95). This current of response flows, then, in the same
direction as the existing current of reference--the current of
injury--and may therefore be called _positive_. Unfortunately the
current of injury is very often apt to change its sign; it then flows
through the eye from the cornea to the nerve. And now, though the
current of response due to light may remain unchanged in direction,
still, owing to the reversal of the current of reference, it will appear
as _negative_. That is to say, though its absolute direction is the same
as before, its relative direction is altered.
I have already advocated the use of the term _positive_ for currents
which flow towards the stimulated, and _negative_ for those whose flow
is away from the stimulated. If such a convention be adopted, no
confusion can arise, even when, as in the given cases, the currents of
injury undergo a change of direction.
#Normal response positive.#--The normal effect of light on the retina, as
noticed by all the observers already mentioned, is a positive variation,
during exposure to light of not too long duration. Cessation of light is
followed by recovery. On these points there is general agreement amongst
investigators. Deviations are regarded as due to abnormal conditions of
the eye, owing to rough usage, or to the rapid approach of death. For
just as in the dying plant we found occasional reversals from negative
to positive response, so in the dying retina the response may undergo
changes from the normal positive to negative.
The sign of response, as we have already seen in numerous cases, depends
very much on the molecular condition of the sensitive substance, and if
this condition be in any way changed, it is not surprising that the
character of the response should also undergo alteration.
Unlike muscle in this, successive retinal responses exhibit little
change, for, generally speaking, fatigue is very slight, the retina
recovering quickly even under strong light if the exposure be not too
long. In exceptional cases, however, fatigue, or its converse, the
staircase effect, may be observed.
#Inorganic response under the stimulus of light.#--It may now be asked
whether such a complex vital phenomenon as retinal response could have
its counterpart in non-living response. Taking a rod of silver, we may
beat out one end into the form of a hollow cup, sensitising the inside
by exposing it for a short time to vapour of bromine. The cup may now be
filled with water, and connection made with a galvanometer by
non-polarisable electrodes. There will now be a current due to
difference between the inner surface and the rod. This may be balanced,
however, by a compensating E.M.F.
[Illustration: FIG. 96.--RECORD OF RESPONSES TO LIGHT GIVEN BY THE
SENSITIVE CELL
Thick lines represent the effect during illumination, dotted lines the
recovery in darkness. Note the preliminary negative twitch, which is
sometimes also observed in responses of frog's retina.]
We have thus an arrangement somewhat resembling the eye, with a
sensitive layer corresponding to the retina, and the less sensitive rod
corresponding to the conducting nerve-stump (fig. 96, _a_).
The apparatus is next placed inside a black box, with an aperture at the
top. By means of an inclined mirror, light may be thrown down upon the
sensitive surface through the opening.
On exposing the sensitive surface to light, the balance is at once
disturbed, and a responsive current of positive character produced. The
current, that is to say, is from the less to the more stimulated
sensitive layer. On the cessation of light, there is fairly quick
recovery (fig. 96, _b_).
The character and the intensity of E.M. variation of the sensitive cell
depend to some extent on the process of preparation. The particular cell
with which most of the following experiments were carried out usually
gave rise to a positive variation of about ·008 volt when acted on for
one minute by the light of an incandescent gas-burner which was placed
at a distance of 50 cm.
#Typical experiment on the electrical effect induced by light.#--This
subject of the production of an electrical current by the stimulus of
light would appear at first sight very complex. But we shall be able to
advance naturally to a clear understanding of its most complicated
phenomena if we go through a preliminary consideration of an ideally
simple case. We have seen, in our experiments on the mechanical
stimulation of, for example, tin, that a difference of electric
potential was induced between the more stimulated and less stimulated
parts of the same rod, and that an action current could thus be
obtained, on making suitable electrolytic connections. Whether the more
excited was zincoid or cuproid depended on the substance and its
molecular condition.
Let us now imagine the metal rod flattened into a plate, and one face
stimulated by light, while the other is protected. Would there be a
difference of potential induced between the two faces of this same sheet
of metal?
Let two blocks of paraffin be taken and a large hole drilled through
both. Next, place a sheet of metal between the blocks, and pour melted
paraffin round the edge to seal up the junction, the two open ends being
also closed by panes of glass. We shall have then two compartments
separated by the sheet of metal, and these compartments may be filled
with water through the small apertures at the top (fig. 97, _a_).
[Illustration: FIG. 97 (_a_)
A, B are the two faces of a brominated sheet of silver. One face, say A,
is acted on by light. The current of response is from B to A, across
the plate.]
[Illustration: FIG. 97 (_b_).--RECORD OF RESPONSES OBTAINED FROM THE
ABOVE CELL
Ten seconds' exposure to light followed by fifty seconds' recovery in
the dark. Thick lines represent action in light, dotted lines
represent recovery.]
The two liquid masses in the separated chambers thus make perfect
electrolytic contacts with the two faces A and B of the sheet of metal.
These two faces may be put in connection with a galvanometer by means of
two non-polarisable electrodes, whose ends dip into the two chambers. If
the sheet of metal have been properly annealed, there will now be no
difference of potential between the two faces, and no current in the
galvanometer. If the two faces are not molecularly similar, however,
there will be a current, and the electrical effects to be subsequently
described will act additively, in an algebraical sense. Let one face now
be exposed to the stimulus of light. A responsive current will be found
to flow, from the less to the more stimulated face, in some cases, and
in others in an opposite direction.
It appears at first very curious that this difference of electric
potential should be maintained between opposite faces of a very thin and
highly conducting sheet of metal, the intervening distance between the
opposed surfaces being so extremely small, and the electrical resistance
quite infinitesimal. A homogeneous sheet of metal has become by the
unequal action of light, molecularly speaking, heterogeneous. The two
opposed surfaces are thrown into opposite kinds of electric condition,
the result of which is as if a certain thickness of the sheet,
electrically speaking, were made zinc-like, and the rest copper-like.
From such unfamiliar conceptions, we shall now pass easily to others to
which we are more accustomed. Instead of two opposed surfaces, we may
obtain a similar response by unequally lighting different portions of
the same surface. Taking a sheet of metal, we may expose one half, say
A, to light, the other half, B, being screened. Electrolytic contacts
are made by plunging the two limbs in two vessels which are in
connection with the two non-polarisable electrodes E and E' (fig. 98,
_a_). On illumination of A and B alternately, we shall now obtain
currents flowing alternately in opposite directions.
[Illustration: FIG. 98.--MODIFICATION OF THE SENSITIVE CELL]
Just as in the strain cells the galvanometer contact was transferred
from the electrolytic part to the metallic part of the circuit, so we
may next, in an exactly similar manner, cut this plate into two, and
connect these directly to the galvanometer, electrolytic connection
being made by partially plunging them into a cell containing water. The
posterior surfaces of the two half-plates may be covered with a
non-conducting coating. And we arrive at a typical photo-electric cell
(fig. 98, _b_). These considerations will show that the eye is
practically a photo-electric cell.
[Illustration: FIG. 99.--RESPONSES TO LIGHT IN FROG'S RETINA
Illumination L for one minute, recovery in dark for two minutes during
obscurity D. (Waller.)]
We shall now give detailed experimental results obtained with the
sensitive silver-bromide cell, and compare its response-curve with those
of the retina. A series of uniform light stimuli gives rise to uniform
responses, which show very little sign of fatigue. How similar these
response-curves are to those of the retina will be seen from a pair of
records given below, where fig. 99 shows responses of frog's retina, and
fig. 100 gives the responses obtained with the sensitive silver cell
(fig. 100).
It was said that the responses of the retina are uniform. This is only
approximately true. In addition to numerous cases of uniform responses,
Waller finds instances of 'staircase' increase, and its opposite, slight
fatigue. In the record here given of the silver cell, the staircase
effect is seen at the beginning, and followed by slight fatigue. I have
other records where for a very long time the responses are perfectly
uniform, there being no sign of fatigue.
[Illustration: FIG. 100.--RESPONSES IN SENSITIVE SILVER CELL
Illumination for one minute and obscurity for one minute. Thick line
represents record during illumination, dotted line recovery during
obscurity.]
Another curious phenomenon sometimes observed in the response of retina
is an occasional slight increase of response immediately on the
cessation of light, after which there is the final recovery. An
indication of this is seen in the second and fourth curves in fig. 99.
Curiously enough, this abnormality is also occasionally met with in the
responses of the silver cell, as seen in the first two curves of
fig. 100. Other instances will be given later.