Night Vision Current Research and Future Directions, Symposium Proceedings (1987) / Chapter Skim
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Photoreceptor Properties
Photoreceptor Properties
Pages 23-80
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From page 25... ...
The incidence of these conditions, sometimes grouped under the heading of retinitis pigmentosa, has been estimated to be 1 in 3,500 births in the United States. Affected patients can be asymptomatic and have normal visual acuity and yet have considerable compromise in visual function due to abnormalities in dark adaptation and loss of midperipheral visual field.
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From page 26... ...
These outer segments function as the sole lock s for transduction. The inner segments of the rods are connected via a ciliary bridge to the outer segments.
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From page 27... ...
) cGMP ~ ~ I 4 Dark Current NatI,/ ¢~ C>,/ Synaptic / O 1° Tern~inal ~ Oo fig _ Horizontal Cells K+ Bipolar Cells FIGURE 1 Schematic diagram of rod and mechanisms under ying the light responses.
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From page 28... ...
The transmembrane potential in the dark is typically about -40 mV. The K and C1- permeability is conf ined primarily to the inner segment, while the flat permeabili ty is loca ~ ized to channels in the plasma membrane of the outer segment.
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From page 29... ...
that the membrane potential is -40 mV, and that the reversal potential for Nat ions is 0 my, this would indicate that the 20-pA dark current is conducted through about 5 x 103 open ionic channels in the plasma membrane of the outer segment. The absorption of an individual photon by a single rhocopsin molecule causes an isomerization of the rhodopsin molecule from a cis to a bans configuration.
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From page 30... ...
, that cam injected into the outer segment increases the dark current, and that the injection of PDE evokes a change in the rod's dark current (and membrane potential) which mimics light.
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From page 31... ...
(1942) , in their classical experiments measuring the absolute dark-adapted sensitivity of human vision, found close agreement between the randomness of seeing, as would be predicted by photon noise, and the estimated number of photons reaching the roast Their results implicitly assumed that vision at absolute threshold was limited strictly by photon noise.
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From page 32... ...
In an attempt to substantiate or rule out the Barlow (1956) hypothesis that these dark events limited detection in the retina, an endeavor was made to record threshold responses from ganglion cells in the retina of a rod-dominated animal and test whether the detection of dim lights was indeed influenced or limited by dark events.
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From page 33... ...
For Figure 2, the best fit was obtained for a dark rate equivalent to 0.05 dark events/rod and s. These data and the results from other cells indicate that threshold detection is not limited strictly by photon noise and that a second source of noise exists which can be attributed to spontaneous dark events in the rods.
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From page 34... ...
Dow - +D: - + - + _ ~ _ ~ 'it Il11111117111111il~ 0.1 1 10 FREQUENCY (Hz) FIGURE 3 Horizontal cell membrane fluctuations during darkness and under backg round illumination.
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From page 35... ...
It is concluded, therefore, that the dark events play a major role in limiting detection of dim lights. If the dark events are evident in the human rods, then these may, by extrapolation from experiments in animals, play a major role in limiting human night vision.
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From page 36... ...
Lyubarsky 1985 Induction by cyclic GMP of cationic conductance in plasma membrane of retinal rod outer segment. Nature 313: 310-313.
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From page 37... ...
A third way to optimize information is by testing rod thresholds at several points in the visual field. At the University of Illinois, absolute thresholds are measured at 15, 30, and 45 degrees superior and inferior to the fovea.
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From page 38... ...
Early studies on this disorder have suggested that the majority of patients show a monophasic dark adaptation curve, implying that cones mediate thresholds. If white light stimuli are utilized for testing, the examiner may draw an erroneous conclusion
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From page 39... ...
Some patients may present in an atypical fashion and not complain of night blindness. If dark adaptation testing is done at several loci, it is possible to find isolated retinal regions where thresholds are elevated by less than 1 log unit.
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From page 40... ...
Pigmentary Retinal Degenerations Not Associated with Diffuse Photoreceptor Disease In some instances, patients who are carriers of hereditary retina disorders, such as ocular albinisms or choroideremia, and others with inflammatory disorders, such as rubella, are evaluated for night blinding disorders. Normal dark adaptation thresholds as well as normal ERG recordings in the majority of instances help determine that these patients, with variable degrees of fundus pigmentary changes, have a benign or at least a nonprogressive retinal disorder.
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From page 41... ...
In the early stages patients can have minimal, if any, changes on ophthalmoscopic examination. In more advanced stages, the characteristic fundus abnormalities include narrowed retinal vessels, depigmentation of the retinal pigment epithelium, intraretinal bone spicule pigment distributed around the midperipheral fundus, and waxy pallor of the optic disks.
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From page 42... ...
Hereditary cerebroretinal degenerations (9) Vitreo-retinal degeneration of Goldmann-Favre Congenital amaurosis of Leber Choroideremia Generalized choroidal sclerosis Gyrate atrophy of the choroid and retina Prog ressive albipunctate dystrophy .
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From page 43... ...
·-SNB -- NORMAL -L 1 1 1 0 5 10 15 20 25 TIME (MIN) 1 1 .1 30 35 43 FIGURE 1 Representative dark adaptation curves for a normal subject; a patient with congenital stationary night blindness, SNB; two patients with moderately advanced retinitis pigmentosa, RP(1)
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From page 44... ...
These subnormal responses with delayed implicit times and elevated final dark-adapted thresholds across the retina, seen in these widespread progressive forms of retinitis pigmentosa, contrast with the subnormal responses with normal lo-wave implicit times and normal final dark-adapted threshold in some areas and elevations in all other areas seen in self-limited sector retinitis pigmentosa. These delays in cone lo-wave implicit time in progressive forms of retinitis pigmentosa with night blindness also contrast with the normal cone lo-wave implicit time seen in stationary forms of night blindness (Berson, 1976b)
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From page 45... ...
At this time no patient, age 6 or over, has been observed with normal cone and rod amplitudes and normal cone and rod lo-wave implicit times who later developed widespread retinitis pigmentosa. Young adults with a positive family history of retinitis pigmentosa in a parent, grandparent, uncle, aunt, or sibling are at higher risk than the general population for having this condition, and therefore, dark adaptation and ERG testing should be done to confirm or negate the diagnosis in these patients.
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From page 46... ...
Patients with these refractive errors should be questioned closely with regard to whether or not they have symptoms of night blindness, and their peripheral funds should be carefully examined. Dark adaptation and ERG testing should be considered if symptoms or signs raise the possibility that these patients are affected.
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From page 47... ...
Possible pathogenetic mechanisms include a defect in the capacity of photoreceptors to synthesize outer segments, an abnormality in the capacity of the pigment epithelium to ingest outer segment tips, a deficiency in phosphodiesterase activity in photoreceptor cells with elevation of cyclic guanosine monophosphate (cGMP) and consequent cell death, a deficiency in the interphotoreceptor matrix, or some yet to be defined metabolic defect in these cells.
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From page 48... ...
Other probes that identify restr lotion f ragment length polymorphisms (RFLPs) are designated with asterisks.
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From page 49... ...
Large doses of vitamin A have resulted in a return of dark adaptation thresholds and ERG responses to normal in the early stages (Gouras et al., 1977; Carr, 1976~. More advanced cases have not responded to therapy, but in one such case in which the retina was examined after the death of the patient, widespread loss of photoreceptor cells was observed (von Sallmann et al., 1969~.
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From page 50... ...
Improvement in muscle morphology has been reported following creative supplementation, although no evidence exists that creative will alter the course of the chorioret~nal degeneration (Sip~la et al., 1981~. Although no treatments are known for most types of retinitis pigmentosa and allied diseases, the symptom of night blindness can be alleviated with a night vision pocketscope (Berson et al., 19~4; Berson, 1976a)
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From page 51... ...
(C) Single-stage image intensifier tube, 3 cm in length, contained in the night vision pocketscope.
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From page 52... ...
Rabin 1974 A night vision pocketscope for patients with retinitis pigmentosa. Archives of Ophthalmology 91:495-500.
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From page 53... ...
Sandberg 1984 Diurnal rhythm in the human rod ERG. Investigative Opthalmology and Visual Science 25: 236-238.
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From page 54... ...
1941 Dark adaptation: Some physiological and clinical considerations. Archives of Ophthalmology 26 :203.
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From page 55... ...
Berson 1984 Abnormal rod ERG diurnal rhythm in a family with dominant retinitis pigmentosa. Investigative Ophthalmology and Visual Science 25(Suppl)
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From page 56... ...
Buist 1978 Vitamin Be in management of gyrate atrophy of choroid and retina. Lancet 2~8101~:1213.
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From page 57... ...
I cannot accept responsibility for anything wholly novel here, except the particular collation of evidence on the roles of the different mechanisms of light adaptation shown in Figures 7 and 8, and possibly the analysis of Sakitt's (1972) data.
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From page 58... ...
This is not ideal, for an ideal quantum counter of light entering the cornea would detect a flash on 60 percent of the trials when the mean flash intensity were only 0.9 quanta per trial. This conclusion is worth mentioning because it is the only conclusion one can make from this experiment without estimating unknown quantities.
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From page 59... ...
Barlow, 1956; Ahumada and Watson, 1985~. Sakitt Experiment The equivalent noise under conditions optimal for human performance is not known.
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From page 60... ...
. When the mean number of noise events is below 7 or above 10, fewer data can be reconciled with the model; 7 fits best.
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From page 61... ...
An equivalent noise of 7 to 11 quanta best describes the performance of these two observers, but the deviant observation from BS and the deviant observer indicate deviations f rem a true representation. Noise Coincident with the Stimulus For the present, disregard the anomalous data and consider the implications of the conclusion that the performance of Sakitt's observers is like an ideal quantum detector that is subject to a mean of 7-11 spontaneous, quantumlike events on each trial.
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From page 62... ...
Evidently, both pressure blinding and light adaptation reduce that noise. The pair of dark adaptation curves in Figure 2 appears to approach different asymptotes.
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From page 63... ...
FIGURE 2 Dark adaptation following monocular or binocular light adaptation. The nontest eye was pressure bl inded dur ing minute 13 .
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From page 64... ...
The lower curve shows the threshold under the same conditions, except that the test flash followed removal of an annulus surrounding the location of the test flash. The point here is that removal of the annulus lowers the absolute threshold to a value lower than that when it is tested conventionally, that is, with no annulus at all.
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From page 65... ...
Geisler and coworkers applied this approach to a comparison of human observers with more complex observers on more complex visual tasks than those broached above. Ideal observers subject only to physical constraints use quanta 5 to 20 times more efficiently than typical humans on the tasks tested so far; namely, increment threshold, resolution, and separation discrimination.
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From page 66... ...
FIGURE 6 Temporal modulation sensitivity curves. Source: Kelly (1972)
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From page 67... ...
The last curve represents the signal passing up the optic nerve; the difference between it and the curve above is due to postreceptor adaptation.
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From page 68... ...
Pupil Pteceptor Saturation Receptor Adaptation I Post Receptor Adaptation FIGURE 8 Response magnitudes (R) under different illumination at several levels of the rod system.
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From page 69... ...
If noise limits sensitivity during steady illumination, then its sudden removal by reduction of illumination should allow a sudden increase of sensitivity that is proportional to the square root of the decrease of illumination. Such increases are shown in Figure 9.
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From page 70... ...
It was argued above that even when the eye is at its best, it adds noise to that which inevitably accost panics the signal, and that in a variety of tasks human observers do about 10 times worse than an ideal quantum detector. This suggests that the signal-to-noise ratio can be increased by amplifying the stimulus, noise and all, for it would reduce the relative contribution of the noi se that is intr insic to the visual system.
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From page 71... ...
Screening Procedures Human Factors and Testing Some of the issues relating to screening for night vision are not specific to vision and can be handled by specialists in testing or human factors or by reference to data in the literature on human factors, such as the text by Bailey (1982) or the reference works of Van Cott and Kinkade (1972)
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From page 72... ...
My own impression is that cone sensitivity, as reflected by the level of the cone plateau of dark adaptation curves, is more variable than rod sensitivity, as reflected by the dark-adapted absolute threshold. Equivalent Backgrounds One of the significant findings about light and dark adaptation is that the state of the visual system under an enormous range of conditions can be characterized by a single parameter referred to as the equivalent background (Crawford, 19471.
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From page 73... ...
Peachey 1984 Interocular transfer and dark adaptation to long-wave test lights. Vision Research 24:1043-1048.
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From page 74... ...
Makous 1985 Difference frequency gratings above the resolution limit. Investigative Ophthalmology and Visual Science 26(Suppl.)
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From page 75... ...
van Norren 1983 Light adaptation of primate cones: An analysis based on extracellular data. Vision Research 23:1539-1547.
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From page 76... ...
There is some dispute still over whether we are photon-limited at the absolute threshold. But the point is that you cannot get any better from the physics of the light.
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From page 77... ...
Another issue has to do with the relationship between cone dark adaptation and rod dark adaptation, but I don't have any data on it. I expect from our research that rod adaptation wouldn't tell you much about cone sensitivity, or rate of cone dark adaptation or vice versa.
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From page 78... ...
There must be an underlying reason for that, in terms of a fluctuation over time and the dynamics of a given individual's dark adaptation process. I think it would be very interesting to try to do some analytical work on the systemic factors that underlie this fluctuation with time in a given individual's dark adaptation characteristics, whether it's related to eating patterns, caffeine, or diurnal rhythms and body temperature, and in the latter case, whether sensitivity is optimal during the night phase or during the daytime phase when body temperature is higher.
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From page 79... ...
With a quick test you have very limited information. Since very little is presently known about the relative parameters of night vision for job performance, I think that a quick screening test is a bit preliminary.
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From page 80... ...
The patient can be led into the room to record a dark adaptation threshold, then have an ERG lens placed on the topically anesthetized cornea, have a few flashes of light administered, thus doing a very comprehensive and definitive examination in 10 or 15 minutes. So I think screening programs could be very definitive and run in a short period of time.
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