Advances in Photoreception Proceedings of a Symposium on Frontiers of Visual Science (1990) / Chapter Skim
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The ERG and Sites and Mechanisms of Retinal Disease, Adaptation, and Development
The ERG and Sites and Mechanisms of Retinal Disease, Adaptation, and Development
Pages 41-58
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From page 41... ...
The analysis is similar to Granit's classic analysis (Granit, 1947~. One of the two main potentials contributing to the ERG is a corneally negative potential generated by the receptors.
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From page 42... ...
The log of the flash intensity (log cd/m2) is shown next to each record.
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From page 43... ...
To simplifier the presentation, it is assumed that n is equal to 1.0. Retinal diseases produce both an increase in K and a decrease in Rm.
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From page 44... ...
In Figure 3 the change in log Rm is plotted against the change in log K Each data point is for a different adapting field intensity.
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From page 45... ...
. -0.5 0.0 0.5 1.0 1.5 2.0 Log Relative K 45 FIGURE 3 Log relative Rm versus log relative K is shown for a range of adapting intensities from the Fulton and Rushton (1978)
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From page 46... ...
Both K and Rm are affected by the ratio of Am to Kb. The dashed vertical and horizontal lines in Figure 4 show the predicted changes in K and Rm for increases in Ka (a decrease in receptor sensitivity)
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From page 47... ...
The three solid curves in Figure 4 show the predicted changes in log K and log Rm for increases in Kb or decreases in Am for three values of the ratio Kb/Am. The middle curve was derived assuming a K/Ka ratio of 10-2 5.
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From page 48... ...
For this incremental response function, response compression yields an increase in K and a decrease in Rm (Normann and Perlman, 1979~. If a particular value of the dark-adapted Ka is assumed, the response compression model provides predicted values of Ka and Am as a function of adapting intensity.
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From page 49... ...
The upper panel plots the total response amplitude versus the total light, flash plus adapting field, for adapting fields of different intensities (dotted lines)
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From page 50... ...
Most of the variation from the model is caused by the dark-adapted data point.) Assessing the Approach With both procedures for estimating Ka and Am, we reject the receptor as the primary site of sensitivity change for adapting fields up to 1.4 log scotopic trolands.
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From page 51... ...
The solid curves are the predicted changes in log K and log Rm based on the response compression model of the rod receptors. The crosses show the predicted change in log K and log Rm derived from the Fulton and Rushton a-wave data and Equations (6)
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From page 52... ...
RETINAL DISEASE AND ERG CHANGES Various retinal diseases have been shown to increase log K and decrease log Rm of the lo-wave. Figure 7 shows data from three clinical studies.
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From page 53... ...
O ~ .10 ~ on 0= ~ ° ~ ~ a O0 0 0 ~ a o a a a · , · I · I to 0.5 1.0 1.5 2.0 Log Relative K 53 FIGURE 7 Log relative Rm versus log relative K for ERG intensity-response functions recorded from patients with retinal disease.
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From page 54... ...
Under these assumptions a regional loss of one-half of the visual field, by itself, would reduce log Rm by 0.3 log unit.
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From page 55... ...
This is particularly interesting since central vein occlusion disrupts blood flow to the inner nuclear layer while leaving the choroidal blood supply of the receptors intact. Disrupting the blood flow to the inner nuclear layer appears to compromise receptor responsiveness.
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From page 56... ...
SUMMARY The ERG is a complex potential; we should be cautious in interpreting changes it brought about by disease or development. However, carefully measured intensity-response functions combined with explicit models of the ERG and with data from psychophysical paradigms (e.g., visual fields, matching, increment threshold)
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From page 57... ...
Fish 1987 Rod ERGs in retinitis pigmentosa and cone-rod degeneration. Investigative Ophthalmologic and Visual Science 28:14~150.
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From page 58... ...
Massof 1988 Photoreceptor sensitivity loss in patients with central retinal vein occlusion and iris neovasularization. Investigative Ophthalmology and Jovial Science (Suppl.~:179.
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