Electroretinography measures the electrical responses of various cell types in the retina, including the photoreceptors (rods and cones), inner retinal cells. Basic mechanisms of electrical field generation in the tissue. Recording protocols Electroretinography (ERG) alone does not give you necessarily a diagnosis. Electroretinogram: An electrical diagnostic test of retinal function in situ. Electro – part Show you the basic clinical test; Show some research examples. The Eye .
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They replaced the vitreous humor of cats with heavy oil to abolish current flow from the retina to distant sites and thereby ensured large potential recordings of local ERG from the retinal surface.
Amplitude and Implicit Time Measurements The most common ERG response from a human, which is elicited with a full-field Ganzfeld flash of light, contains the a-wave and the b-wave as shown in Fig.
The multifocal ERG mfERG simultaneously measures local retinal responses from up to retinal locations within the central 30 degrees mapped topographically. Such analysis can also be used to compare ERG data between laboratories as shown in Fig.
For the fraction of the unit of energy erg, see micro-erg.
In this equation, P 3the amplitude of Granit’s P-III component, is a function of flash energy i and time of measurement t after light onset. Figure 10 A typical early receptor potential ERP of the human eye that was elicited by a very bright light flash arrow.
It is normally masked throughout most of its period by the large, positive b-wave. Bsaics to the massed retinal electrical response, small retinal lesions may not be revealed in ffERG recordings. The properties and nature of the R membrane of the frog’s eye.
Wave III was equivalent to the c-wave. The fit of the theoretical curves to the actual data is quite good for the first 25 ms of the ERG responses.
The b-wave is also eliminated when the blood flow through the central retinal artery is blocked either intentionally in laboratory animals 1531 or in human patients The basic waveform of the ERG a-wave b-wave complex was not changed by these drugs; however, after bicuculline and strychnine, the b-wave was augmented, indicating removal of inhibitory contributions.
This new technology was developed by Erich Sutter in the early s and involves powerful computers and high —intensity display monitors. The luminance-response function of the dark-adapted human electroretinogram.
Therefore, the former is a slow potential at stimulus onset recorded in the dark-adapted retina, whereas the latter contains ON and OFF components and is recorded in light-adapted conditions compare Fig. It has been argued, based baics recent findings on the rod system in mammals and primates, that rod signals can be transmitted via two pathways. With this procedure, cone-mediated function can be isolated from the large-amplitude rod ERG and allows analysis of the cone versus rod system in the dark-adapted state.
The Electroretinogram: ERG – Webvision – NCBI Bookshelf
However, the amplitude of the a-wave also depends upon the development of the positive Granit’s P-II component. Pharmacological studies and depth recordings lead us to believe they reflect extracellular electrical currents generated by negative feedback pathways between amacrine cells, ganglion cells, and bipolar cells Therefore, any change in the magnitude of this resistor will affect the distribution of currents between the retinal pathway I A and the remote pathway I B.
A, the ERG responses that were elicited in the dark-adapted state using light stimuli of different intensities. The best estimate of the b-wave is by measuring it from the trough of the a-wave to the peak of the b-wave; electrorftinogram, this method will always underestimate the amplitude of P-II. Correspondences in the dlectroretinogram of the electroretinogram and of the potentials evoked at the visual cortex. Therefore, in situations where the a- and b-waves remain normal in waveform and elcetroretinogram, OP recording scans indicate mild retinal ischemia in the inner retina Normal scotopic a-wave with selectively reduced b-wave; implicit time of b-wave is approximately the same under scotopic and photopic conditions.
Signal transmission along retinal rods and the origin of the electroretinographic a-wave. Extent of reduced a- and b-wave amplitudes depends on extent of fundus pigmentary changes; longer duration of dark-adaptation may be necessary for scotopic amplitudes to reach normal values. This is not true for the cone photoreceptors, probably because of the in-foldings of the plasma membrane that make the outer segment discs and contribute to membrane capacitance. Now, the electroretinogram response is commonly abbreviated to the ERG.
With the rods, the membrane capacitance is relatively small and, therefore, the development of membrane potential follows quite accurately the development of photocurrent.
These responses reflect mainly cone contribution and can be subtracted from the responses to white light stimuli and thus isolate the rod ERG responses. The Early Receptor Potential ERP This component of the light-induced activity of the eye was first revealed when a very bright light flash was used to stimulate the monkey eye 2263the rat eye 64or the human eye The early receptor potential in sex-linked retinitis pigmentosa. On the basis of the ionic mechanism.
In patients, these parameters provide quantitative assessment of retinal function during the progress of retinal diseases and the success of treatment Oscillatory potential and nyctometry in insulin-dependent diabetics. Measurements obtained before and after intravitreal injection of l-aspartate are compared open circles and filled symbols, respectively more The size of basiics a-wave is measured from the baseline to the trough of the wave.
Assigned status Up to Date by Peter A. Such an experiment is illustrated in Fig. Visual pigment and photoreceptor sensitivity in the isolated skate retina. The a-waves elicited by probe flashes at different time intervals after the test flash are shown in Fig.
In the future, more sophisticated analytical approaches may allow separation of the different contributions and allow identity of more exact sites of retinal diseases. Figure 21 The ERG parameters that are customarily measured in the ophthalmic clinic for electrodiagnosis. Yonemura D, Kawasaki K. Figure 13 The effects of BaCl 2 on the STR and extracellular potassium ion concentration recorded from the proximal retina of the dark-adapted cat.
His dark-adapted a-wave is smaller in amplitude than that of the normal range and when scaled up exhibits considerably slower kinetics. The amplitude of the ERP depends directly upon stimulus intensity and the concentration of visual pigment in the outer segments of the photoreceptors.
A theoretical interpretation of ERG abnormalities in central retinal vein occlusion. University of Utah Health Sciences Center ; According to Piper, the first two waves, I and II, were characterized by different latencies and temporal properties so that electroretinpgram interaction between them resulted in the formation of the a- and b-waves.
However, with short inter-flash intervals, the probe flash can only shutdown the “dark” current that exists at that time, and therefore, the amplitude of the a-wave will be smaller. In humans, ERP recording have been done only occasionally to estimate rhodopsin density in patients suffering from retinitis pigmentosa, a disease of the photoreceptors 69 The reduction in the extracellular concentration of potassium ions near the apical membrane of baskcs pigment epithelial electroreitnogram is expressed as an increase in the trans-epithelial potential with the retinal side becoming more positive relative to the choroidal side.