Frequency & Amplitude Ranges for Bioelectric Signals

Transcription

1 Frequency & Amplitude Ranges for Bioelectric Signals Signal Frequency range (Hz) Amplitude range(mv) ECG EEG EOG EMG

2 Electro-oculogram (EOG) The clinical electro-oculogram is an electrophysiological test of function of the outer retina and retinal pigment epithelium in which the change in the electrical potential between the cornea and the fundus is recorded during successive periods of dark and light adaptation. Emil du Bois-Reymond (1848) observed that the cornea of the eye is electrically positive relative to the back of the eye. Elwin Marg named the electro-oculogram in 1951 and Geoffrey Arden (Arden et al. 1962) developed the first clinical application.

3 Electro-oculogram (EOG) The electro-oculogram measures the potential that exists between the cornea and Bruch s membrane at the back of the eye. The potential produces a dipole field with the cornea approximately 5 mv positive compared to the back of the eye, in a normally illuminated room. The EOG consists of two potentials: the standing potential (resting potential, dark phase, dark current) which is evoked by moving the eyes in the dark and originates from the retinal pigment epithelium and the light potential (light rise) which is evoked by moving the eyes in a lighted environment and originates from the photoreceptors. Clinically, the ratio between the light and dark potentials (sometimes also called the Arden index or Arden ratio) is assessed. If that ratio is less than 1.8 it indicates a malfunction of the structures from which the potential originates. The EOG is also used to monitor eye movements.

4 Electro-oculogram (EOG) The eye has a standing electrical potential between front and back, sometimes called the corneo-fundal potential. The potential is mainly derived from the retinal pigment epithelium (RPE), and it changes in response to retinal illumination. The potential decreases for 8 10 min in darkness. Subsequent retinal illumination causes an initial fall in the standing potential over s, followed by a slow rise for 7 14 min (the light response). These phenomena arise from ion permeability changes across the basal RPE membrane.

5 By attaching skin electrodes on both sides of an eye the potential can be measured by having the subject move his or her eyes horizontally a set distance. Ganzfeld

6 Electro-oculogram (EOG) The clinical EOG makes an indirect measurement of the minimum amplitude of the standing potential in the dark and then again at its peak after the light rise. This is usually expressed as a ratio of light peak to dark trough and referred to as the Arden ratio.

7 Electro-oculogram (EOG) Typical signal magnitudes range from 5-20 µv/degree.

8 Electro-oculogram (EOG) Best disease is an eye disease that is slowly progressive and usually starts becoming an issue when someone is a child or young adult. It is characterized by a loss of central vision. The severity of vision loss is quite variable. Usually, Best disease affects both eyes, but occasionally it can be unilateral.

9 Electro-oculogram (EOG) Egg-Yolk Lesion

10 Electroretinogram (ERG) Electroretinography (ERG) is a non-invasive electrophysiological method which provides objective information about the function of the retina. Advanced ERG allows to assay the different types of retinal receptors and neurons in human and animal models.

11 Electroretinogram (ERG) a-wave : derived from photoreceptors (rods & cones) b-wave : derived from ON bipolar cells OPs : oscillatory potentials, derived from the inner retina (amacrine and G-cells) OPs

12 E R G

13 Electroretinogram (ERG) Electroretinography (ERG) measures the electrical responses of various cell types in the retina, including the photoreceptors (rods and cones), inner retinal cells (bipolar and amacrine cells), and the ganglion cells. Electrodes are usually placed on the cornea and the skin near the eye, although it is possible to record the ERG from skin electrodes. During a recording, the patient's eyes are exposed to standardized stimuli and the resulting signal is displayed showing the time course of the signal's amplitude (voltage). Signals are very small, and typically are measured in microvolts or nanovolts. The ERG is composed of electrical potentials contributed by different cell types within the retina, and the stimulus conditions (flash or pattern stimulus, whether a background light is present, and the colors of the stimulus and background) can elicit stronger response from certain components.

14 Electroretinogram (ERG) Dark adapted, dim (blue) flash response: Isolated rod-driven response Dark adapted, bright (white) flash response: Generates a-wave, b-wave, and OPs Light adapted, bright flash: Isolated cone-driven response 30 Hz Flicker: Another method of isolating cone responses Rod Rod & Cone Cone

15 E R G Retina ya da ağkatman, görmeyi sağlayan ışığa ve renge duyarlı hücrelerin bulunduğu göz tabakasıdır. Retina, göz küremizin iç yüzeyini kaplar, ince yarı saydam ve hafif pembe-kırmızı renkli bir zardır. Retina, göz küresi boşluğuna bakan iç kısımda duyusal (nörosensoriyel) tabaka ve dışa doğru kısımda pigmentli tabakadan oluşan iki katmanlı bir yapıdır.