An electroretinogram (ERG) is used as both a research tool and a diagnostic procedure that can detect certain retinal diseases. The retina is flooded with light and the resulting electrical activity of photosensitive cells is recorded through active peripheral transdermal electrodes and a retinal electrode. These recorded patterns demonstrate a distinctive series of negative deflections, called A waves, and positive deflections termed B waves.
The ERG will be used in our study to determine the spectral thresholds of scotopic and photopic vision and measure changes in these thresholds as rhodopsin is replaced by porphyropsin.
The scotopic function is tested by exposing the eye to various frequencies of light stimuli in the NIR range, after achieving a dark-adapted state via 30 minutes of darkness. The resulting pattern (if any of the NIR light sources are able to stimulate photoreceptors) will appear as a slow corneal negative potential, termed a scotopic threshold response (STR). This STR pattern indicates that the photoreceptive cells are receiving a light stimuli near rod threshold. Once it has been determined likely that threshold is being achieved by NIR light, this sensitivity will be confirmed via strobing of the light source at that NIR frequency. This recorded signal should then appear as a series of B waves matching the intervals of the stimuli.
An ERG unit consists of an EEG receiver, passive and active transdermal electrodes, a corneal electrode, and a PC with the appropriate software to record signals. Additional supplies needed include a headnet, Ten20 conductive gel, normal saline, and an optically safe topical anesthetic gel. The suppliers that were chosen for each component are listed below along with the costs at time of purchase and part numbers.
|USB –A-B Cable||1||2.00 Euro|
|ERG-Jet Item # ERGACC7||1||30.00 US|
|Radioshack||3.5mm stereo plug M/M||1||10.00 US|
|Weaver and Companyhttp://www.doweaver.com/ten20.html
|1020 conductive adhesion paste||1||45.00|
|Misc. supplied by subject||Surgical tapeAlcohol pads
Initial setup of the device is to be done according to Olimex Documentation.
Connect the EEG-SMT unit to the pc. Attach electrodes to the EEG-SMT unit as follows
CH1+ will be connected to an ERG jet contact lens electrode.
CH1- will be connected to a
CH+2 will be connected to an EEG-AE active electrode.
CH2- will be connected to a
DRL will be connected to an EEG-PE passive electrode or ear clip.
Open Openvibe acquisition Server.
Set Driver to OpenEEG Modular EEG P2
Set the connection port to 1024
ample count per sent block set to 32
Push driver properties.
Configure the identifier to 0.
Set number of channels to 2.
Set the device to the appropriate Com source.
Set the sampling frequency to 256.
Push connect and then push play.
Open program Openvibe designer.
Access the acquisition folder in the right pane and drag acquisition client to the left pane.
Access the File Reading and Writing folder in the right pane. Access the EDF subfolder and drag the EDG file writer to the left pane.
Access the Visualization folder in the right pane and then access the Basic subfolder. Drag Signal display to the left pane.
Connect the pink signal stream flag on the acquisition client icon to the green flag of signal streamed matrix of the Signal display icon.
Connect the pink signal stream flag on the acquisition client icon to the pink signal flag of the EDF File writer.
Double left click on the EDF file writer and set the save location for the EDF file.
The ERG unit and PC are now ready to make a recording.
Prepare the skin of the forehead using an alcohol pad and then use 1020 conductive paste to position the electrode. Secure electrode using tape or strap. Plug the electrode into CH1-.
Prep the skin of the earlobe with an alcohol pad and use 1020 conductive paste on electrode contact surface. Clip electrode to ear lobe. If using a passive electrode use 1020 conductive paste to position the electrode behind the ear. Plug electrode into DRL.
Additional electrodes can be used to monitor EMG signals through CH2+ and CH2-.
Push play in OpenVibe designer
Isolation subject from all other light sources except Red Light.
Attach isolation mask stimulator and dark adjust eyes for 25 minutes.
At minute 25, instill ocular anesthetic into the eye to be tested according to directions. Occlude the lacrimal sinus while keeping both eyes shut.
At minute 30, add a drop of ocular lubricant to the contact lens electrode and then insert. Secure the electrode by taping the wire to the cheek. Plug electrode into CH +.
Activate the stimulator, and run stimulator dark sequence to detect wave oscillatory potentials.
After dark sequence is finished, eyes are light adapted for ten minutes. Then the stimulator light sequence is run.
In Openvibe, push stop.
Access EDF software in order to analyze signal and convert to a PDF file.
Salmon, Thomas. OD, PhD, FAAO, Northeastern State University College of Optometry, lecture outlines
Olimex, EEG SMT Users Guide and Quickstart Guides: EEG-SMT
Also see the hardware schematics for the EEG-SMT, AE, and PE
OpenVibe Project: Openvibe