Electroretinogram Via EEG-SMT

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.

Photopic function will then be measured after allowing the eyes to light adapt for a five-minute period. The eyes will then be exposed to a strobing light stimuli in the NIR spectrum. If photoreceptors are stimulated by NIR under light conditions, this will be made evident by a series of A and B waves that vary from normal deviations by changes in the waveform, such as an increased amplitude of the waves, or duration of the waves that match the frequency of the strobe.

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.

Olimex Part Qty. Cost
EEG-SMT 1 99.00 Euro
EEG-AE 3 9.00 Euro
EEG-PE 1 5.00 Euro
USB –A-B Cable 1 2.00 Euro
Occuscience Part Qty. Cost
ERG-Jet Item # ERGACC7 1 30.00 US
Shipping 1 15.00 US
Total 45.00 US
Radioshack 3.5mm stereo plug M/M 1 10.00 US
Software Part Qty. Cost







OpenVIBE Free Download
EDFbrowser Free Download
MaXimus Free Download
Weaver and Companyhttp://www.doweaver.com/ten20.html



Part Qty. Cost
1020 conductive adhesion paste 1 45.00
Shipping Free
Optic Anaesthetic  Part  Qty.1 Variable
Misc. supplied by subject Surgical tapeAlcohol pads










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This is the EEG-SMT unit as purchased from Olimex. The unit is an inexpensive opensource EEG at home unit. Both software and hardware were designed by the OpenEEG project, and a good resource for information regarding this device is available at the projects site OpenEEG.
The unit itself is a grey box with 6 ports. The large port on the left side of the device in the picture above is a USB – B port, which is accessed via the USB A B plug and attached to the PC. The device is powered through this USB connection.
The rest of the ports, CH1 through DRL are 3.5mm stereo female plugs. This plug size is important to note as we will need to modify the contact lens electrode in order to attach it to the EEG unit.
EEG Electrodes
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The above pictures shows, from left to right, the contact surface of the Olimex EEG electrodes. This is where the Ten20 adhesive is applied and the side of the electrode which contacts skin. Second from the left shows the dorsal surface of an Olimex EEG-AE. This can be differentiated from the EEG-PE immediately to its right by the components on the board. The EEG-PE has no other parts beyond the cable and the board. The final item on the right is the 3.5mm male audio jacks by which the electrodes are connected to the EEG-SMT unit.
ERG – Jet Contact Lens Electrodes
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The above image shows the ERG-Jet Contact Lens electrode in the midst of being modified in order to allow interface with the EEG unit. The 1.5mm pin connector is cut, and the wire lead is soldered to the red wire from the 3.5mm male audio jack.
Openvibe is a free, open source software which allows one to acquire, record, analyze, and visualize brain signals. Openvibe has a vast number of features. The interface is a graphical one which requires researchers to drag and drop various component icons and connect them with lines which designate how each component will interact. Once you get familiar with this software it’s great, but at first it isn’t very intuitive. OpenVibe can export files into five different file types. The format we’ve chosen to use is EDF, or European Data Format, as it’s an open and non-proprietary file type.
When first using OpenVibe it may be most useful to simply follow the directions provided below. However, this is an excellent piece of software and familiarity with it will make your ERG experience much easier. The OpenVibe project provides a manual here: (http://openvibe.inria.fr/documentation-index/#User+Documentation)
EDFbrowser is another free open source software solution. It provides a number of options which assist in analysis of our collected data, and it allows for exportation of the ERG wave signal as a pdf document.
Although, a later series of instructions will be provided regarding the analysis of collected ERG signals, familiarizing oneself with the software now will make the process simpler. An instruction manual for EDFbrowser is available here: http://www.teuniz.net/edfbrowser/EDFbrowser%20manual.html
MaXimus is another EDF reader that’s both open source and free. This software provides a simple interface by which to analyze and annotate the EDF files.
MaXimus does not appear to have an instructions manual available; however, it’s pretty simple and one can familiarize themselves with it easily. Use of this program will be discussed in the ERG interpretation segment.
Optic Anesthetic
The eye is densely innervated and thus is easily irritated by the hard CL electrode. Use of this electrode requires a lubricating saline solution and a topical anesthetic. Lidocaine is available in a 4% optical gel, which is optimal as mere eye drops drain through the lacrimal ducts nearly immediately. The 4% gel is usually administered in 0.5ml doses, giving a total of 2mg of lidocaine. A 1% lidocaine solution such as is used for injection may be administered via eyedropper. After applying the appropriate number of drops or gel, subject is to lie prone with a finger occluding the lacrimal glad for 5 minutes. After five minutes, the anesthesia will be sufficient to allow the use of the CL electrode. Keep in mind that when the eye is numbed, extreme care must be taken not to scratch the surface. The eye should never be rubbed. Any use of Anesthetics, even topical, come with certain risks and should be dispensed under the order of a physician only.
Setting Up The Unit

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 age.

Set gender.

Set number of channels to 2.

Set the device to the appropriate Com source.

Set the sampling frequency to 256.

Hit apply.

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.


Testing and Recording
  1. 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-.
  2. 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.
  3. Additional electrodes can be used to monitor EMG signals through CH2+ and CH2-.
  4. Push play in OpenVibe designer
  5. Isolation subject from all other light sources except Red Light.
  6. Attach isolation mask stimulator and dark adjust eyes for 25 minutes.
  7. At minute 25, instill ocular anesthetic into the eye to be tested according to directions. Occlude the lacrimal sinus while keeping both eyes shut.
  8. 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 +.
  9. Activate the stimulator, and run stimulator dark sequence to detect wave oscillatory potentials.
  10. After dark sequence is finished, eyes are light adapted for ten minutes. Then the stimulator light sequence is run.
  11. In Openvibe, push stop.
  12. Access EDF software in order to analyze signal and convert to a PDF file.
The output at this time should appear similar to the following image:
E3 - frontal
Further Testing Information
At this point, you’ve set up your ERG unit and gathered some sample data. Interpretation of data and analysis of signals will be discussed in further posts. The next post will deal with the other hardware component needed for ERG testing: the optic stimulator. A good practice to keep is to do a practice run of the ERG testing once per week even while still in the preparation phase. This will guarantee familiarity and the ability to troubleshoot your ERG once the gathering and analysis of data goes live.
Resources and Citations

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






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