Timothy C. Hain, MD Page last modified: May 31, 2012
Intended audience: Clinicians and researchers that want to record eye movements
Eye movement recording devices are an essential component to an ENG lab. This document discusses the pros and cons of several methods. The key parameters to consider with eye movement recording devices are:
A perfect system would be the following:
Such a system does not presently exist, but the closest in the author's opinion is the video EOG system (see later).
EOG works by quantifying the corneo-retinal potential, a small voltage between the front and back of the eye. Basically EOG consists of an isolated instrumentation amplifier coupled to a chart recorder or a computer.
If you are not using your EOG on humans, you can get a very reasonable EOG system by purchasing a used Tektronix differential scope with amplifiers. These can be found on the surplus market. When testing humans, especially in a hospital environment, UL approval of your equipment is important. This will likely force you to purchase a commercial, UL approved system. There are a variety of vendors of UL approved physiological amplifiers including Micromedical technology, LifeTech, Nicolet, and Neurokinetics (among others). At this writing, we recommend going with video, so the details of what the vendors offer are not terribly important. We prefer Beckman miniature silver/silver chloride electrodes (which cost quite a bit). Snap on electrodes are simply not reasonable. Disposable electrodes can be used, but tend to be noisy.
Noise is the big problem -- this is an electrical method so you can get muscle artifact as well as 60 cycle artifact. As an example of 60 cycle noise, see below. The vertical recording is useless.
If you are prepared to make certain compromises, you can get by very cheaply. If you filter your signal so that you pass roughly 1-20 hz, you do not need a DC EOG amplifier, and you will filter out most of the line noise and EMG noise. With this type of EOG, you can use a surplus EEG machine, which may cost next to nothing and which is certainly UL approved. Given that your main objective is to quantify positional and caloric nystagmus, there is nothing wrong with this approach. However, you should realize that saccade and pursuit tests will not be very useful.
As a bottom line, this methodology is for people on a very tight budget.
Infrared (IR) generally works by quantifying the difference between the amount of infrared light reflected by the sclera between a sensor (phototransistor) pair. Infrared is used so that one can test subjects in darkness. IR has less noise than EOG. The problem with IR is that it only works well with the eyes close (i.e. +- 10 deg) from center.
For certain applications, such as sinusoidal rotatory chair testing, IR works well. Also, IR may be a good choice if you are studying micro-eye movements. IR is not a reasonable technology to measure pursuit or saccades because of the nonlinearity problem. IR is also not a good technology to look at binocular alignment. Skalar and Microguide are vendors of IR systems.
In our opinion, video-EOG is preferable to IR reflection in nearly all situations.
Supplemental material on the site DVD: Video of BPPV nystagmus using Micromedical hardware from 2006
Video-eye movement recording systems, such as those sold by Micromedical Technologies (Chatham Ill), are currently the author's #1 choice for clinical testing. These devices usually combine frame-grabber software, with a small video camera set in a SCUBA goggle or swim-goggle (e.g. SPEEDO) type frame.
|Goggles typical for monocular (single eye viewing) video-ENG systems. This particular pair is made by Neurokinetics. Nearly all vendors listed below make very similar devices. Note that this goggle design has no method of positioning the camera.||Goggles typical for binocular (both eyes viewing) video-ENG systems. This particular system is made by Micromedical Technology. We mainly use the monocular Micromedical goggles in our clinical laboratory in Chicago.|
These devices are not perfect, but all in all, in the author's view, are the most practical of the devices listed here for clinical testing. The spatial resolution is adequate for saccade and pursuit testing. The temporal resolution is also adequate for nearly everything but saccadic oscillations (where it fails completely -- you need an analog system -- ideally an eye coil -- to record these). Linearity is reasonable. There is less noise than EOG.
The main problem with this the VNG type system, compared to EOG or the eye coil is that one cannot record under closed eyelids. Also, VENG software tends to be generally proprietary rather than open source and consequently it is nearly always extremely buggy. These cottage industry companies evidently just don't have enough resources to debug their software.
A very common flaw to these devices is that they have no method of putting the camera in front of the eye. You can spot these devices because they have no adjustment knobs. Examples from the list below are the Neurokinetics device, the Synapsys device, and the Vestibular technologies device. Micromedical's device as well as SMI have ways to adjust the camera angle.
If you can live with an eccentric camera (I don't recommend living with it because you can't see torsion), you should as least get a considerable discount for this type of device as opposed to one of the models that can position the camera, and save your money for a better design.
Vendors (also see bottom of this page):
Several of these vendors are selling equipment developed by other vendors, so the variety is not as large as it may seem. We use the Micromedical system in our clinical lab.
The scleral magnetic eye coil, invented by Dr. David A. Robinson, is a wonderful technology if your subjects are willing to let you put a contact lens on their eye with a little wire coming out of it. Because most people are unenthusiastic about this, this is not a clinically relevant technology. Nevertheless, it is a truly great research methodology.
Eye coil systems, at this writing, are research tools. To use an eye-coil system, all of your subjects must sign a consent because of the risk of corneal abrasion. This technology is certainly the most expensive of all, because of the cost of the eye-coils. The intrinsic cost of the equipment is actually less than video.
The eye-coil technology seems to us most suitable for animal research. In animals, the coil can be permanently implanted, eliminating the prohibitive expense of contact lenses and also the limitation on the duration of recording.
Vendors -- manufacture of these devices is still basically a cottage industry. The first two vendors offer somewhat expensive commercial systems. The third offers somewhat inexpensive systems that require more effort on your part to get it to work.:
Because video eye movement tracking is much easier to use and rapidly improving, we expect that scleral eye coil recordings for humans will be gone by 2010, except for use in animals.
Torsion eye trackers might be very useful for BPPV testing, or perhaps in diagnosing the "utricular syndrome", but at this writing (2008), these systems are just too expensive and not in general use. For clinical use, one can get far quicker and generally more reliable answers using a carefully configured analog system (no computer needed!). Note that for this method you need to use a camera that has an analog output and also one that can be positioned in front of the eye. We use the Micromedical system ourselves. For this solution you should avoid "digital" output cameras such as ones that only have IEEE interfaces.
Anyway, if you are an early adopter of torsional technology, or are doing a research project that requires torsion, we know of four commercial torsional eye movement video systems. The vendors are Neurokinetics, SMI, Synapsys, and Vestibtech. We have not had much experience with any of these with the exception of the Synapsys system which we tried for a month. We were not impressed enough with the Synapsys system to purchase one. Our main problem was that we didn't follow how "neural network" technology had anything to do with torsional eye tracking and we also that we couldn't read the error messages (software was written in French).
Miscellaneous eye movement devices:
The "Ober" system is a variant IR reflectance system, incorporating some novel technology. Some labs are supposedly quite happy with it, although nobody we know. It costs roughly $15,000.
The House Eye Dynamics system (marketed by MedTrak) is a pupil tracking system somewhat similar to the ISCAN pupil tracker. A target array is built into the goggles for this device. We think that this is a bad idea.
There is considerably more information about eye trackers under the link in "additional information".
EYE DYNAMICS, INC. (also see MedTrak)
2301 WEST 205TH ST., # 102
TORRANCE, CA 90501 US
310-328-0477 / 310-328-0477
ICS Medical (also see GN otometrics) (Video ENG)
125 Commerce Drive
Schaumburg Il 60173
MedTrak Technologies, Inc.
14700 N. Frank Lloyd Wright Blvd.
Scottsdale, AZ 85260PHONE: 480-993-3701 or 800-293-1858
MICROMEDICAL TECHNOLOGIES, INC. (Video ENG with nice goggles and various other devices)
10 KEMP DR.
CHATHAM, IL 62629 US
217-483-2122 / 217-483-2122 122
SENSOMOTORIC INSTRUMENTS, INC. (Video ENG with nice goggles)
97 CHAPEL ST.
NEEDHAM, MA 02492 US
781-453-1377 / 781-453-1377
Neuro Kinetics, Inc. (ENG and rotatory chair)
128 Gamma Drive
Pittsburgh, PA 15238
Phone: (412) 963-6649
Fax: (412) 963-6722
REMMEL LABS (Scleral Eye coil)
26 Bay Colony Dr, Ashland MA 01721-1840 USA
SYNAPSYS, INC. (ENG including Torsion)
7000 BEE CAVES RD.
AUSTIN, TX 78746 US
011-334-9117575 / 512-301-9890
Vestibular Technologies (ENG and various other systems)
Phone: (618) 993-7554
Direct Line: (307) 637-5711
Toll free (800) 752-6178 (US and Canada Only)
FAX: (312) 896-5856