Timothy C. Hain,
MD, Please read our disclaimer.
Eye movement recording devices are an essential component to an ENG lab. This document discusses the pros and cons of several methods. The key paramaters 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 Tektronics 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.
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.
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. In general, IR is not a reasonable technology because of the nonlinearity problem. IR is not a good technology to look at binocular alignment because of the nonlinearity situation. Scalar is a vendor of IR systems. Because we suggest Video (see below) for new laboratory setups, we will not discuss IR further.
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 combine a pupil-tracking card made by ISCAN Inc, with a small video camera set in a swim-goggle type frame.
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 adequete for saccade and pursuit testing. The temporal resolution is also adequate. Linearity is reasonable. There is less noise than EOG. The main problem with this system, compared to EOG or the Eye coil is that one cannot record under closed eyelids.
The scleral magnetic eye coil 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.
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 and the intrinsic cost of the equipment.
The eye-coil technology seems to us most suitable for animal research. In animals, the coil can be permanently implanted, eliminating the 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.:
Miscellaneous eye movement and ENG devices:
The "Ober" system is a variant IR reflectance system, incoporating some novel technology. Some labs are quite happy with it. It costs roughly $15,000.
We know of only two commercial torsional eye movement video systems. The vendors are SMI and Synapsys.
The Jedmed/House IR system is a pupil tracking system somewhat similar to the ISCAN pupil tracker. A target array is built into the goggles for this device. We do not think the proprietary and inflexible design of this device is reasonable for reserach or clinical purposes.
ICS Medical (Schaumberg IL) makes a commercial ENG system. This system performs very well, but we hesitate to recommend it because it uses a highly proprietary design. We feel that ENG technology should be more open.
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