A Patient’s Guide to Visual Field Testing

Visual Field Testing – What It Is

One’s visual field is the space in front and to the side of an individual in which seeing occurs. Eye doctors call this the “island of Traquair”. A normal human visual field spans approximately 160 degrees of space[1]. In the center of this “island” our vision tends to be more sensitive than out toward the edges of the field of vision. 

Clinical testing of visual fields has been done for over a 100 years. The first documented field loss from glaucoma was by Dr. Bjerrum who tested his patients using the back of his clinic door. Such testing could only detect the most severe (or “dense”) field deficits. Over time, far more sensitive methods of visual field testing have been developed. 

Automated Perimetry 

For the last few decades most glaucoma field testing has been done using automated perimetry. Essentially an individual sits facing a white bowl onto which small spots of light (usually white) are projected. These spots of light vary in intensity in order to detect the threshold of vision which is the weakest light that can be seen. Light that is projected at an intensity lower than the threshold of vision will not be seen. Multiple points throughout the field of vision are tested in a seemingly random pattern. Some positions in the visual field are tested more than once as the software algorithm attempts to discover the threshold of vision for a particular location of the visual field. 

Patient Experience 

With most modern automated perimeters (visual field testers), the individual to be tested will be seated and asked to lean forward toward the perimeter. Usually there is a chin rest and forehead rest to properly position the head. The individual being tested will be asked to hold onto an object with a button on it and instructed to press the button whenever a light is seen on the surface of the testing bowl. It is important to focus on the fixation object throughout the testing. If one is looking around then the test will be of poor quality. For glaucoma testing each eye is tested separately. The test will last between four and eight minutes. If one gets tired and needs to take a break during the test it is usually possible to pause the test with the handheld device. 

Reasons Why Automated Perimetry Can be a Challenging Testing Experience 

Visual field testing using automated perimetry can be quite frustrating. There are multiple reasons for this. Some include:

It’s Boring

Automated perimetry is essentially like playing the world’s most boring video game. You sit in front of a white screen, focus on an uninteresting fixation target, and wait for spots of light to appear after which you click a button. No colorful graphics. No way to score points. No bonus round (not that you’d want one). Simple, but very, very boring. 

It Seems to Last a Very Long Time

Four to eight minutes may not seem like a very long time, but if you’re spending it as described above time seems to slow down. And then you get to repeat the experience for the second eye. Oh joy! 

You May Feel as if You Are Failing the Test

There will be times when you seem to miss multiple spots of light in a row or when nothing seems to be happening for a long stretch of time. This can make one feel as if you are failing the test. It’s helpful to know in these situations that the test is designed such that you will likely not see about half of the stimuli. That’s because the testing algorithm is trying to straddle what you can and can’t see for each point in the field. Additionally, the testing unit will occasionally project a spot onto your blind spot just to check to see if you are trying to cheat. By definition, you can’t see something projected onto your blind spot. So don’t feel badly that you are not seeing so many spots. You’re not supposed to see them all.

Your Field of Vision may “White Out” During the Testing

On occasion after a few minutes of automated visual field testing you may notice that your vision seems to “white out”. Some people notice that the entire field of vision takes on the appearance of a blizzard or the screen of an old cathode ray tube television set to a channel without a broadcast. Don’t worry. There’s nothing wrong with your eyes. Sometimes when the brain is not getting any input from the eyes (as with a static white screen) it will put an image there. If this happens just press the pause button on the handheld controller. Sit back, look around, and when your vision returns to normal then resume testing. 

It Can Be Confusing

There will definitely be times when you’re not sure whether you saw something or not. You’ll think “Do I press the button or not?” When that happens you’ve probably just been presented with a spot of light at the very edge of your threshold of vision. At that threshold something very interesting happens. When light stimulates the retinal photoreceptors (cells that detect light) a signal is sent through the optic nerve to the brain. There the signal splits. One pathway is directed to the subconscious area of the brain. This is where we become aware that something is present in our vision. The other signal heads to our visual cortex where “seeing” is processed. At the edge of what we are capable of seeing, the signal to the visual cortex gets filtered out as “noise”. Essentially, our brains decide that we didn’t really see anything beyond what was on the background. Yet the other signal, the one to our center of awareness, is not filtered. So what we end up with is the sense that “something was there”, yet “I didn’t actually see anything.” Now that you know this I hope that instead of being frustrated by this experience, you’ll appreciate the coolness of it when it happens to you. 

So, what should you do when you have this sense that something was presented to you but you’re not sure that you actually saw anything? As a general rule only press the button when you are certain you saw something. 

Reference:

[1] Anderson DR, Patella VM. Automated Static Perimetry. St. Louis: Mosby;1999.

David Richardson, MD

David Richardson, MD

Medical Director, San Marino Eye

David Richardson, M.D. is recognized as one of the top cataract and glaucoma surgeons in the US and is among an elite group of glaucoma surgeons in the country performing the highly specialized canaloplasty procedure. Morever, Dr. Richardson is one of only a few surgeons in the greater Los Angeles area that performs MicroPulse P3™ "Cyclophotocoagulation" (MP3) glaucoma laser surgery. Dr. Richardson graduated Magna Cum Laude from the University of Southern California and earned his Medical Degree from Harvard Medical School. He completed his ophthalmology residency at the LAC+USC Medical Center/ Doheny Eye Institute. 

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