Blind Spot Mapping: A Dubious Aspect of “Chiropractic Neurology”

Several hundred practitioners, mostly chiropractors, are offering a simple paper-and-pencil test that they say can tell how your brain is functioning [1]. They call it “blind spot mapping,” “brain function testing,” “brain mapping,” or “cortical mapping.” They claim that this test can detect an enlarged blind spot in one eye, that the enlarged blind spot reflects a malfunction in the brain, and that they can shrink the blind spot back to normal by manipulating the neck on the same side. Some chiropractors see this as the long-awaited proof that spinal manipulation affects more than just the musculoskeletal system. A typical ad [2] reads:


Brain Trauma and Car Accidents?

Memory, Visual, Balance And Other Problems May Be From An Accident!
There is a quick way to get your initial window into how your brain is functioning. In a recent study, the researchers concluded: “Accurate reproducible maps of cortical (i.e. brain) responses can be used to measure the neurological consequences of spinal joint manipulation. Cervical manipulation activates specific neurological pathways. Manipulation of the cervical spine may be associated with an increase or decrease in brain function depending on the side of the manipulation and the cortical hemisphericity (i.e, side of decreased brain function) of a patient. If you have problems that go undetected you will suffer future neurological breakdown. Call today.
Consultations and Initial Brain Mapping Is Free.

Note the wording: if you have undetected problems, you “will [not “may”] suffer future neurological breakdown.” Other ads imply that selective spinal manipulation based on blind-spot testing is effective in treating ADHD, balance problems, learning disability, pain, eye problems, etc. This chiropractor chooses not to identify himself in his ads, providing only a phone number. At least one major chiropractic organization has an ethics policy prohibiting “blind” ads such as this.

What is the Blind Spot?

Every eye has a blind spot where the optic nerve enters the eye and where photoreceptors are absent. We normally do not notice this because the brain fills in the missing information. The brain can give us the illusion of an uninterrupted visual field, but it cannot change the size of the blind spot because the area where photoreceptors are absent is a fixed anatomical feature. Diseases of the retina can cause photoreceptors to malfunction, which may cause an apparent enlargement of the anatomical blind spot or the appearance of additional blind spots at other locations. In the illustration to the right, the black spot to the right is a normal blind spot; the central black ring is a field defect (scotoma) caused by a disease.

How is Blind Spot Mapping Done?

You can easily do your own test at home. Tape a sheet of paper to the wall and make a small mark at eye level. From about a foot away, fixate on the mark with one eye while covering the other eye. Take a small target (the researchers used a red dot on a plastic extender, but you can use any small target you can see easily) and slowly move it horizontally from the mark to the edge of the paper. At some point, the target will disappear from view, and then it will reappear. You have now located the left and right borders of the blind spot. By returning to the blind spot and moving the target out in other directions, you can map its other dimensions by marking the paper at the points where the target seems to reappear. Your finished map will look something like this:


    Left Eye

Right Eye

You will probably notice that this is not a very precise test, and results may vary depending on the size of the target, the lighting, whether you are wearing glasses, how slowly you move the target, how patient you are, whether your head moves, and how reliably you manage to stay fixated on the central mark. Scientists prefer to test visual fields with standardized instruments to minimize such possible sources of error.

Ophthalmologists regularly map the visual fields, including the blind spot, to follow patients with glaucoma and to diagnose other diseases. They rarely find that the blind spot is enlarged on one side, and when they do they can almost always find retinal disease to explain it. Chiropractors are claiming to find an enlarged blind spot in almost everyone. Something is wrong here.

Where’s the Evidence?

Brain function testing by blind spot mapping has been reported in only one published study [3]. The author, Frederick R. Carrick, D.C., Ph.D., heads the Carrick Institute for Graduate Studies where he teaches clinical neurology to chiropractors. Carrick has also served for several years as president of the ACA Council on Neurology.

Carrick’s report states that 500 volunteers who were taking postgraduate neurology courses were tested and all were found to have a significantly enlarged blind spot in one eye, averaging about 50% larger in diameter than the blind spot on the normal side.

Since nonelderly people chosen at random should not have abormally enlarged blind spots, the logical next step would have been to compare the pencil-and-paper test to standardized tests to see if something was wrong with the measurement method. If Carrick had taken their temperatures, for example, and found that all were above 105 degrees F., he probably would have thought to check the accuracy of his thermometer. But instead of checking the accuracy of his initial tests, he manipulated each volunteer’s neck on the side of the enlarged blind spot, remeasured the spots, and reported that they were no longer enlarged. He concluded that (a) the blind spot must represent brain function or hemisphericity in the cerebral cortex or it wouldn’t have changed after manipulation, and (b) manipulation must affect brain function or the blind spot would not have changed. Neither conclusion is warranted, and the reasoning is circular.

The study has additional flaws, several of which have been pointed out by other chiropractors in letters to the journal’s editor [4-10].

Blind spot mapping is also described on the Web site of Bio-Kinetics, a “total body-brain integration system.” [11] Bio-Kinetics utilizes a patented device called the New-Stim Stimulator, a spring-loaded mallet that delivers a 4-ounce to 5-pound thrust to a 3/8 inch area over the spine in lieu of manual manipulation. Its practitioners use blind spot testing, electrocephagraph [sic] tests, structure balance tests, and surface electromyography to evaluate their patients’ progress. The Web site’s proprietors state that they have done blind spot testing on thousands of people and found less than 1% to be normal.

It is hard to imagine how 99% of people who seek treatment for a variety of problems could have an identical abnormal test result. The normal range for most tests is determined by testing a large normal population, graphing the values obtained (which usually form a bell-shaped curve), and cutting off the extreme ends (beyond 2 standard deviations). They claim the New-Stim is 95% effective in restoring the blind spot to its normal size, but they provide no other assessment of clinical results. Their proposed explanations for how the test may work are fanciful and not consistent with the known facts of neuroanatomy. For instance, it was suggested than nerve impulses from the spine are summated with impulses from the eye in the thalamus and the resultant integrated signal goes from there to the optic area of the cerebral cortex.[3] This hypothesis would require the presence of pathways that are not known to exist [4], and even if they did, a difference in signal strength could not change the size of the blind spot.

The Bio-Kinetics Web site shows a lack of understanding of basic principles.

  • It states, “The larger the blind spot the less peripheral vision we have.” (The blind spot is located about halfway between the center and the periphery of the visual field and has no significant effect on peripheral vision because the same area is visible from the other eye).
  • It claims that if the blind spot “is smaller than a nickel or quarter, it indicates that the brain is being over stimulated from input somewhere in the body.” This is ridiculous because the blind spot can never be smaller than the optic disc, and no stimulation from anywhere in the body can make it so.
  • It confuses the blind spot in the eye with the blind spot outside your car when you are driving. But the driving-related “blind spot” is not a visual defect. It occurs because the rear-view mirror can’t be positioned so that the driver has an unobstructed view of everything behind and alongside of the car.
  • It describes “blind spots” as large as a dinner plate; but that situation would be more than just a blind spot, it would qualify as a blind eye, even if some far peripheral vision remained. Only the most optimistic practitioner would hope to restore such an eye to normal vision by a tap on the neck. Moreover, anyone with such a defect would be blind and know it without blind-spot testing.

Another chiropractor who was one of Carrick’s students has stated:

Realize that if you have a large blind spot in your left eye compared to your right eye you cannot perceive vision as well from that left eye and you will have to tilt your head to compensate for this. The changes in head position have major effects on body mechanics. Patients that have a large blind spot often remark that they have a tendency to walk into walls or worse, end up in your office because they were involved in an auto accident because they didn’t see the other car coming [12].

The above statement is not supported by the scientific literature related to either chiropractic care or to vision [13]. It is also illogical if you remember that the invisible area of each blind spot is visible to the other eye.

James Randi Educational Foundation has a standing offer of $1,000,000 to anyone who can demonstrate the existence of the paranormal under controlled conditions to eliminate the possibility of either deliberate trickery or self-delusion. He directly contacted Carrick and personally offered him the prize if he could demonstrate that his blind spot test worked. Carrick refused [14]..

The Bottom Line

Blind-spot testing is worthless. It does not reflect brain function, and it does not validate claims of chiropractors that manipulation affects the brain.


  1. Carrick FR. Personal communication to Harriet Hall, M.D., Dec 26, 2002.
  2. Burns D. The News Tribune, Tacoma WA, Dec 31, 2002.
  3. Carrick FR. Changes in brain function after manipulation of the cervical spine. Journal of Manipulative and Physiological Therapeutics 20:529-545, 1997.
  4. Seaman D. Letter to the editor. Journal of Manipulative and Physiological Therapeutics 21:295-6, 1998.
  5. Troyanovich S and others. Letter to the editor. Journal of Manipulative and Physiological Therapeutics 21:297-99, 1998.
  6. Noone P. Letter to the editor. Journal of Manipulative and Physiological Therapeutics 21:302-3, 1998.
  7. Henry G. Letter to the editor. Journal of Manipulative and Physiological Therapeutics 21:303-4, 1998.
  8. Turk D. Letter to the editor. Journal of Manipulative and Physiological Therapeutics 21:497, 1998.
  9. Meyer J. Letter to the editor. Journal of Manipulative and Physiological Therapeutics 21:498, 1998.
  10. Ahadpour A. Letter to the editor. Journal of Manipulative and Physiological Therapeutics 21:495, 1998.
  11. Frequently asked questions. Bio-Kinetics Health Systems Web site, accessed Dec 31, 2002.
  12. Burdorf M. Blind spots and chiropractic: Chiropractic adjustments have a direct effect on brain function. Archived Web site, dated Jan 20, 1998.
  13. Seaman D. Philosophy and science versus dogmatism in the practice of chiropractic, accessed Dec 31, 2002.
  14. Carrick FR, Randi J. E-mail correspondence, Jan 15-16, 2003.

This article was originally published in the Chirobase section of Quackwatch.

Dr. Hall is a contributing editor to both Skeptic magazine and the Skeptical Inquirer. She is a weekly contributor to the Science-Based Medicine Blog and is one of its editors. She has also contributed to Quackwatch and to a number of other respected journals and publications. She is the author of Women Aren’t Supposed to Fly: The Memoirs of a Female Flight Surgeon and co-author of the textbook, Consumer Health: A Guide to Intelligent Decisions.

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