Blind Spots, Brain Maps, and Backaches: A New Chiropractic Delusion

My trip down the rabbit hole started when an ad in the newspaper caught my eye: “Do you have a good brain or a bad brain? One simple test may tell you… call today for a free brain exam.” I started to worry. I had always thought I had a pretty good brain, but what if I was wrong? Would a bad brain mean I was evil, or just that I was developing Alzheimer’s? What kind of brain exam could tell me that? I was intrigued, so I called the number. It turned out to be a chiropractor’s office. Now I was even more intrigued: I thought they did backs, not brains. They explained that they could tell if one side of your brain was malfunctioning by mapping the blind spot in each eye. Then they could adjust the proper side of your neck to correct the problem.

That didn’t make any sense to me, but maybe I slept through that lecture in medical school. I am always eager to learn new things, so I asked the chiropractor where I could read more. He pointed me to a 1997 article in a chiropractic journal.[i] I later found out this one article was the only thing in print on the subject.

The article was very impressive. It looked like real science. It was by Frederick Carrick, a PhD as well as a DC (Doctor of Chiropractic). It was seventeen pages long, with diagrams, charts, statistical analyses, and forty-seven references. It described an elaborate experiment in which 500 volunteers were found to have a significant, reproducible difference in the size of the blind spots between their left and right eyes. It demonstrated that chiropractic manipulation of the proper side of the neck could shrink the enlarged blind spot; if you manipulated the wrong side, nothing happened. It concluded that it had found a new test of cortical (brain) function.

Figure 1. Visual field with abnormal pericentral scotoma (black circle at center) and normal blind spot (black dot at right).

OK, I know what the blind spot is. The retina of the eye is covered with light receptors (rods and cones) except in the area where the optic nerve comes into the eye: that spot, lacking photoreceptors, is known as the blind spot. You don’t notice it, because your brain fools you by filling in the vacant area with more of whatever the other photoreceptors nearly are sensing. The blind spot is a fixed anatomical feature. The only way it could physically enlarge is if the optic nerve itself enlarged. There are two ways it could seem to enlarge: damage to the retinal photoreceptors around the optic nerve, so that no light signals were picked up; or damage to the brain or the nerves connecting the eye and the brain, so that signals received were not processed properly. I couldn’t imagine how adjusting the neck could affect any of those factors.

Ophthalmologists have been mapping visual field for many years. They get a picture for each eye that looks something like Figure 1. In this example, the white areas represent what the patient could see, the black ring around the center is an abnormal finding, and the smaller black spot to the right represents the normal blind spot that everyone has.

In a normal person, the blind spots are approximately equal in size because the optic nerves are equal in size. Oodles of ophthalmologists, numerous neurologists, many medical students, and untold others have looked at plenty of pairs of normal eyes, and have always found blind spots that matched. In Carrick’s study, they found 500 lopsided people, with one blind spot averaging half again as large as the other. This worried me. How could all of those previous observers have missed such an obvious difference?

An ophthalmologist has to look at a lot of visual field tests to find one with an abnormally enlarged blind spot, and when he finds one he can almost always find retinal or other pathology to explain it. How many people did Carrick have to test to find 500 with abnormal blind spots?  I went Web surfing, found his e-mail address, and asked him. All he could tell me was that they tested more than 500. All were students in Carrick’s postgraduate neurology courses for chiropractors; how many students could he have had altogether? Even the most generous estimate would suggest that asymmetrical blind spots are common.

Another source I found on the Internet said they found abnormal blind spots in 99 percent of those tested. How could 99 percent of people be abnormal? That Web site also left me wondering about a lot of other things, such as how they managed to confuse the anatomical blind spot on the retina with the blind spot where you can’t see other cars in your rear-view mirrors. And how could a rap on the spine with the product they sell, a little spring-loaded “activator,” fix anything in the spine, much less in the eye?

Maybe Carrick’s test was better and he could pick up things the ophthalmologists’ tests missed? Not likely. Ophthalmologists use sophisticated equipment; Carrick’s test uses a paper and pencil. You can easily do your own rest at home following these simple instructions:

  1. Tape a piece of typing paper co the wall and stand twenty-eight centimeters (eleven inches) from the wall.
  2. Place a dot in the center of the paper.
  3. Cover one eye and fixate on the dot with the other eye.
  4. Move a small target (the eraser on a pencil will do) from the dot horizontally in each direction.
  5. Make a pencil mark where the target seems to disappear, and another mark where it seems to reappear.
  6. Starting in the area where the target disappeared, move the target up, down, and at 45 degree angles until it appears again, and mark those spots.
  7. Draw a rough circle by connecting the eight dots you have made.
  8. Repeat for the other eye.

You now have a map of your blind spots (only your blind spots, not your brain function). If you find that one of your blind spots is half again as wide as the other, I would advise you to run, not walk, to the nearest eye doctor.

On the basis of this one study, more than 200 practitioners are now using this test clinically. Carrick heads the Carrick Institute for Graduate Studies and teaches the test in his postgraduate chiropractic neurology courses in various cities around the world. I asked him if there was anything else in print to justify using the test. He said other experiments were in progress, and in the meantime, I could learn more by reading the forty-seven references cited in his paper. I didn’t find them very enlightening: I found most of them irrelevant or misinterpreted, and one said clearly that the blind spots were equal in size.

Carrick did not seem to notice that he had found something no other observer had ever found. When I tried to point this out to him, his answer was that his test was reproducible: two independent examiners had found the same results. I tried to explain to him that “reproducible” does not mean “valid.” For instance, if you use a broken thermometer, you might reproducibly get the same results every time, but they wouldn’t be valid – they wouldn’t agree with a functioning thermometer. He didn’t seem to think this concept applied to his research.

I tried the test at home on friends and family and none of them were lopsided. If I had found any significant difference, my next step would have been to cross-check my results with one of the standard visual field tests used by ophthalmologists. Carrick did no cross-checking. He reminds me of my great­grandfather: when he was in his nineties and almost completely blind, the kids handed him a basketball as a joke. He threw it once and, amazingly, made a perfect basket. They said, “Do it again, Grandpa!” He replied, “Nope. I don’t want to break a perfect record.”

I tried to convince Carrick that if he found “X” where every other observer had always found “not X,” logically either he was wrong or all the other observers were wrong. Either both blind spots are the same size, or one is larger than the other; both statements can’t be true at the same time. This is a problem. Until the conflict is resolved, it doesn’t seem reasonable to forge ahead and use the new test in further experiments. But that’s exactly what he did. He used his new blind spot test to show the effect of chiropractic manipulation.

He manipulated the volunteers’ necks on the side of the enlarged blind spot and nothing changed. He manipulated their necks on the side of the smaller blind spot, and follow-up tests showed that the enlarged blind spot was smaller after treatment, essentially back to normal size. Results were reproducible and statistically significant.

If current medical science is right, nothing in the neck should have any effect on the size of the optic nerve where it enters the retina. What was going on here?

He concluded (without any supporting evidence) that he had found a way to map cortical (brain) function. One blind spot could be affected by manipulation on one side, so manipulation must affect the brain on one side, and the activity of that side of the brain must affect the size of the blind spot. Never mind the fact that medical science has never found any anatomical connections that would allow for that, or any rationale that would explain it. Never mind logic. You can express Carrick’s reasoning as an illogical sillygism:

Let A=manipulation

Let B=change in brain function

Let C=decrease in size of enlarged blind spot

A is followed by C, therefore A causes B and B causes C.

Sorry, but this doesn’t compute. It is no more logical than saying “washing my car is always followed by rain (eventually), therefore clean cars make the Weather God angry and anger makes the Weather God pee rain.”

I pointed out the logical fallacy to Carrick. He insisted that manipulation must affect the brain and the brain must affect the blind spot because nothing else would explain his results. I told him the fact that he couldn’t think of another explanation didn’t prove there wasn’t one. I could think of a lot of other possible explanations for his results, and I gave him a long list of them, including lack of proper blinding and conscious or unconscious bias on the part of experimenters or subjects. After all, the subjects were chiropractors and were Carrick’s students; perhaps they knew what to expect and wanted to please the teacher.

An example of how scientists can get erroneous results was Jacques Benveniste’s study published in Nature in 1988.[ii] It seemed to provide incontrovertible evidence that homeopathic dilutions left an imprint in water after all the molecules of the active ingredient had been diluted out. He had done a series of experiments that sounded infallible: his studies were double blinded, carefully controlled, and highly statistically significant. A team including the editor of Nature and James Randi later went to Benveniste’s lab to observe his procedures. Under their watchful eyes, with proper precautions to prevent tampering and bias, the experimental results could not be replicated.

James Randi, through the James Randi Educational Foundation, has a standing offer of a million-dollar prize to anyone who can demonstrate a paranormal phenomenon under carefully controlled conditions. On a BBC program in 2002, he challenged British researchers to repeat another homeopathy experiment on live TV; it had worked fine in their lab, but failed under the camera’s scrutiny.[iii]

I asked Randi if Carrick’s experiment would qualify for the million dollars; he not only said yes, but he personally contacted Carrick to offer him a chance at the prize. I pointed out to Carrick that this was a great opportunity: all he had to do was to demonstrate his test under careful controls that he and Randi would agree to ahead of time. Randi even offered to be in another state while independent observers judged the testing. Carrick stood to gain not only a million dollars, but international renown. His research would be validated and he would be a hero. He would have proved that generations of medical scientists had missed easily demonstrated facts, and he would be the first chiropractor to present really good evidence that manipulation of the spine affects other parts of the body.

Carrick chose to consider the offer an insult and is no longer speaking to either of us. The local chiropractor who placed the newspaper ad was unconvinced by my arguments and is still using the test, because he believes the test helps him treat patients with various conditions including ADHD (attention deficit hyperactivity disorder), balance problems, learning disability, eye problems, and pain.

Chiropractic has been with us since 1895, when D.D. Palmer first adjusted a janitor’s back and supposedly cured his deafness. Palmer said that all disease was due to subluxations (bones out of place), that subluxations interfered with the activity of the nerves that controlled all body functions, and that the body would be kept in perfect health by Innate if the spine was properly aligned. Innate (always capitalized) was his personification of the body’s mystic ability to function properly without conscious guidance. He rejected the germ theory of disease.

As the twentieth century progressed, observers discovered that different chiropractors found subluxations in different places, and that you couldn’t see any of them on x-rays. So chiropractic had to change its definition of subluxation from BOOP (bone out of place) to “a complex of functional and/or structural and/or pathological articular changes that compromise neural integrity and may influence organ system and general health.”[iv] If you find that definition vague and confusing, so do I. I think it means “subluxation is a good excuse for treating anything we want to treat with chiropractic adjustments.”

They have never been able to convince medical science that these subluxations exist, that they impair nerve function, or that such impaired nerve function would lead to disease. Critical thinkers among chiropractors have given up the theory of subluxation and have elected to limit their practice to musculoskeletal conditions and evidence-based treatments. Other chiropractors still believe they can treat asthma, ear infections, colic, and a host of other “somatovisceral” problems by manipulating the spine. They would love to have objective proof that their adjustments have far-flung effects on the nervous system and on all the organs of the body. The blind spot test would have been just what they were looking for. Unfortunately, logic forbids.

The most fascinating aspect of my journey down the rabbit hole was the human psychology of those who live in Wonderland. I encountered direct refusal to accept logic. l could not get a PhD scientist to admit that reproducibility did not equate with validity. I found a researcher who did not realize that any single experiment can reach an incorrect conclusion for a variety of reasons and that unexpected results must be always confirmed in another lab. I encountered a high tolerance for cognitive dissonance. In fact, you might say I encountered a different kind of “blind spot” in the human mind, one that allows clinging to a belief and ignoring the facts. It was an interesting trip, but I was ready to have Scotty beam me back up to where there was intelligent life. I reached for my Skeptical Inquirer and breathed a sigh of relief.

Further Reading

Hall, H. Blind spot mapping, cortical function, and chiropractic manipulation. Scientific Review of Alternative Medicine, in press.

Hall, H. Blind spot mapping. Available online at


[i]Carrick, F. 1997. Changes in brain function after manipulation of the cervical spine. Journal of Manipulative and Physiological Therapeutics20(8):529-545

[ii]Davenas, E. et al. 1988. Human basophil degranulation triggered by very dilute antiserum against I.e. NatureJune 30; 333(6176):816-8.

[iii]Available online at shcrnl; accessed May 29, 2004.

[iv]Available online at subluxacion.shrml; accessed May 29, 2004


This article was originally published in Skeptical Inquirer.

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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