routine blind spot testing the blind spot leading the blind joseph s. ferezy, d.c

Routine Blind Spot Testing

The Blind Spot Leading The Blind

Joseph S. Ferezy, D.C


I writing this letter to the editor in response to article an which appeared in the JMPT Volume 20, No. 8, Oct., 1997 entitled, "Changes in Brain Function after Manipulation of the Cervical Spine" by Frederick R. Carrick, D.C.


First, allow me to congratulate Dr. Carrick on his effort to relate his ideas in a public, scientific forum, such as the JMPT. Only through scrutiny, discussion and ultimately criticism by one's peers, can new ideas be "fleshed out" and their value to the chiropractic profession accurately be assessed.


Unfortunately, after spending a great deal of time when this paper, gathering Dr. Carrick's references, and speaking to experts in the field of visual physiology, I find far too many inaccuracies, unsupported conclusions, problems with scientific methodology, confusing and inappropriate terminology, and unanswered questions to make this paper of any practical use to either the chiropractic profession, or to the neuroscience's in general.


In essence, it appears that Dr. Carrick is performing a test known as manual perimetry blind spot mapping, though he alternately refers to it as cortical perceptual mapping. I find no other references in the scientific literature, which refer to this test as cortical perceptual mapping. Typically, this test is relegated to ophthalmologists and optometrists who are trained in performing this procedure.


Based upon his discussion of the procedure in his paper, and the fact that eye health professionals were not utilized, and that asymmetry was apparently found in all 500 subjects tested, it is likely that the test was performed inaccurately. In any event, manual perimetry does not appear to be the most accurate variety of this test, when compared to automated perimetry.


He then assumes that this measurement somehow relates to something that the test was never designed to measure, that is neurological activity effecting the occipital cortex. To compound this confusion, he then assumes that pre-and post manipulation changes in this measurement are attributable to activation and/or inhibition of specific neurological pathways.


This is analogous to using a home pregnancy test to diagnose AIDS. By paralleling some of the actual parameters used in Dr. Carrick paper, I believe the following analogy further clarifies my point. Suppose you were looking at kidney function by performing manual red blood cell counts (using remedially trained laboratory technicians) in 500 subjects.


Then, assuming that the low red cell count (found in all subjects) is a measure of abnormal kidney function, you repeat the red blood cell count after performing cervical manipulation in 439 subjects, again noting changes in all 439 subjects re-tested. Would you then conclude that kidney function can be improved due to activation and/or inhibition of specific neurological pathways related to the cervical manipulation?


Keep in mind that no "control group" was kept, therefore no second measurement was made on any individuals not subjected to cervical manipulation. Also, no "placebo group" was kept, therefore no second measurement was made on any individuals who received a sham treatment such as a squeeze of the big toe, a shake of the hand, a slap in the face, etc.


To further support my above contentions, I offer the following discussion. However, due to the extensive number of problematic areas in this paper, I will limit my discussion only to the most serious and obvious flaws.


Inaccuracies and unsupported conclusions: Dr. Carrick's conclusion that "... manual perimetry mapping is an inexpensive, accurate (94.6 percent) and reproducible method of recording the physiological blind spot..." or that it is "simple" to perform is simply not supported by current literature on this subject. Textbooks such as, "Synopsis of Ophthalmology" (Havener WH.)


Also "Contribution of manual and computerized perimetry to the differential diagnosis of optic neuropathies" (Zingirian M., Dorigo MT, et al,) and "Automated perimetry detects visual field loss before manual Goldmann perimetry" (Katz J., Tielsch JM., et al) discuss the limitations, difficulties and pitfalls of manual perimetry. In many cases automated perimetry can detect visual field losses over one year prior to manual perimetry detection.


Probably the most glaring and important example of inaccurate and unsupported conclusions comes with Dr. Carrick's contention that the size of the blind spot is, in fact, a function of cortical activity.

The Blind Spot Leading The Blind Statements such as, "Blind spots are the consequence of global brain activity and not representative of the anatomical size of the optic disk." and "Manual perimetry blind spot mapping is a simple and cost effective way to measure integrated cortical activity. It is a highly reproducible method of measuring cortical activity..." and "... it (manual perimetry) was therefore chosen as a sensitive integer of brain activity that could be used to measure changes in cortical activity..." and "...


manual perimetry mapping is an inexpensive, accurate (94.6 percent) and reproducible method of recording the physiological blind spot and, thus, cortical activity" can be found throughout Dr. Carrick's paper.


In fact, quite the opposite is true. Scientific studies have repeatedly demonstrated the association between optic disk morphology and blind spot size. Every physician should know that swelling of the optic disk (papilledema), optic disk atrophy, and increased intra-ocular pressure associated with glaucoma, all cause retinal and optic disk changes that may be visualized upon ophthalmoscopy. Often, these changes are associated with an enlargement of the physiological blind spot.


In Dr. Carrick's own paper he identifies no less than seven separate disorders which appear to be local to the retina and effect blind spot size. This is including, but not limited to diseases of the choroid layer and local vascular diseases. Recent papers such as, "Blind spot size depends on the optic disk topography: a study using SL0 controlled scotometry and the Heidelberg retina tomograph" (Meyer JH, Guhlmann M, et al)


and "Correlation of the blind spot size to the area of the optic disk and parapapillary atrophy" further support long held clinical observations regarding the close association between the optic disk and blind spot size.


Finally, there appears to be another serious problem with Dr. Carrick's hypothesis. In order to support his ideas, Dr. Carrick references a paper by Tripathy and Levi entitled "Long-range dichoptic interactions in the human visual cortex in the region corresponding to the blind spot."


This paper suggests that "... the cortical representation of the region of the visual field that corresponds to the contralateral eye's blind spot is not strictly monocular. (This)... suggests the involvement of horizontal cortical connections...".


Dr. Carrick uses this paper to bolster his argument that "the frequency of firing of these cortical connections affects visual perception and enlargement of the blind spot might be attributable to any mechanism and decreases the firing rate of horizontal cortical connections...". However the paper is actually suggesting that it is the contralateral eye which contributes to blind spot perception.


In Dr. Carrick's experiment "the other (contralateral) eye was patched to prevent binocular vision." Therefore, the horizontal cortical connections cited could not be responsible for the alleged results obtained by Dr. Carrick.


Problems with scientific methodology: See discussion by Dr. John Meyer.


Confusing and inappropriate terminology: Throughout the paper, Dr. Carrick continually uses terminology which is either not consistent with accepted terminology in the health sciences, or is unnecessarily obtuse. One example is stated above wherein Dr. Carrick often refers to manual perimetry blind spot mapping as "cortical perceptual mapping".


A Medline search for the term "cortical perceptual mapping" yields no responses. Therefore, I believe that this terminology in reference to this test is peculiar to Dr. Carrick. This is particularly confusing, because a blind spot recorded in the right eye, is actually perceived (or not perceived) in the left cortex.


Therefore, an enlarged blind spot in the right eye would be measured in the right visual field via perimetry mapping, but cortical perception would be in the left hemisphere defined by Dr. Carrick as "decreased left cortical hemisphericity".


Additionally, Dr. Carrick uses the term "hemisphericity" throughout his paper. In the neurosciences the term hemisphericity usually refers to the dominant cerebral hemisphere of an individual. Sentences such as "... on the side opposite decreased brain hemisphericity..." are unacceptably confusing, and seem almost deliberately obtuse.


Unanswered questions: What were the qualifications of the examiners in this study? What was the tolerance for a match of the "cortical maps" and how was this determined? Why was no control group utilized? Why was no placebo group utilized? Why was direct fundoscopy not performed, to help rule out local optic disk pathology that might affect blind spot size? Why is the test not reproduced using computerized perimetry?


In closing, while I always try to support genuine scientific endeavor, and do not wish to appear hypercritical, there are simply too many problems with this paper to give it any credence whatsoever. My comments above really only scratch the surface in regard to the multiplicity of problems with this work. In fact, the real question lies with the JMPT decision to publish this paper.


Certainly, JMPT peer reviewers should have noticed all of these problems, and deemed this paper unfit for publication in its current form. Allowing this paper to appear in the JMPT severely diminishes the long earned prestige of this journal, and therefore damages the scientific credibility of the chiropractic profession.


Scientific journals, as well as post doctoral neurology programs in the chiropractic profession must remain ever vigilant in screening out poor, unsubstantiated, or incomplete attempts at the presentation of clinical and scientific information, or the profession at-large will continue to suffer the branding of "unscientific cult."

Subj: Carrick paperDate: 98-02-26 03:06:25 ESTFrom: JOHN MEYERTo: Joseph S Ferezy (E-mail))


Sorry it's taken awhile to get back to you regarding this paper. I did read you letter, and while it is very good and very thorough, it is a bit too long for a letter to the editor (which makes writing a good letter a challenge).


The first difficulty encounter in evaluating the Carrick paper is his absence of a stated overall hypothesis or theory. He states several research hypotheses, which he then tests, but no overall theory. It is fairly universal to provide the hypothesis or theory tested at the end of the introduction. Carrick's last statement, while sounding as grand as a political rhetoric, it is equally void of meaning or real substance.


I have two major concerns regarding the reliability and validity of the 'cortical perceptual maps' used by Dr. Carrick to arrive at his conclusions, and perhaps this should be the heart of the letter.


First, regarding the reliability of his measurement, , Dr. Carrick repeatedly declares his measurement reproducible. This is a misnomer. Experiments are reproducible, measurements are reliable. Dr. Carrick provides no evidence of acceptable reliability for 'cortical perceptual maps', either by the work of other investigators, or more importantly, in his hands and under his experimental conditions.


Holding up two ink blots, and having someone declare them reproducible does not constitute adequate scientific evidence of measurement test-retest reliability.


Beyond this, the second major problem encountered is with the validity of this measurement. This problem is not with the test itself, for its actual name is manual perimetry testing, and it was intended to map the perimeter of the retinal blind spot. This test measures the perimeter of the retinal receptors immediately surrounding the optic disk. This is exactly what it measures and it is incapable of measuring anything other than the blind spot perimeter.


Dr. Carrick's attempt to rename the measurement 'cortical perceptual mapping' does nothing to alter its measurement properties or capabilities. Dr. Carrick never recording from the cortex, so there is absolutely no way he could have obtained a 'cortical map'. How he can infer obtaining cortical maps without cortical recordings is beyond all logic.

The Blind Spot Leading The BlindOne cannot record an EKG, rename it 'cortical cardiac EEG', and infer alterations in EEG activity within the cardiac center of the brain, based upon alterations in EKG. EKG will always measure EKG activity, regardless of the misnomer applied. The same applies to manual perimetry test. There is absolutely no validity to Dr. Carrick's assertion that he recorded 'cortical perceptual maps' of the occipital cortex, or for that matter, any other region within the brain.


Having established that there is no evidence of reliability or validity of the dependent variable as utilized by Dr. Carrick, no further evaluation of this manuscript is necessary. Without measurement, no statistical analysis can be performed and no conclusions can be drawn.


If Dr. Carrick has any literature that supports his unique assertions of measurement reliability, and especially its validity, we would welcome the opportunity to evaluate these further. There is no shame in admitting ones mistakes, only in compounding them. We hope that this letter will serve to encourage Dr. Carrick to more critically evaluate his assertions prior to espousing them in the future.


Well, it seems that as I've written this, it begins to sound more and more like the actual letter as I approach the end. Feel free to use any or all of it in the letter. If you need more help or would like me to edit the next draft, let me know. I hope this is helpful.

The Amazing Randi

Personally Offered Dr. Carrick $1,000,000.00 If He Could Prove His Blind Spot Mapping Was Valid In A Controlled Environment.

Carrick Declined.

routine blind spot testing the blind spot leading the blind joseph s. ferezy, d.c

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