Brock String and the with neurological disorders such as cerebral 2 3 Horopter: A Perspective palsy and traumatic brain injury. A question that has been posed to us on Kenneth J. Ciuffreda, OD, PhD, more than one occasion, frequently with a FAAO, FCOVD-A, FARVO look ranging from inquisitiveness to distain, Diana P. Ludlam, BS, COVT is, “Why the horopter?” Certainly, this is a fair question. Unfortunately, the horopter seems Barry Tannen, OD, FCOVD to be misunderstood by many, and hence Naveen K. Yadav, BS Optom, the question. In our lectures, we have always MS, PhD, FAAO emphasized the concept of the horopter and its clinical importance, with its clinical analog being the all-important “Brock String.”4 Our Over the past number of years, we immediate reply to this question has been, have been involved in the teaching “Why not, as the concept of the horopter is of both normal and abnormal the underpinning for, and basis of, nearly ALL binocular vision at several optometry aspects of normal and abnormal binocular colleges, national and international vision”. This answer usually results in a pause, vision meetings, and vision research sometimes followed by the request for an institutes and hospitals. This has explanation, which we gladly provide in great involved both the underlying detail with numerous clinical examples. theory and its multitude of clinical The horopter is a wonderful pedagogical implications, including the sensory, and clinical tool, as it blends and integrates PERSPECTIVE motor, and perceptual domains and basic aspects of physiological optics, binocular their interactions. The area of “binocular visual information processing, and binocular vision” is a critical topic in the field of vision visual perception, with optometric clinical care, care, since binocular vision abnormalities are especially with respect to the understanding prevalent in the general population,1 as well and conceptualization of both normal and as in ‘special’ populations including those abnormal binocular vision.4,5 For example, the horopter’s clinical analog, the Brock string, Correspondence regarding this article should be emailed can be readily used to treat convergence to Kenneth J. Ciuffreda, OD, PhD, at kciuffreda@sunyopt. insufficiency in the clinic, and it can also be edu. All state ments are the authors’ personal opinions and may not reflect the opinions of the College of used as a simple home therapy tool. Optometrists in Vision Development, Vision Development The term horopter translates as “boundary & Rehabilitation or any institu tion or organization to which of the observer”, which is a nice way to the authors may be affiliated. Permission to use reprints of this article must be obtained from the editor. Copyright conceptualize a ‘virtual’, binocularly-based 2016 College of Optometrists in Vision Development. region of visual perceptual and physical VDR is indexed in the Directory of Open Access Journals. space. It represents the directional projection Online access is available at www.covd.org. https://doi. org/10.31707/VDR2016.2.4.p208 of corresponding retinal points (CRPs) into the visual field/visual space of the observer, Ciuffreda K, Ludlam D, Tannen B, Yadav N. Brock string horizontally (and vertically along the midline, and the horopter: A perspective. Vision Dev & Rehab although this aspect is rarely used clinically),4 2016;2(4):208-10. with respect to one’s ‘egocenter’ which serves as the visual-directional, or “zero sensory-motor- perceptual”, visuo-spatial reference point. Keywords: Brock string, binocular vision, Some key examples of this in the basic egocentric localization, horopter, vergence and clinical domains of the horopter and the 208 Vision Development & Rehabilitation Volume 2, Issue 4 • December 2016 angular extent of PFAs in visual space and at the retina, respectively. Thus, targets should be tested both along the midline, as is the usual case, and perhaps as well at different retinal eccentricities, for example with a second Brock string, while the patient bifixates on the midline bead of the first one (Figure 2). Figure 1: Brock string and the horopter. Symbols: BS = Brock string, HOR = horopter, l = bifixation target/midline BS bead, PFA = Panum’s fusional areas, 50% P = 50% proximal limit for diplopia 50% of the time, 50% D = 50% distal limit for diplopia 50% of the time, R = right eye’s image of either the far or near bead, L = left eye’s image of either the far or near bead, CR = center of rotation of the eye, LE = Left Eye, RE = Right Eye, f = fovea, and shaded area = PFA region for 50% or greater haplopia. related Brock string, respectively, and their Figure 2: Brock string and the horopter. Same as for Figure interdependence, are provided below (See 1 except : BS1 = midline Brock string, and BS2 = eccentric Figure 1): Brock string. 1. They provide a psychophysically and 3. They provide a reference point for perceptually-based spatial map of stereopsis. Stereoacuity is best at the correspondence, i.e., CRPs. horopter, as the number of overlapping binocular receptive fields from the two 2. They provide a reflection and sense of eyes is maximal here.6 Further away from Panum’s fusional areas (PFAs) projected the horopter, as well as with greater into visual space, as those regions both retinal eccentricity, stereoacuity is not immediately in front of and behind the as good, and this is also true for BS/BS1 horopter for which single vision (i.e., and BS2 at their respective horopter haplopia) is typically present represent regions. PFAs. Within this region, haplopia would be present 50% of the time at 4. They provide an indicator of vergence its proximal and distal limits, and 100% accuracy. Vergence is most accurate of the time at the horopter itself, as it when the two perceived Brock strings is a probability distribution function. intersect precisely with the intended That is, the farther a target is from the fixation bead. This is also true for the horopter or bifixation bead, the greater horopter, but its resolution is greater the probability that diplopia would be yielding one’s resultant fixation disparity, perceived. And, the greater the retinal or steady-state vergence error, of a few eccentricity, the great the linear and minutes of arc. 209 Vision Development & Rehabilitation Volume 2, Issue 4 • December 2016 5. They can be used to assess for the proprioceptive eye-muscle-based signal presence of normal retinal correspond - provide information related to vergence ence (NRC) versus abnormal retinal magnitude (and perhaps also vergence correspondence (ARC) in strabismus. ‘effort’). Such information can be used to With the Brock string in ARC, one obtains assess for, and then train, proper visual the red-green, split-field response, binocular localization in depth. while with the horopter, one obtains the “horopter notch” reflecting marked For example, as the patient attempts to spatial/directional discontinuity. bifixate and focus upon the middle bead of the Brock string, the clinician can ascertain 6. They can be used for visual-feedback- information related to the sensory state, such based remediation training when as the perceived and simultaneous presence of diplopia is present to guide the appro- diplopia of the far and near beads, split-field ARC priate vergence response to result in response, intermittent suppression, perceived the desired haplopia. Similarly, they absolute and relative distance of the beads and can be used to demonstrate normal other targets in space, as well as gross vergence physiological diplopia. accuracy, to name a few. The possibilities for creativity in this area are limitless. 7. They can be used to identify regions So remember, both the Brock string and of binocular suppression, especially in the horopter can be the clinician’s best friend! strabismus. This clinical information can be used to “break down” the regions REFERENCES of binocular suppression via string 1. Hokoda SC. General binocular dysfunctions in an urban movement, string flashing, adding optometry clinic. J Am Optom Assoc 1985; 56: 560-562. 2. Duckman R. The incidence of visual anomalies in a beads, increasing bead size, etc., with population of cerebral palsied children. J Am Optom the Brock string. More accurate and Assoc 1979; 50:1013-1018. quantitative assessment of suppression 3. Ciuffreda KC, Kapoor N, Rutner D, Suchoff IB, Han ME, can be ascertained by use of a Craig S. Occurrence of oculomotor dysfunctions in acquired brain injury: a retrospective analysis. Optometry stereo campimeter, synoptophore, or 2007; 78: 155-161. amblyoscope, or other such instruments 4. Steinman SB, Steinman BA, Garzia RP. Foundations of and devices. It is also possible to do so binocular vision. New York: McGraw-Hill, 2000: 81-119. 5. Ogle KN. Researches in binocular vision. Philadelphia: WB with the horopter. Saunders Company, 1950:10-17. 6. Bishop PO. Binocular vision. In Moses RA, Hart WM (eds) 8. They can be used to demonstrate Adler’s Physiology of the Eye, CV Mosby, St. Louis, MO. the difference between, and assess 1987: 619-689. for normality of, oculocentric (i.e., monocular eye/fovea-based, except in AUTHOR BIOGRAPHY: eccentric fixation) versus egocentric (i.e., Kenneth J. Ciuffreda, OD, PhD New York, New York midline, binocular body-based) visual • OD 1973, Massachusetts localization per the famous Hering College of Optometry 4,5 window demonstration. • PhD 1977, Physiological Optics Uni­ vers ity of California at Berkeley 9. They provide coarse information School of Optometry regard ing relative/absolute perceived • Distinguished Teaching Professor SUNY, State College of distance in visual space. With Optometry changes in vergence, both the cortical • Research Diplomate in Binocular Vision American Academy oculomotor innervational signal and the of Optometry 210 Vision Development & Rehabilitation Volume 2, Issue 4 • December 2016.