Vision Development & Rehabilitation Vision prevalent inthegeneral population, care, sincebinocularvision abnormalitiesare vision” isacriticaltopicinthefieldofvision their interactions.Thearea of“binocular as in‘special’populationsincludingthose 2016;2(4):208-10. and thehoropter: Dev&Rehab Aperspective.Vision Ciuffreda K,LudlamD,Tannen B,Yadav N.Brock string org/10.31707/VDR2016.2.4.p208 Online access is available at VDR isindexedintheDirectory ofOpenAccessJournals. Development. 2016 CollegeofOptometristsinVision this articlemustbeobtainedfrom theeditor. Copyright the authorsmaybeaffiliated. Permissiontouse reprints of & Rehabilitationoranyinstitu­ Development Development, Vision Optometrists in Vision and maynotreflect theopinionsofCollege edu. Allstate­ to Kenneth J. Ciuffreda, OD, PhD, at Correspondence regarding thisarticleshouldbeemailed egocentric localization,, vergence Keywords: motor, andperceptual domainsand implications, including the sensory, theory and its multitude of clinical involved boththeunderlying institutes andhospitals.Thishas vision meetings,and vision research colleges, nationalandinternational binocular visionatseveraloptometry of bothnormalandabnormal have been involved in the teaching

PERSPECTIVEMS, PhD, FAAO Naveen Yadav, K. BS Optom, Tannen,Barry OD, FCOVD Diana P. Ludlam, BS, COVT FARVO FCOVD-A, FAAO, Kenneth J. OD, Ciuffreda, PhD, Perspective A Horopter: StringBrock the and ments are theauthors’personalopinions Over thepastnumberofyears,we Brock string,binocularvision, tion ororganization towhich www.covd.org. kciuffreda@sunyopt. https://doi. 1 aswell

208 although thisaspectisrarely usedclinically), horizontally (and vertically along the midline, the visual field/visual space oftheobserver, of corresponding retinal points(CRPs)into space. Itrepresents thedirectional projection region ofvisualperceptual andphysical conceptualize a‘virtual’,binocularly-based of theobserver”,whichisaniceway to used asasimplehometherapytool. insufficiency intheclinic, anditcanalsobe can bereadily usedto treat convergence horopter’s clinical analog, the Brock string, abnormal binocularvision. and conceptualizationofbothnormal especially withrespect to theunderstanding visual perception, withoptometric clinicalcare, visual informationprocessing, andbinocular basic aspectsofphysiologicaloptics,binocular and clinicaltool,asitblendsintegrates detail withnumerous clinical examples. explanation, whichwegladlyprovide ingreat sometimes followedbytherequest foran vision”. Thisanswerusuallyresults inapause, aspects ofnormalandabnormalbinocular the underpinningfor, andbasisof,nearlyALL “Why not,astheconceptofhoropter is immediate reply tothisquestionhasbeen, and clinical domainsofthehoropter andthe perceptual”, visuo-spatialreference point. as thevisual-directional, or“zero sensory-motor- with respect toone’s ‘egocenter’which serves being theall-important“Brock String.” its clinicalimportance,withanalog emphasized theconceptofhoropter and the question.Inourlectures, wehavealways to be misunderstoodby many, and hence question. Unfortunately, thehoropter seems is, “Whythehoropter?” Certainly, thisisafair look rangingfrom inquisitivenesstodistain, more thanoneoccasion,frequently witha palsy with neurological disorders suchascerebral The termhoropter translates as“boundary The horopter isawonderful pedagogical Some keyexamples ofthisinthebasic A questionthathasbeenposedtouson 2 andtraumaticbraininjury. Volume 2,Issue4• December 2016 4,5 Forexample,the 3 4 Our 4

angular extent of PFAs in visual space and at the , 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 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 . 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 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 , 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 . ‘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 stereo­campimeter, 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 . 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 Uni­ vers­ity of California at Berkeley 9. They provide coarse information School of Optometry regard­ing 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