Brock String Exercises: Uses, Patterns and Studies

Updated 2026-10-23

Written by the Optics Trainer team from published research, with every source linked below. Not yet reviewed by a clinician.

A Brock string is a long cord with a few colored beads on it. Eye care professionals use it to see how well the two eyes aim together, and as part of exercises for convergence and suppression. This guide explains what the patterns on the string mean, then describes the published studies that used it: one randomized trial that tested the Brock string by itself after concussion, and many studies where it was one part of a larger therapy program. For each study we give the design, who took part, what was done, how long people were followed and what was found. Your clinician sets up any Brock string exercises for you.

What is a Brock string?

A Brock string is a cord with several colored beads that can slide along it. One end is held at the bridge of the nose, and the other end is fixed further away. You look at one bead at a time.

It is named after Fred Brock, an optometrist who practiced in the mid-1900s and was known for his work on strabismus. A 2025 article for optometrists in Review of Optometry, citing a strabismus textbook, lists four uses for the string in therapy: building voluntary convergence (turning the eyes in on purpose), checking that both eyes aim at the same point, working on suppression, and practicing switching eye aim between near and far targets.

The string works because of something called physiological diplopia. When you look at a bead, anything closer or farther away is seen double. So the string in front of and behind the bead looks like two strings. That doubling is normal and expected. It is what lets the string show where each eye is pointing.

What the Brock string test shows

When an eye care professional uses the string as a quick test, they ask what you see while you look at a bead. Here is what the main patterns mean, as described in Review of Optometry:

An X at the bead: Two strings come toward the bead and cross right at it. This means both eyes are aimed at the bead.
Only one string: One eye's view is being tuned out, or suppressed. Because the strings cross at the bead, the missing string is on the side opposite the suppressed eye, so let your eye care professional work out which eye it is.
Part of a string missing: For example, two strings coming in but only one going out. This can mean suppression at some distances.
Strings crossing in front of or behind the bead: The eyes tend to aim closer than the bead (an eso posture) or beyond it (an exo posture).

Common Brock string patterns and what an eye care professional may read from them (Review of Optometry, 2025)
What you see	What it can suggest
Strings cross at the bead (X)	Both eyes aimed at the bead
One string only	Suppression of one eye (the missing string is on the opposite side from the suppressed eye)
Strings cross in front of the bead	Eyes tend to aim too close (eso posture)
Strings cross behind the bead	Eyes tend to aim too far (exo posture)

How Brock string exercises appear in research

The same article says the string is one diagnostic test among several. If the pattern is not a clean X, the authors advise more testing, such as how close the eyes can turn in together and how well they focus, or a referral.

In published studies the Brock string shows up in four ways. In one randomized trial it was the whole treatment. In many trials and case series it was one named procedure inside a larger office or home program, so the results describe the whole program. In a few concussion studies it was used as a measurement of convergence. In two convergence trials, a one-eye version was part of the placebo therapy. The tables below say which role it had in each study. We searched Europe PMC in October 2026 for studies that name the Brock string.

Brock string for concussion: what the studies did and found

The only randomized trial of the Brock string by itself that our search found was done in concussion. In that trial, from the UPMC Sports Medicine Concussion Program, the authors concluded that the exercises may be effective for a receded near point of convergence after concussion. Two letters followed in the same journal: one titled "A Brock String Procedure Is Not Comprehensive Optometric Vision Therapy", and a reply from the trial's authors. Eye care professionals can read our clinician article on eye movements after concussion.

In the larger CONCUSS trial, the published protocol lists the Brock string in the second phase of office therapy and among the home exercises. The trial's results paper does not name the Brock string; it reports that use of the home computer program was 16% of prescribed sessions in the immediate group and 11% in the delayed group.

One case series of seven people with concussion and Ehlers-Danlos syndrome reported that one patient found the Brock string exercises uncomfortable and that they caused nausea, and that another could not keep them up for more than a minute at a time.

Concussion and brain injury studies that name the Brock string
Study	Design	Who	What was done	Follow-up	Main result as reported
Trbovich 2025, J Neurotrauma	Single-blind randomized controlled trial	50 people aged 11 to 30, within 10 days of a concussion, near point of convergence 5 cm or more	Brock string alone: 5 repetitions twice a day with a string about 3 m long and three beads, compared with usual care (breaks from reading and other visual tasks, plus sleep, fluids and diet advice)	7 to 14 days	Near point of convergence improved 8.9 cm with the Brock string and 3.8 cm with usual care; neck symptom scores also improved more with the Brock string
CONCUSS, Alvarez 2026, Br J Sports Med	Randomized clinical trial, immediate versus delayed therapy	104 people aged 11 to 25 with concussion-related convergence insufficiency, 4 to 24 weeks after injury	Office vergence and focusing therapy with movement, twice a week; the protocol lists the Brock string in phase 2 and at home	6 weeks, then after all 16 sessions	At 6 weeks, 88% of the immediate group and 8% of the delayed group were successful or improved; after 16 sessions, 87% and 92%
Storey 2017, Optom Vis Sci	Retrospective cohort	275 children aged 5 to 18 at a hospital concussion program; 67 had an abnormal near point of convergence	Standard care; some were referred to vestibular therapy that included the Brock string and pencil push-ups; some to office vision therapy	Until recovery (medians 4.5 to 23 weeks after injury)	26 recovered with standard care (median 4.5 weeks), 23 after vestibular therapy (median 11 weeks) and 7 after office vision therapy (median 23 weeks after injury)
Wong 2021, Int J Sports Phys Ther	Retrospective cohort	38 patients, average age 26.9	Neck treatment first, then combined vision and vestibular therapy; the Brock string was used to measure convergence	10.4 sessions over 57.6 days on average	Brock string convergence scores and symptom scores improved
Ziaks 2026, Ir J Med Sci	Retrospective case series	5 women over 60 with symptoms after falls	Multidomain program including eye movement training; the Brock string was used as a measure	Not stated in the abstract	All had fewer symptoms; Brock string measure improved 19.05 cm
Ross 2023, Neurotrauma Rep	Retrospective chart review	62 people aged 13 to 71 with symptoms an average of 2.2 years after injury	5-day program with many parts; for 22 of the 62, eye exercises included jumping gaze along a Brock string	End of the 5 days	Symptom checklist scores and several timed and visual tests improved
Moller 2020, Brain Sci	Case reports	3 adults after mild traumatic brain injury	Office and home vision therapy that included the Brock string	End of program	Symptom scores improved in two of three; one went from 36 to 19

Brock string for convergence insufficiency

In the CITT and CINAPS trials, placebo therapy used one-eye versions of real procedures, such as the Brock string done with one eye covered, so that people could not tell which group they were in. In CITT, 93% of the real-therapy group and 85% of the placebo group thought they had real therapy.

In CINAPS, near point of convergence and fusional vergence improved more with real therapy than placebo, while symptom scores improved by similar amounts in both groups. CITT-ART reported the same pattern after 16 weeks; in CITT-ART the Brock string is named only in the maintenance therapy that came afterwards.

Convergence insufficiency studies that name the Brock string
Study	Design	Who	What was done	Follow-up	Main result as reported
CITT long-term, 2009, Optom Vis Sci	Follow-up of a randomized trial	79 children aged 9 to 17 with no symptoms after 12 weeks of treatment	Maintenance 15 minutes once a week for 6 months; the office therapy group did one convergence technique (Brock string or barrel card) and one fusional vergence technique	1 year	84.4% of the office therapy group still had no symptoms; 87.5% were still successful or improved
CITT-ART, 2019, Optom Vis Sci	Randomized clinical trial with placebo	311 children aged 9 to 14	16 weeks of office therapy or placebo therapy; the Brock string is named only in the maintenance therapy prescribed after the 16-week results (Morrison 2024)	16 weeks	Near point of convergence improved 10.4 cm vs 6.2 cm; symptom scores improved 11.8 vs 10.4 points, a difference that was not statistically significant
CITT-ART one-year, Morrison 2024, Ophthalmic Physiol Opt	Follow-up of a randomized trial	121 children who had office therapy and no further treatment	For children without symptoms at 16 weeks, home maintenance 15 minutes once a week for 3 months, using one convergence technique such as the Brock string and one fusional technique	1 year	Near point of convergence did not change significantly; fusional vergence fell by 4.7 prism diopters, and similar shares were classed as normal; the authors said the improvements persisted
CINAPS, Alvarez 2020, Optom Vis Sci	Double-masked randomized trial with placebo	50 adults aged 18 to 32	12 one-hour office sessions; the protocol lists the Brock string in phase 1	12 sessions	Near point of convergence improved 6.0 cm vs 3.1 cm; symptom scores improved 12.4 vs 10.1 points, not a significant difference
Li 2022, BMC Ophthalmol	Pilot randomized trial	Adults aged 18 to 35; 33 with convergence insufficiency completed	Virtual reality therapy or office therapy (which included the Brock string), 1 hour a week for 12 weeks	12 weeks	Both groups improved; in the convergence insufficiency patients, the groups did not differ significantly on any measure
Maagaard 2021, Invest Ophthalmol Vis Sci	Randomized crossover trial	44 children aged 9 to 13	Home vergence exercises (including the Brock string) then focusing exercises, or the reverse, 20 minutes a day, 5 days a week, 6 weeks each	12 weeks	After the first 6 weeks, full recovery was more frequent in the group that started with vergence exercises
Alvarez 2010, Optom Vis Sci	Small clinical study	4 adults with convergence insufficiency and 13 controls	18 hours of therapy; home training used the Brock string and a prism	4 months to 1 year in 3 people	Convergence measures improved, and brain scans showed more activity in some areas
Kergoat 2017, Gerontol Geriatr Med	Case series	2 men aged 68 and 80 with Parkinson's disease	3 office visits and 8 weeks of home exercises: pencil push-ups, Brock string and vectograms	8 weeks	Both gained convergence and had fewer symptoms

Brock string for lazy eye and strabismus

In these studies the Brock string was one procedure in a larger program, often alongside patching, glasses or prisms. A 2022 review for pediatricians said anti-suppression work can sometimes lead to double vision that does not go away.

Amblyopia and strabismus studies that name the Brock string
Study	Design	Who	What was done	Follow-up	Main result as reported
ATS12, Lyon 2013, Optom Vis Sci	Pilot randomized trial	19 children aged 7 to under 13 with amblyopia	2 hours of daily patching plus active vision therapy (anti-suppression steps included the Brock string) or placebo therapy, weekly visits for 16 weeks	16 weeks	Not enough children could be enrolled for a full trial
Suwal 2024, J Optom	Two-group comparison; randomization not reported	65 children aged 5 to 16 with amblyopia	Active vision therapy (Brock string used to notice the normal doubling) or patching	3 months	Acuity gains were similar; stereo vision gains were larger with vision therapy
Huang 2022, Children	Retrospective comparison	31 children aged 7 to 10 with amblyopia in both eyes	Vision therapy program that included the Brock string, or no added therapy	To end of treatment	Acuity improved 0.32 logMAR with therapy and 0.003 without
Hsieh 2022, BMC Ophthalmol	Retrospective comparison	36 children aged 7 to 10 with amblyopia in one eye	Vision therapy (Brock string among the tools) plus glasses and patching, or glasses and patching	9 months, plus 3 months after stopping	Acuity gain was larger with vision therapy; no loss 3 months after stopping
Sanchez 2025, J Optom	Retrospective series	53 patients, average age 7.75, with amblyopia that persisted with patching	Clinic therapy (Brock string with prism in the vergence phase, which 43 of 53 completed), home perceptual learning games and patching	6 months after therapy	87% reached 0.1 logMAR or better; stereo vision was measurable in 81%
Ma 2021, J Optom	Retrospective chart review	40 people aged 5 to 22 with intermittent exotropia	Office therapy every 1 to 2 weeks (Brock string in phase one) plus home practice	Last therapy session	Eye control scores improved by 1.1 points at distance and near
Pan 2026, Ophthalmol Ther	Retrospective cohort	636 people aged 4 to 34 with intermittent exotropia	Office therapy; the first module included the Brock string	Within 1 year	46.5% met recovery criteria; more sessions were linked to higher odds; the authors said randomized trials are needed
Xu 2023, J Clin Med	Randomized controlled trial	136 children aged 7 to 17 after exotropia surgery	Orthoptic therapy (the protocol lists the Brock string at home) or no therapy	12 months	Suboptimal surgical outcome in 20.5% with therapy and 42.6% without
Wahid 2024, Hong Kong Physiother J	Randomized controlled trial	40 patients after exotropia surgery	Usual care, with or without eye exercises that included the Brock string, 24 sessions over 3 months	3 months	Eye control scores were better in the exercise group at near and far
Satgunam 2026, PLoS One	Retrospective cohort	14 patients with acute acquired esotropia	Vision therapy, with prisms when needed; Brock string at home, and for some patients the Brock string was the only exercise	Up to 1 year	Eye turn was reduced without surgery in 11 of 14
Portela-Camino 2024, PeerJ	Before-and-after study	48 adults with double vision, average age 62.45	Prism glasses; if prisms alone did not work, vision therapy including the Brock string	End of therapy	13 of 20 who had therapy reached binocular vision, and quality-of-life scores improved

Brock string for sports and athletes

The Review of Optometry article notes that a clinician can set the bead at a distance that fits a person's sport or work. In the 2026 Hemsball pilot, the authors said the results should be treated with caution and that randomized trials are needed.

Sports studies that name the Brock string
Study	Design	Who	What was done	Follow-up	Main result as reported
Tekin 2026, Sportverletz Sportschaden	Single-group pilot, no control group	27 young Hemsball athletes, average age 10.22	One session of Brock string training combined with balance tasks	Right after the session	Balance, eye-hand coordination and reaction scores improved
Tuncel 2026, Front Behav Neurosci	Single-group study, no control group	42 male basketball players aged 14 to 17	One session at three stations; one combined Brock string focus with passing	Right after and 30 minutes later	Jump, sprint, passing and shooting scores improved
Zwierko 2015, J Hum Kinet	Randomized, 12 trained and 12 not trained	24 female university athletes	Eye training including the Brock string, 20 minutes 3 times a week for 8 weeks	4 weeks after training ended	3 of 6 eye alignment and vergence measures improved
Clark 2012, PLoS One	One team compared with its prior season and its conference	University of Cincinnati baseball team	Preseason and in-season vision training that included the Brock string	One season	Team batting average rose from 0.251 to 0.285
Clark 2015, J Vis Exp	Methods paper with team data	College athletes	Vision training program that included the Brock string	6 weeks	Authors reported better stereo vision and reaction scores, and fewer concussions in players who trained

Brock string in other settings

Smaller studies have also listed the Brock string as part of therapy programs in other groups:

Typical binocular vision: In a 2025 randomized study of 34 adults with normal eye teaming, fusional vergence did not differ between the therapy and placebo groups after the first 12 weeks, then rose after a second 12-week phase in which the placebo group also did the program, which started with the Brock string. The authors said these ranges cannot be trained beyond 2 (negative) and 5 (positive) prism diopters in people without a binocular problem.
Visual snow syndrome: A 2022 pilot of 21 people reported better quality-of-life scores after 6 and 12 weeks of therapy. The authors said a drop in specific symptoms had yet to be shown.
Stroke: Three case reports from 2019 described an intense four-week program that included Brock's string.
Developmental delay: A 2026 case series of three kindergarten children used the Brock string in its later phase.
Myopia: A 2026 retrospective study of 208 children listed the Brock string in a training program and reported slower eye growth after training compared with before; many of the children were also using other myopia treatments.

Brock string and double vision

Seeing two strings on a Brock string is normal. That is different from seeing double in everyday life. If you or your child sees double, even if it comes and goes, the NHS advises seeing an optician or doctor. Double vision that starts suddenly, comes with eye pain, or follows a head injury needs urgent care.

An eye turn or a concussion needs a full exam first, because the right plan depends on the cause. Our guides to strabismus exercises, visual suppression and convergence insufficiency exercises explain more.

Brock string instructions: what to ask your clinician

Where the beads go, how long to practice, how often, and what counts as progress depend on your diagnosis. The doses in the studies above were set for those study groups. Ask your eye care professional to show you in the office and watch you do it, and stop and call your clinician if headaches, eye pain or double vision appear during an exercise. Our guide to home vision therapy covers how home practice fits into a clinician's plan. Good questions to bring:

What is my diagnosis, and why is the Brock string part of my plan?
Where should I put the beads and the far end of the string?
How many minutes a day, and on which days?
What should I see if I am doing it right, and what if I only see one string?
What should I do if I get headaches, eye strain or double vision away from the string?
When will you recheck my eyes?

Where Optics Trainer fits

Optics Trainer is a set of game-based vision and brain training apps for iPhone, iPad, Android, web browsers and Meta Quest. Its games include eye teaming practice with convergence and divergence levels, and binocular games where each eye gets its own target while both stay open, using red-blue glasses on a phone, tablet or computer or the built-in separation on Meta Quest. It is not a Brock string replacement, it is not a treatment or diagnosis, it is not FDA cleared, and it was not tested in the studies described above.

Optics Trainer is not a substitute for professional diagnosis or prescribed care. Consult your eye care professional for guidance about your specific condition. If your clinician agrees that home practice fits your plan, mobile and web access is $14.99 a month or $149.99 a year. The phone and tablet apps start with a 7-day free trial, and a free web account includes 3 plays a day. VR access is $19.99 a month or $199 a year after a 7-day free trial.

Frequently asked questions

What is a Brock string used for?

Eye care professionals use it to check whether both eyes aim at the same point and to spot suppression. In vision therapy, it is used as part of programs for convergence and for helping the brain notice both eyes.

What does it mean if I only see one string on a Brock string?

It can mean one eye is being suppressed, or tuned out. Because the strings cross, the missing string is on the side opposite the suppressed eye. Tell your eye care professional, since it calls for more testing.

What does it mean if the strings cross in front of or behind the bead?

Crossing in front suggests your eyes tend to aim closer than the bead. Crossing behind suggests they aim farther. An X right at the bead means both eyes are aimed at it.

Has the Brock string been studied after a concussion?

Yes. A 2025 randomized trial of 50 people aged 11 to 30 enrolled within 10 days of a concussion compared Brock string exercises with usual care, with follow-up 7 to 14 days later. Near point of convergence improved 8.9 cm with the Brock string and 3.8 cm with usual care. It also appears in the CONCUSS trial protocol and in several clinic case series.

Does the Brock string help convergence insufficiency?

In convergence trials such as CITT, CITT-ART and CINAPS, the Brock string was one procedure in office or maintenance therapy, so results describe the whole program. We did not find a trial that tested it alone for convergence insufficiency.

Can I use a Brock string for strabismus?

Ask your eye care professional. Studies in eye turns used it inside a clinic program, and the right plan depends on your type of eye turn.

How long should I do Brock string exercises?

Ask your clinician. The right setup, time and schedule depend on your diagnosis, and they should recheck your eyes to see whether the plan is helping.