A Brock string is an instrument used in vision therapy

Vision therapy

Vision therapy, also known as visual training, vision training, or visual therapy, is a broad group of techniques aimed at correcting and improving binocular, oculomotor, visual processing, and perceptual disorders."-Historical development:...

. It consists of a white string of approximately 10 feet in length with three small wooden beads of different colors.

The Brock string is commonly employed during treatment of convergence insufficiency

Convergence insufficiency

Convergence insufficiency or Convergence Disorder is a sensory and neuromuscular anomaly of the binocular vision system, characterized by an inability of the eyes to turn towards each other, or sustain convergence.-Symptoms:...

 and other anomalies of binocular vision

Binocular vision

Binocular vision is vision in which both eyes are used together. The word binocular comes from two Latin roots, bini for double, and oculus for eye. Having two eyes confers at least four advantages over having one. First, it gives a creature a spare eye in case one is damaged. Second, it gives a...

. It is used to develop skills of convergence

Convergence (eye)

In ophthalmology, convergence is the simultaneous inward movement of both eyes toward each other, usually in an effort to maintain single binocular vision when viewing an object. This action is mediated by the medial rectus muscle, which is innervated by Cranial nerve III...

 as well as to disrupt suppression

Suppression (eye)

Suppression of an eye is a subconscious adaptation by a person's brain to eliminate the symptoms of disorders of binocular vision such as strabismus, convergence insufficiency and aniseikonia. The brain can eliminate double vision by ignoring the images of one of the eyes...

 of one of the eyes.

During therapy, the one end of the Brock string is held on the tip of the nose while the other is tied to a fixed point. The three beads are spaced out a various distances. The patient is asked to focus on one of the beads, while noting the visual input of each eye and sensation of convergence. Techniques may be made more difficult by bringing the beads closer to the nose and by employing lenses

Lens (optics)

A lens is an optical device with perfect or approximate axial symmetry which transmits and refracts light, converging or diverging the beam. A simple lens consists of a single optical element...

 and prisms

Prism (optics)

In optics, a prism is a transparent optical element with flat, polished surfaces that refract light. The exact angles between the surfaces depend on the application. The traditional geometrical shape is that of a triangular prism with a triangular base and rectangular sides, and in colloquial use...

.