Vergence
A vergence is the simultaneous movement of both eyes in opposite directions to obtain or maintain single binocular vision.[1]
When a creature with binocular vision looks at an object, the eyes must rotate around a vertical axis so that the projection of the image is in the centre of the retina in both eyes. To look at an object closer by, the eyes rotate towards each other (convergence), while for an object farther away they rotate away from each other (divergence). Exaggerated convergence is called cross eyed viewing (focusing on the nose for example) . When looking into the distance, the eyes diverge until parallel, effectively fixating the same point at infinity (or very far away).
Vergence movements are closely connected to accommodation of the eye. Under normal conditions, changing the focus of the eyes to look at an object at a different distance will automatically cause vergence and accommodation, sometimes known as the accommodation-convergence reflex.
As opposed to the 500°/s velocity of saccade movements, vergence movements are far slower, around 25°/s. The extraocular muscles may have two types of fiber each with its own nerve supply, hence a dual mechanism.
The vergence of light is defined by:
Vergence = V = n/L
where n is the index of refraction of the medium and L is the distance in accordance with the Cartesian sign convention. The standard use of vergence expresses the distance L in meters, so the unit of vergence is m-1, often called "diopters".
Since the distance L1 is measured from the wavefront and light is traveling left to right, it is a negative distance and the vergence is negative (divergent). L2 is positive since it is directed to the right from the wavefront (convergent).
The change in vergence when the light encounters a refracting surface is equal to the power of the surface Ps:
V + Ps = V'
Example:
The example below shows a positive lens followed by a negative one, but the powers can be given any value, positive or negative. At each surface the relationship V + Ps = V' is applied, and the change in vergence between surfaces is calculated.
The following types of vergence are considered to act in superposition:
- Tonic vergence: vergence due to normal extraocular muscle tone, with no accommodation and no stimulus to binocular fusion. Tonic vergence is considered to move the eyes from an anatomical position of rest (which would be the eye's position if it were not innervated) to the physiological position of rest.[2]
- Accommodative vergence: blur-driven vergence.
- Fusional vergence (also: disparity vergence, disparity-driven vergence, or reflex vergence[2]): vergence induced by a stimulus to binocular fusion.
- Proximal vergence: vergence due to the awareness of a fixation object being near or far in the absence of disparity and of cues for accommodation. This includes also vergence that is due to a subject's intent to fixate an object in the dark.[3]
Acommotative vergence is measured as the ratio between how much convergence takes place for a given accommodation (AC/A ratio, CA/C ratio).
Proximal vergence is sometimes also called voluntary vergence, which however more generally means vergence under voluntary control and is sometimes considered a fifth type of vergence.[4] Voluntary vergence is also required for viewing autostereograms as well as for voluntary crossing of the eyes. Voluntary convergence is normally accompanied by accommodation and miosis (constriction of the pupil); often however, with extended practice, individuals can learn to dissociate accommodation and vergence.[5]
Vergence is also denoted according to its direction: horizontal vergence, vertical vergence, and torsional vergence (cyclovergence). Horizontal vergence is further distinguished into convergence (also: positive vergence) or divergence (also: negative vergence). Vergence eye movements result from the activity of six extraocular muscles. These are innerved from three cranial nerves: the abducens nerve, the trochlear nerve and the oculomotor nerve. Horizontal vergence involves mainly the medial and lateral rectus.
References:
- Cassin, B (1990). Dictionary of Eye Terminology. Solomon S. Gainesville, Fl: Triad Publishing Company.
- George K. Hung; Kenneth J. Ciuffreda (31 January 2002). Models of the Visual System. Springer Science & Business Media. p. 342. ISBN 978-0-306-46715-8.
- Jump up ^ Ian P. Howard (January 1995). Binocular Vision and Stereopsis. Oxford University Press. p. 399. ISBN 978-0-19-508476-4.
- Jump up ^ Penelope S. Suter; Lisa H. Harvey (2 February 2011). Vision Rehabilitation: Multidisciplinary Care of the Patient Following Brain Injury. CRC Press. p. 97. ISBN 978-1-4398-3656-9.
- Jump up ^ C. Keith Barnes (May 1949). "Voluntary dissociation of the accommodation and the convergence faculty: Two observations". Arch Ophthalmol. 41 (5): 599–606. doi:10.1001/archopht.1949.00900040615008.
- http://hyperphysics.phy-astr.gsu.edu/hbase/geoopt/vergence.html