|
Volume 1 - Why Two Eyes?
|
Lecture 24 of 24 NEXT»
|
Cyclops got along with one eye until it was put out. We have two - just to have a spare or is there some other reason? Yes, there is more to it. There are other reasons for and benefits to be derived from having two eyes. The two eyes provide vision which is additive by enabling a wider field of vision when using two eyes compared to one. The two eyes also cooperate in the overlapping visual fields enabling stereoscopic vision by blending slightly dissimilar views of an object which is then seen singly and with depth.
Field of Vision: In the normal person with good vision and normal binocularity
|
Left Eye Monocular Binocular Right Eye Monocular
 |
 |
|
Total binocular field is nearly 170 degrees (varies according to configuration of orbits) |
You can demonstrate to a patient the difference in their field or their child's field with one eye compared to two. With two eyes you can also demonstrate the peripheral field and the central fusion. |
Try this: With both eyes open notice how bright the room light is. Now close your eyes alternately. The light level in the room remains about the same. That is because the eyes really see one at a time alternating rapidly - faster than you can detect. This is called retinal rivalry which is the basis for suppression and to some extent retinal correspondence.
Encouragement for the Amblyope
This demonstration of peripheral vision is valuable for parents of children with poor vision in one eye from amblyopia, especially those with unilateral congenital cataract and very poor vision of 20/200 or less in one eye. The peripheral field is more or less intact and is very useful to these patients provided the eyes are aligned or nearly so. It should be kept in mind that peripheral visual acuity in the normal is very low - less than 20/400, but at the same time this low level vision provides useful information. This poorly seeing amblyopic eye can euphemistically be called a "helper" eye rather than a "blind" eye. Actually, a child with 20/20 vision in each eye but with a strabismus and alternation uses one good eye and one helper eye alternating. The principal advantage for the alternater is that he/she has a "spare" good eye. This type of information can be very encouraging to parents of children with one good eye. However, individuals with amblyopia at any age should always wear safety glasses.
The Field of Vision in Exotropia and Esotropia
 People with exotropia can have an enlarged peripheral field - which "shrinks" after the eyes are straightened surgically - warn these exotropic patients before surgery! Then reassure them that they will stop noticing the reduction in field in a few weeks.
 |
Larger Peripheral
Smaller Binocular Overlap but with Suppression |
 We took care of a football player who had a large angle X(T). He did not want surgery because he said he could see "behind" and this helped him avoid tackles.
Later, after dropping a few passes, he asked to have his eyes straightened so he could follow the ball better. A True Story!
 While less dramatic, esotropes have reported an expanded visual field of vision after their eyes were aligned. This has been used by ophthalmologists to support a functional result even in adult esotropia surgery. But, since it is legitimate that every human deserves to look like a human, with straight eyes, the enlarged field is just an added benefit.
The Phi Phenomenon
Look at a pencil held at arm's length and close your eyes alternately - the pencil will jump - this is the Phi Phenomenon. This demonstrates the different views from each eye which are combined to produce stereopsis. It can be useful to demonstrate this to patients (parents) while explaining stereopsis.
Seen in Stereo Close left eye - finger jumps Left Close right eye - finger jumps right
The Two Pencil Test of Lang
|
With both eyes open the patient who uses both eyes producing stereopsis can put his pencil accurately on the examiner's pencil if stereopsis is present. |
 |
The same person with one eye closed or with manifest strabismus or no stereopsis will miss the examiner's pencil initially and place it correctly only after the second or third try. |
 |
Stereopsis
|
Panum's Fusional Space |
 |
Horoptor |
Definitions:
Horoptor - the sum of points in space seen by corresponding retinal points. (Seen singly but not in depth)
Panum's Fusional Space - the sum of the points seen by disparate retinal points singly and with depth. (This is the basis of stereopsis.)
| The Titmus Test uses polarized real images viewed with polarized glasses. Monocular clues exist especially with the first three sets of circles. The test circle is displaced when seen monocularly. A patient without stereopsis may point to this circle not because it "projects" from the page as it would with stereopsis, but because it is displaced laterally. |
 |
The Random Dot Test (Julez) provides a virtual image imbedded in random dots which can be seen only with stereopsis.
No Monocular Clues As seen with Stereopsis |
The Strabismus Minute, Vol.1, No. 4 Copyright © Eugene M. Helveston All Rights Reserved
Editor-in-Chief: Eugene M. Helveston, M.D.
Associate Editor: Faruk H. Orge, M.D.
Editorial Board: Bradley C. Black, M.D.
Edward O'Malley, M.D.
David A. Plager, M.D.
Derek T. Sprunger, M.D.
Daniel E. Neely, M.D.
Naval Sondhi, M.D.
Senior Editorial Consultant: Gunter K. vonNoorden, M.D.
Graphics: Michelle L. Harmon
Technical Support: George J. Sheplock, M.D.
|
Print this Page