VS 117
2/23/99

Announcement
Quiz : next week. Cover material up to tonic vergence.
Midterm : the week before spring break. Format: multiple choice and ?

Lecture Outline
Maddox Component of Vergence
Tonic
Anatomical and Physiological position of rest
phoria - vergence demand - physiological position of rest
adaptation - development
environment
optometric
concomitant
non-concomitant
Proximal
Disparity - driven (Fusional) Vergence
Accommodative Vergence

Today we are going to cover the Maddox component of vergence.

To get a picture of patients' binocular system we need to know their phoria, convergence amplitude and amplitude of accommodation. That all comes from the Maddox' classification of the vergence hierarchy; there are several components: tonic, proximal, fusional and accommodative. The all sum up linearly and that is the basis of case analysis.

Vergence can occur in all three axis: Horizontal, Vertical and Cyclo. The vertical vergence is also called skew movements.

Tonic Vergence

Anatomical and Physiological Position of Rest
In the absence of tonic vergence, the tension of the muscles will exert an eye posture called anatomical position of rest, which is about 20 degrees divergent. Anatomical position of rest occurs in sleeping infants, deep coma and death. (However, a former opto student who worked as a mortician also observed all kinds of eye positions in dead people.) After six weeks of age the eye positions develop into the new position called physiological position of rest, which is usually around 0 degree. That is the position of the eye in the absence of external stimuli. The physiological position is determined solely by intrinsic innervation. Tonic vergence brings the eyes from anatomical position to the physiological position.

Vergence Demand

The vergence demand is the angle subtended by the object with the visual lines and is independent of where the eyes are pointing. Phoria is the error between where the eyes are pointing and where they should be pointing.
Ideally we want to see single. That requires the convergence response equal to convergence demand. When looking at distant object, the C.D. is 0 and our physiological position is slightly diverging. Phoria is the difference between convergence demand and physiological position.

Example:
The left eye is pointing at the target and the right eye is turned out by 3 prism diopters. This is the physiological position for this person.
Phoria = C.D. - PPR = 0 -(-3D) = + 3D <= exophoria

Another example:
Given:
PPR = 12 D
Vd = 40 cm
PD = 6 cm

C.D. = 2.5 MA ( meter angle) [ MA = 1/Vd ]
C.D. = 15D [D = MA* PD ]
Phoria = 15D -12D = 3D <= exophoria

When we measure phoria in clinic, we are measuring the error; we are not measuring the rest position. For far targets at infinity, the phoria equals the PPR, but at near distances the PPR and phoria differ.

Tonic Adaptation
Tonic vergence is very adaptable. It can be modified to compensate for overall changes in binocular alignment caused by normal development, injury, spectacle magnification, etc. Under tonic vergence you increase until you get single vision. This tells the tonic system to stop converging. If some condition of the eyes prevents fusion, e.g. cataract, vertical skew, the eyes will continue to converge and become esotropic. Almost all strabismus found in infants are esotropes. As infants develop, their PD increases from 4cm to 5.5 cm, and for a target at a given distance, more vergence is required as the PD increases. Tonic vergence is responsible to bring in this required amount of vergence to overcome the anatomical position of rest. Also when some innervation to the extraocular muscles is damaged, most commonly, lateral rectus VI, the muscle is weakened, therefore more innervation is required to align the eyes. Tonic vergence is also responsible for this compensation. Compensation for IV nerve damage is another example. In the case of spectacle magnification, if you are anisometropic, the eye that receives the bigger image will have to converge more in order to fuse.

Looking at the plot of prevalence of phoria, we note that the overwhelming percentage of the population is orthophoric at distance or slightly exo. Considering all the independent variables, e.g. 12 extraocular muscles, that play a part in maintaining such eye position, this fine tuning of eye position can not be genetically rooted but relies on post-developmental adaptation. The near phorias are a lot more distributed. The reason is that our AC/A ratio is more varied. People having the same distant phoria can have different AC/A ratios.

paste figure on p171 `prevalence of phoria' here

Prism adaptation
Place a 6D BO prism in front of a person's eye for 5 sec, then occlude the eye. The vergence relaxes.Keep the prism for 1 min, the eye doesn't relax but continue to converge. We can see that the adaptation is fairly rapid ~ 1min. Phoria changes by the amount of prism you put in. This is called prism adaptation. This can be frustrating clinically. When your patient has a phoria, you put a prism there to eliminate the phoria. When they come back in a week they still have the phoria while wearing the prism. However, most people do allow the prism to change their phoria. People who have phoria tend to have poor prism adaptation and as a result, prisms do work for them (i.e. they don't adapt to them). People who have the best prism adaptation tend be orthophoric and don't need prism anyway.

Normally when you are binocular, your disparity vergence will converge your eyes, and internally, you will allow your disparity vergence to relax and let you tonic vergence take over by adapting to compensate for the prism. In clinic we know there is the base in range and base out range of adaptation. The BO range is much bigger that the BI range. The difference is because that the adaptation process works really well in the converging direction and weakly in the diverging direction. There is a good reason for it; the anatomical position is diverging and the eyes don't really anticipate diverging movement.

When you have two different powers in your two eyes, one is myopic and the other is hyperopic. You put a plus lens in front of the right hyperopic eye. Since it is a plus lens you get a magnified image. When you look to your right, your right eye has to diverge more than the left eye. When you look to your left, your right eye has to converge more. Your phoria changes to exo in right gaze and eso in left gaze. This is called non-comitant adaptation - the phoria is different for different direction of gaze.
Another example, if my lateral rectus in my left eye is damaged, when I look to the left I have a esophoria and since my medial rectus is fine, when I look to the right I am orthophoric.
Say I am a presbyope who has more add in the right eye, when I look down throught my bifocal the image for the right eye will be bigger and thus deviated down more. So I have to move my right eye down more than my left eye, i.e. a vertical phoria. So the optometrist decided to add a little prism to correct the vertical phoria. It turns out that it is not needed. I tonically adapt to the vertical vergence demand in down gaze and this adjustment is effortless. (Other vergence adjustments are really hard on you. Headaches, sore eye, etc..) This non-comitant adaptation takes hours.

Proximal Vergence
Proximal vergence is what I called `coarse adjustment'. It is a response to the perceived distance. When you look a far object, your eyes diverge without noting the retinal disparity. When you cover one eye and look at a far object and then a near object, your eye can converge very quickly and precisely. It is not extremely precise. That is why you need disparity vergence, which senses whatever residual errors left over from proximal vergence. Proximal vergence takes care of 99% of your vergence demand. When you look at a two-dimensional painting that has a 3 D rendition, you eyes diverge a little when you look at the background and converge a little when you look at the foreground.

Disparity Vergence
Disparity vergence is the fine adjustment. There are two different kinds of disparity vergence, symmetric and asymmetric. In asymmetric vergence you change both visual distance and direction. The eyes converge symmetrically and then make versions.

Saccade facilitates vergence; makes vergence much much faster.
Blink facilitates vergence; also makes accommodation faster. To make strabismus patient to look straight just tell them to blink.
Asymmetric vergence is a combination of lateral saccades with horizontal vergence. The velocity of vergence is combined with saccades to yield some very high velocity vergence responses that appear to result from yoked saccades of unequal amplitude. The vergence velocity resulting from differences in the two saccades can approach 50 deg/sec. It is not clear if these movements are the result of a combination of symmetrical vergence with equal saccades, or if we are able to make saccades of unequal amplitude. It is likely that both of these explanations are correct.

Symmetrical Convergence
Symmetrical vergence movements tend to be more sluggish than asymmetric vergence. They have latencies of 160 msec which is shorter than a saccade and about the same as a pursuit. Their response time is sluggish taking .8 to over 1 second for completion. The velocity is proportional to disparity. It is about 3 degrees per second per degree of disparity. As your eyes converging on the target, the disparity is getting smaller and smaller, the velocity gets slower. When you get to the target, the disparity goes to zero, so does the velocity.
The disparity vergence system has some prediction capability. Let's say we have a prism in the vertical prism, we tell the subject to converge. The vergence response starts 160msec later. As the eyes approaching the target, the vergence system would overshoot due to latency if it had not anticipated to stop. Sometimes we do get some oscillation. However, for the most part the eyes are able to slow down as they approach their final destination because we anticipate when the disparity will go to zero. This anticipatory function is performed by the cerebellum for most body movements.

Disparity is encoded by the binocular cells in the primary visual cortex. We don't know very much until we get to the brain stem. Very close to the third and sixth motor nuclei, which control the medial and lateral rectus, there are groups of cells called near response cells. They signal for convergence and accommodation. The near response cells only fire when the eyes converge or accommodate; no firing when eyes make vertical movements or saccades.