VS117
3/2/99
Dr. Schor
Notetaker: Ryan Nakamura

Today we had our second quiz. Afterwards there was a question on the term "myodiopter". Dr. Schor defined it as the increased effort required to accommodate. As the lens becomes more brittle (it takes more of an effort to accommodate and the myodiopter increases).

Another question asked by "Big Luke": Can you explain why you get a difference between the calculated and gradient AC/ A? Has to do with proximal vergence. When you do the gradient AC/A your doing it at one distance so your proximal stimulus is constant, but when your doing the calculated AC/A you are taking into consideration the proximal vergence as you go from far to near. As a result, the near phoria is increased in the ESO direction and that will cause a difference in the near and far phorias by the amount of proximal vergence (from text: the calculated AC/A is slightly higher because of the addition of proximal convergence to the measurement of the phoria).

Q: What is meant by Accommodative Loop? What we are really talking about here is whether we are using Feedback: the error signal of blur to correct for errors in accommodation or not. For example, if you put a pinhole pupil in front of your eye, and you see the object getting bigger, the proximal is stimulating an accommodative response. But since you don't know how much you are accommodating, the response is 'open loop' (you don't get feedback to tell you how accurate your response was). The amount of blur will tell you if you are accommodating the correct amount. Referring to the diagram, the loop refers to the fact that you have negative feedback where you are comparing the target distance and the accommodative response-the difference is the error, in this case the error (feedback) is blur. Blur is what stimulates the central nervous system. Note that we can also open the accommodative loop by looking at an out of focus photograph, for which the eye won't accommodate because no matter how much we accommodate, the picture looks the same, so there won't be any feedback.

In the case of vergences, the error (feedback) is the difference in the convergence response and the convergence stimulus. Under closed loop conditions, this error is called fixation disparity. Under open loop conditions the error is the phoria. The phoria is what stimulates fusional vergence to converge more. When we are measuring the phorias (Maddox, cover test, prism dissociation -each test disrupts fusion by taking away the disparity stimulus) the loop is open and there is no negative feedback, so the CNS won't respond by converging to fuse the images and we can measure the phorias. From VS118, we know that the 'disparity' is the deviation of retinal images from corresponding points. When images are on corresponding points, they appear single and in the same visual direction. The two foveas are corresponding points, so if one image falls on the fovea and the other falls off the fovea, this is a non-zero disparity that stimulates vergence eye movements.

Today's outline:

Cross-link convergence
Accommodative convergence (AC/A)
Convergence Accommodation (CA/C)

Measurement of AC/A
Calculated= PD (cm) + (Pnear - Pfar)/MA
Gradient= Change in phoria/ change in accommodation

Normal amplitude
Response Ratio
Stimulus Ratio

Linearity

Stability
Age
Drugs
Dynamics

Stimulus AC/A vs. Response AC/A

In clinic we measure the stimulus gradient AC/A=Convergence Response/Accommodative Stimulus. In our 117lab we measure both the stimulus AC/A and the response gradient AC/A=Convergence Response/Accommodative Response. So in clinic, even though we click in -1.00D, the patient may not accommodate the full +1.00D (Accommodative Stimulus) due to the lag of accommodation. He may actually only be accommodating +0.5D (Accommodative Response) which we will measure in 117lab. So we see for the same patient, the Response AC/A (from the text the normal value is 4/1) will be higher than the Stimulus AC/A (normal value 3.6/1).

Measurement of AC/A

2 techniques for measuring AC/A ratios:

1) Gradient (measured) AC/A
a) stimulus AC/A
b) response AC/A

2) Calculated AC/A

= PD (in cm) + (Pnear-Pfar)/MA
- MA is the change in stimulus to accommodation in Meter Angles (usually measured from far to 40cm, but could also be the change from 1m to 40cm, etc.).
- PD is the interpupillary distance, and represents the ideal AC/A.
- (Pnear - Pfar/MA) represents the change in phoria as you go from far to near
- ESO (+), EXO (-)

Ex: 2 eso @ near (normally 40 cm which is a 2.5 MA stimulus for convergence), 3 exo @ far, PD= 6cm
- we get 2 - (-3) = 5 prism diopters more eso at near
- 5/2.5=2, we add this to the PD and get an AC/A= 8/1

In the above example, the subject is getting more esophoric at near compared to far. Now, lets say the phoria is the same at far and near (e.g. ortho at far and near). Here, AC/A = PD and we have an ideal AC/A. This means when you're looking far (while dissociated) your eyes are parallel and when your looking near (while dissociated) they are converged exactly at the target your looking at.
Why is your PD the ideal AC/A? For any viewing distance:

Convergence Demand = 1/ viewing distance (m) = MA

- for 1m, the CD=1MA
- recall, to convert from MA to prism diopters ---- prism diopters = MA x PD(cm)
- prism diopters = 1MA x 6cm = 6 prism diopters
- this means if we want to converge from far to 1m, we just want to converge 6 prism diopters to see singly (AC/A = 6/1)

Thus we see that your ideal AC/A is your PD in cm. If your phoria is becoming more ESO at near, then your AC/A is higher than the ideal. If your phoria is becoming more EXO at near, then your AC/A is lower than the ideal.

Back to the question on why the calculated AC/A is higher than the gradient AC/A. Due to proximal convergence, the near phoria will be a little higher (will be a little more ESO) for the calculated AC/A, causing the calculated AC/A to be higher.

Convergence Accommodation (CA/C)

These same two methods can be used to measure the CA/C ratios. Not only do efforts of accommodation cause convergence, but efforts of convergence cause accommodation (one of the Maddox components of accommodation).

A figure was shown that is not in the reader, but the basics of it were as follows. As you age, your lens becomes more brittle. Thus as you age, your efforts to converging has less of an influence on accommodation and your CA/C ratios decrease. It showed that under the age of 20 years the CA/C is one diopter/meter angle of convergence, and that the ratio declines over the age of 20.

How do we measure the CA/C ratio? Turns out we use similar techniques to the gradient AC/A's. For the gradient technique, patient is shown an out of focus picture (e.g. an out of focus green bar). Have patient converge on the blurry green bar at 40cm and do near retinoscopy and measure the lag of accommodation. Then give the patient some BO prism (e.g. 8BO) to fuse through and now they are converging more, and you remeasure the lag, which should be less when they are converging through the BO prism. The change in the lag of accommodation is the change in Convergence Accommodation (numerator). The change in the amount of BO prism is the denominator, and you have your CA/C.
How is this similar to the gradient AC/A? In gradient, AC/A you change the stimulus to accommodation, and measure the change in the phoria. In gradient CA/C, you change the stimulus to convergence and measure the change in the lag of accommodation. Here, accommodation is open loop because we are using a blurry target, so we can't respond with more or less accommodation because there are no contrast cues.

Ideal vs. Normal Values for AC/A

Ideal values for AC/A: 6/1
Normal values for AC/A : 3.6/1 (stimulus gradient AC/A)shown in figure below
4/1 (response gradient AC/A)
Nobody knows why the ideal is so far from the normal values, as Dr. Schor puts it, 'its one of the mysteries of binocular vision'. For some reason we like to have a little more EXOphoria at near than at far (recall from 100B the minimum norms for phorias are 0-2 EXO at far, and 0-6 EXO at near). One theory is that there is a lot of proximal convergence at near, so we cut back the AC/A ratio so that we don't overconverge with a proximal stimulus. In the figure below you can see there is a big range of tolerable values.

Linearity in the AC/A

The AC/A ratios are pretty linear. By this we mean if you have, for example, a 4/1 AC/A, then if the accommodation stimulus is increased by one diopter, then the amount of convergence should increase by 4 prism diopters.

In the above figure, A is the Stimulus to Accommodation and Accommodative Response, which is what we measured in lab #2. B is the Stimulus to Acc and the Convergence Response function - we see that initially when you don't accommodate, you don't converge; then as you start accommodating you start converging linearly; until you reach your amplitude of accommodation (when your trying to accommodate more but you can't), and that forces your convergence way up and you get an apparent increase in your AC/A ratio. C tells you pretty much the same thing. It shows the response AC/A ratio. The point of the entire figure is that AC/A stays linear over the entire range of accommodation, until you reach your peak of accommodation where your AC/A increases rapidly.
For people above 40 years of age, they reach their amplitude of accommodation much faster and when we measure the AC/A we might be measuring near their amplitude of accommodation, so we might see an apparent increase in AC/A.

Affect of Drugs on AC/A

When we cycloplege patients (with Mydriacyl, Tropicamide, or Cyclogyl, which all wear off in about 2 hrs) the AC/A increases because there is partial paralysis of the ciliary body, so there is a big decrease in accommodative amplitude, but the patient still tries to accommodate which is means he is still converging; as a result the AC/A ratio increases. We usually don't use Atropine (shown in the figure below) because it takes about 24 hrs to wear off.

Dynamics of AC/A

In clinic we mainly measure 'static' values - measure AC/A when the eyes are looking at one distance and not changing. But our vision is really dynamic, we are constantly changing viewing distance. Does the same AC/A operate dynamically and statically? The answer is NO!! The figure below shows measures of accommodative convergence as a function of the speed of accommodation:
- when we look from far to near very slowly (8 second period) there is no accommodative convergence
- when we look far to near quickly (2 second period) we do have accommodative convergence. Shows that rapid changes in accommodation stimulate accommodative convergence.

Why is this? When viewing distance changes slowly, we use tonic accommodation to control accommodation, and when viewing distance changes quickly we tend to use optical reflex accommodation. It turns out that tonic accommodation does not stimulate accommodative convergence, but the fast optical reflex accommodation does stimulate accommodative convergence. So which Maddox component we use determines whether we get accommodative vergence. This is illustrated in the block diagram below.

If we note where the 'X' is in the diagram, we see that a "fast" accommodative stimulus not only causes a response of Accommodation, but also crosses to the vergence system and causes a response of Convergence as well. In contrast, the "tonic adaptation" falls after the 'X' and, therefore, only causes a response of Accommodation (doesn't cross to the vergence system).