VS117 LABORATORY V:
ACCOMMODATIVE CONVERGENCE AND ACCOMMODATION (AC/A)

INTRODUCTION

When accommodation occurs, it is accompanied by a tendency for the eyes to converge. The convergence driven by accommodation is called accommodative convergence. The rate at which open-loop convergence (phoria) increases with a unit change in accommodation (1D) is described as the AC/A ratio. Similarly, convergence can drive accommodation and this is called convergence accommodation. The rate at which open-loop accommodation occurs with a unit change in convergence (1 meter angle) is described as the CA/C ratio. AC/A is routinely measured clinically. The convergence loop can be opened to measure the AC/A by covering one eye so that the disparity mechanism does over-ride convergence that is due to the accommodative system. Proximal stimuli to convergence can be eliminated by stimulating accommodation with lenses rather than changes in viewing distance, however clinically, the AC/A can be computed from measures of the near and far phoria. The AC/A ratio computed from phoria measurements is generally larger than the AC/A ratio measured from lenses (gradient technique). Clinically, the accommodative response is not measured; instead, it is assumed to be equal to the accommodative stimulus. The resulting measure is called the stimulus AC/A ratio. The change in accommodative convergence divided by the accommodative response is called the response AC/A ratio . Generally, the stimulus AC/A is lower than the response AC/A ratio because the accommodative stimulus is usually greater than the accommodative response, and the accommodative stimulus is the denominator of the stimulus AC/A ratio. Tonic stimuli to accommodation are considered to be constant in this experiment when changes in accommodation occur.

Review of stereoscopes: Stereoscopes are instruments which allow each eye to see a separate target. The right eye target and the left eye target may be identical or different in whole or in part. When a normal observer looks through a properly adjusted stereoscope, the two targets are fused (usually in an additive fashion) into a single percept.

There are two types of stereoscopes used in optometry. One of these is the Lenticular or Brewster stereoscope. The Brewster stereoscope contains +5.0 D convex lenses before the right and left eye. The other type of stereoscope is the mirror or Wheatstone stereoscope. In this type, the observer looks at the images of the right and left objects as created by reflection.

The stigmascope-haploscope used in this exercise is a special adaptation of the wheatstone stereoscope in which the mirrors are half-silvered. The mirrors and stigmas are mounted on two arms that are located on an adjustable base so that when the observer is properly positioned and the base is adjusted to the intraocular distance, each arm rotates about a point almost directly beneath the center of rotation of its associated eye. Thus rotation of the haploscope arm produces a pure rotation of the retinal image of the stigma without any translational artifact (i.e. the experiment measures binocular convergence without any artifact). Each half-silvered mirror located on each arm of the haploscope is directly in front of its associated eye. When the eye fixates the stigma, the angle of incidence in the horizontal plane which the visual axis of each eye makes with the mirror is 45 degrees. As a result, each convergence arm is bent 90 degrees temporalward from the visual axis at the place where the mirror is located. Reflected images of stigmas are seen superimposed on distant targets when the arms are properly adjusted.

In this experiment, only one eye (the right eye) will view the distant reduced Snellen target but both eyes will view their respective stigma. The stigmas are used to measure accommodative response of the right eye to a monocularly viewed Snellen chart and convergence of the covered left eye. As convergence of the covered left eye increases as a result of the associated accommodative response, the left arm of the haploscope is converged inward to place the stigma near the viewed target. The subject will need to make two types of response adjustments. The first is to measure the accommodative response of the right eye by adjusting the right Badal optometer. The second is to measure the convergence response of the left eye associated with accommodation by adjusting the horizontal position of the left eye's stigma until it appears directly beneath the stigma before the eye viewing the Snellen chart. The amount of convergence to fixate the common object (Snellen chart) in real space can be determined from the protractor associated with the converging arms when the objects, usually two pinholes of light, on the converging arms appear to be imaged one above the other and superimposed upon the common target.

APPARATUS

haploscope meter stick
distance Snellen chart reduced Snellen chart
trial lens set millimeter ruler
white image screen

EXPERIMENTAL PROCEDURE

Align the height of the light sources and mirrors so they are equal.

Adjust the protractor so that the '0' on the scale is aligned with the mark on the haploscope bench. The pointers on the convergence arms when placed at the '0' on the scale should place the arms in a straight line parallel to the base.

Determine the interpupillary distance (PD) of the chosen subject with the millimeter rule. Set the instrument interpupillary distance to equal that of the observer.

Adjust the forehead rest to bring the head either forward or backward until the apex of each cornea is in line with the sighting device located near the lens cells. Looking through the small aperture from the side, one can see a cross-hair. The head should be moved forward or backward until this sighting-line is aligned with the front surface of the cornea. This will place the cornea 14 mm away from the center lens cell.

Move the right source along with the haploscope arm and determine whether the observer detects considerable lateral or vertical motion of the light. If the calibration is correct, the light source should only go out of focus. Any lateral or vertical motion, if present, may be minimized by increasing or decreasing the interpupillary setting a millimeter or two.

Place any necessary refractive correction in the haploscope lens cell before the right eye. With the arms of the haploscope set parallel to the baseline, make sure that the protractor scales on the base of the haploscope are aligned properly before the two eyes (in most machines the '0' on the scale should be lined up with the small mark on the stage).

Place an occluder on the half-silvered mirror on the left side. The occluder should be placed on the side of the mirror away from the observer so that the light from the point-source is allowed to reflect off the mirror and enter the subject's left eye. The occluder should block any light from the fixated target.

PART 1: FAR TARGET MEASUREMENTS

Place a Snellen chart at least 3 m away and straight ahead along the fixating eye. Record this distance and the corresponding accommodative stimulus in the first row of Table 1 on your Data sheet. Determine the accommodative response using the equations from Lab III and IV. The point source should be slightly displaced from the fixated feature on the Snellen chart so the subject does not accommodate to the point source.

Use the point light source on the left eye to determine the amount of convergence. This can be done by aligning the point source on the feature being detected (usually a small letter on the chart). You can adjust the point light source itself to make it appear superimposed with the target feature being observed. For a distance target, the reading on the protractor arm for both eyes should be '0'. Naturally the point source should be in sharp focus when this is done. However, in order to again prevent the subject from accommodating to the point source, the point source should be swung into place only after having determined the accommodative response for the right eye. You do not need to measure the accommodative response for the left eye. Record this reading on the protractor scale in Table 1. The subject should be very careful to continue fixating some small letter on the chart while the settings are being made.

Repeat the series of three optometer settings for the right eye and three convergence settings for the left eye with each of the following lenses before the right eye: -1 D, -2 D, -3 D, -4 D, -5 D, and -6 D. In determining the position of the swinging haploscope arm before the left eye, make sure to align the light spot on the same position in each case so that there is consistency in the reference point. Record all values again in Table 1.

PART 2: NEAR TARGET MEASUREMENTS

Place the reduced Snellen chart 50 cm from the back lens cell along a straight line in front of the fixating eye. Repeat the above measurements with the following lenses before the eyes: +2 D, +1 D, no lens, -1 D, -2 D, -3 d, and -4 D. Record all data in Table 2.

REPORT

For this lab, turn in a single group report containing:
A. Graph
B. Data sheet

A. Graph:

PART 1: Plot the accommodative vergence response (y-axis) versus the accommodative stimulus (x-axis) for the far target condition. On the same graph, plot the accommodative vergence response (y-axis) versus the accommodative response (x-axis) for the far target position. The vergence response should be plotted in units of prism diopters. Use the following formula to convert the vergence response from deg (which you measured on the haploscope) to prism diopters.

Prism diopters [D]= 100 tan q, where q equals convergence in degrees.

Determine the stimulus AC/A ratio and the response AC/A ratio for the far target condition. The slope of the line can be measured by fitting a linear regression to the data in the form of Y=aX +b where (a) is the slope and (b) is the Y intercept. The AC/A is measured by the slope.

PART 2: Repeat Part 1 for the near target condition.

B: Data sheet:

On Tables 1 and 2, compute the vergence response in units of meter angles (MA). Use the following formula:

MA = D/PD(cm)