#253300
In adults visual acuity is measured as "recognition acuity", which uses standard line tests. This type of test cannot be used in examining infants and children with multiple impairments. Instead of recognition visual acuity, grating acuity is measured with grating acuity tests either as detection or discrimination acuity. LEA GRATINGS test is a detection test.
In the LEA GRATINGS test, the infant or child detects the presence of parallel lines of decreasing width, a task simpler than recognizing optotypes. When a striped pattern is presented in front of an infant simultaneously with a gray surface of the same size and luminance, the infant is likely to look at the striped pattern because there is more to see than on a gray surface. What the child sees on the grating cannot be known. From children who can describe what they see we have learned that the lines on gratings can be seen distorted, only in a small part of the grating or no lines are perceived.
The LEA GRATINGS test uses paddles to present gratings.
The handle allows the tester to hold the test easily.
The gratings are defined by the frequency, i.e., the number
of pairs of black-and-white stripes or cycles, within one
degree of visual angle. When grating is printed on a surface,
it can be defined also as the number of cycles per centimetre
of surface.
When a grating is held at 57 cm (~ 2 ft) distance from
the infant's face, one centimetre equals one degree of
visual angle. This is a convenient test distance because number
of cycles/cm corresponds to grating acuity as cycles per degree.
Infants and children at an early developmental level may
not respond to stimuli placed at 57 cm distance. Their
visual sphere may be limited to less than 30 cm (~ 1 ft).
When the gratings are held at half the 57 cm distance, the
number of cycles per degree (cpd) is half of that at 57 cm.
If the infant's response can be elicited only at 15 cm
(~ 1/2 ft), 1/4 of the original distance, the frequency
of the grating is 1/4 of the value printed on the test.
If the child responds to the stimuli at about 1 meter
(exactly 114 cm or ~ 4 ft), the grating acuity values
are twice the value printed on the test.
In the examination of infants it is advisable to choose test distances that are parts or multiples of 57 cm, i.e.
28 cm, 43 cm, 85 cm or 115 cm. Longer distances are rarely used.
Infants at the age of 8-12 months may have had their immunisation injections and learned that unknown people can be friendly and nice for a while but then they stick. These infants look worried as long as the tester is within her arm’s length from the infant and relax when the measurement is made at 115 cm distance.
The 1 cpcm grating is placed at 57 cm and at
114 cm from the camera. At the 57 cm distance the grating
is an 1cpd stimulus and at the 114 cm distance the
same grating is a 2 cpd stimulus. The distance of 57 cm is
derived from the formula 2 p r. A circle has 360° and the
circumference of a circle is equal to 2 p r (where r = the
radius). In this case, "r" is equivalent to the distance
between the child’s eye and the paddle. If the circumference
of a circle measures 360 cm, then each degree of
angle subtends to a distance of 1 cm on the circumference.
The radius of such a circle, r, is then : r = 360 cm /
2 p = 57.2 cm.
Instructions
Make sure the background setting (including your clothing) is either evenly light gray or even dark colour to avoid patterns that could distract the infant. If the infant's visual sphere is limited, the surrounding visual information does not affect the child. However, these children are often disturbed by even weak noises and uncomfortable or unusual body postures.
Start with the coarsest grating. Show the infant the grating simultaneously with the gray stimulus. Then show every other grating in succession.
If the infant responds to the 0.25 and the 1.0 cpcm grating, but not to the 4.0 cpcm grating, present the
2.0 cpcm grating. The threshold is found quickly before habituation occurs. If the infant or child seems to
lose interest, show a face figure (#253100 Medium paddle) or colourful toys to motivate
the infant to respond again.
PRESENTATION OF THE STIMULI
The measurement is based on observing the child's eye movements
when the grating paddles are presented to the child.
Different ways to present the LEA GRATINGS to children. The grating is kept behind the grey surface while moving it in
the midline to the testing distance. When the grating and the grey surface are moved in opposite directions (A-C), motion perception
is an additional factor in perception of the grating. When the grating and the grey surface are kept motionless (D) in
front of the child, which resembles presentation of the Teller Acuity cards, motion perception does not affect the test situation.
When the grey surface
and the grating are moved in opposite directions, some children with brain damage look confused and do not follow the
movement of the grating but make a quick shift of gaze to the grating when it stops. This may be a sign of
difficulties in seeing moving objects. On the other hand, some children have normal tracking movements,
either eye movements or a combination of eye and head movements, but look surprised and confused
when the grating stops. They may not have visual functions to perceive objects that stand still. Both observations
need to be confirmed in other test situations.
When grating acuity has been measured the result needs to
be expressed in cycles per degree (cpd) Some tests express
the result as optotype (Snellen) acuity values. In visually
impaired children it is impossible to predict what the optotype
acuity might be if only the grating acuity can be measured.
Grating acuity tests measure function of the visual field in a much larger area than do the optotype tests
(letters, numbers, symbols); ie, the tests measure function of different retinal areas. Recognition of an
optotype, except E and C, is a much higher and more demanding visual task than resolving straight lines.
The responses come from different functions of the brain. Therefore there is no correct way of converting
grating acuity values to optotype acuity values. Physically, resolution of a 30 cpd grating requires the same
resolution as an 1.0, 20/20, 6/6 Snellen-E. However, this is true only in normal adult foveal vision, and
even there it is not exactly the same. Outside the fovea toward the periphery, grating acuity decreases
more slowly than do optotype acuities.
In low vision the relationship between grating acuity and optotype acuity varies as much as: between 1:1; ie,
the two values are equal, and 1:20; ie, grating acuity is 20 times better than the optotype acuity. Knowing
the type of lesion, an experienced clinician can make a fairly accurate guess what the optotype acuity could
be. However, the error may be sizeable. Therefore it is not wise to convert grating acuity values into optotype
acuity values.
When explaining the results to people who are not familiar with grating acuity measurements, you can show
the grating that the infant or child responded to and say: "As you saw, your child could respond to these
fine lines at this distance. This kind of grating is called .. (eg 2 cycles per cm, which means that there are
two pairs of lines in each centimeter of the surface). When this kind of grating was shown at approximately
57 cm distance, there are 4 lines, 2 cycles per degree." If you explain grating acuity this well, the
people will understand grating acuity much better than they will ever understand optotype acuities.
What to do when the parents and the fellow teachers do
not understand the "cpd"?
They do not understand the optotype acuity value either if
you do not show how big an optotype is e.g. a 0.3, 20/63,
3/18 optotype. Similarly, you can show the grating that
the infant or child responded to and say: "As you saw, your
child could respond to so fine lines at this distance.
This kind of grating is called .. (eg 2 cycles per cm,
which means that there are two pairs of lines in each centimetre
of the surface). When this kind of grating was shown at
approximately 57 cm distance, there are 4 lines, 2 cycles
per degree."
If you explain grating acuity this well, the parents understand
grating acuity much better than they will ever understand
optotype acuities. (How many of the readers can explain,
what a certain optotype acuity value means?)
Why do we need to use "cycles per degree", why not the visual
acuity values?
What is normal at different ages?
The drawing above shows the range of normalcy at different
ages. Values that are clearly below the lower line are highly
likely to be deviant (except when the infant or child was
tired or fuzzy). Values that are above the upper line
are likely to be good normal values. Values within the
range of normal are normal but do not mean that vision
would be developing normally.
Grating acuity alone is a poor depictor of visual function.
Therefore, never say that the child's vision was measured
to be normal. Say that "grating acuity value was within the
range of normal, other observations and measurements are
needed to give a more complete picture of the child's visual function."
PREREQUISITES FOR MEASUREMENT OF GRATING ACUITY
During the measurement of grating acuity, we expect the
infant/child to respond with smooth tracking or rapid eye
turn to the grating when it is presented. This response requires that:
- The infant or child sees the grating in that part of the
visual field;
- The infant or child can direct his or her attention to
the stimulus;
- The infant or child has the ability to plan tracking or
the saccade toward the target;
- The infant or child has the motor function of the eye
muscles to execute the plan;
- The stimulus is presented within the visual sphere of
the infant or child.
On each grating paddle the frequency of the printed grating
is given as cycles per centimetre (cpcm). At the distance
of 57 cm (22.5 in), 1 centimetre equals 1 degree of visual
angle*. Thus, only at that distance the cycles per degree
value of each grating is equal to the cpcm printed on the
paddle. For example, at 57 cm, the 0.25 cpcm paddle is equal
to 0.25 cpd. When the paddle is brought closer, the number
of cycles per degree decreases. When used at a distance longer
than 57 cm, the number of cpd increases. In the table below,
cpd values are calculated at some common distances. If another
distance is used, the cpd results can be calculated using this
formula:
|
Distance Used 57.2 cm |
x cpcm = cpd |
*NOTE: This is derived from the formula below. A circle
has 360° and the circumference of a circle is equal to 2 p r (where r = the radius). In this case, "r" is equivalent
to the distance between the child's eye and the paddle. If
the circumference of a circle measures 360 cm, then each
degree of angle subtends to a distance of 1 cm on the
circumference. The radius of such a circle is then calculated
as follows:
| r = |
360 cm 2 
|
= 57.2 cm |
