Visual screening criteria
Criteria for Passing Visual Screening (Revised April 2008)
For children at least 8 years old:
20/20 vision in each eye on the Lighthouse eye chart for tests of acuity at far. We use this chart instead of the standard Snellen eye chart (the one with the big E at the top) because the Snellen chart has a different number of letters in each line and therefore does not assess the effect of crowding equally for each level of acuity. In fact, for the 20/200 line, there is only one letter (typically an E). The Lighthouse chart is similar to the traditional Snellen chart but has 5 letters per line and therefore is equally sensitive to the effects of crowding at every level of acuity.
Method for testing the right eye (always tested first).
The gray tape on the floor of the waiting room marked "10 feet" is actually 4 metres away from the eye chart (which is the correct testing distance). Prior to patching one eye, keep the child further away from the chart than this marker so he cannot practice the letters with both eyes open. Child has the left eye patched and then is moved up so that his toes are lined up with the beginning of the gray tape. The child reads the first letter in each line in sequence until you get to the 20/40 line or until he makes one mistake, whichever comes first. After that, he reads every line. You may point to the line you want him to read and when the going gets tough, you may cover the line below the line he is reading to help him keep his place. Smallest letters he can read provide an estimate of acuity. When the child makes one or more errors on a line, give the child another chance to read that line. Keep track of the number of errors on the second try for each line. To pass, the child must get at least 20/20-2. Note that all errors prior to the 20/20 line are counted into this calculation. For example, if the child made 1 error on the second try for the 20/30 line, 1 error on the second try for the 20/25 line, and 1 error on the second try for the 20/20 line, his acuity is 20/20-3 and he fails. Repeat for the left eye.
- Checking for near-sightedness.
Although children may be nearsighted, they might be able to see clearly up to the required testing distance for the experiment without any optical correction. For example, near-sighted children who need up to a -2 dioptre add can see clearly out to 1/2 metre with no optical correction. Thus, if those children can pass the 20/20 line with up to a -2 dioptre add, they can participate in an experiment with stimuli at 1/2 meter (50 cm). Similarly, if the stimuli for the experiment are at 1 metre, children can be included if they need no more than the -1.0 D lens to pass the eye chart. Children tested at 4m, must pass screening with no more than a -0.25 dioptre add. In general, the distance (in metres) up to which near-sighted children can see clearly without optical correction is the inverse of the optical correction (in dioptres) that they need to pass the 20/20 line on the eye chart.
If children fail the 20/20 line on the eye chart, give them negative adds starting with -0.5 D and increase in half-dioptre steps until you reach the inverse of the testing distance (e.g., up to -2D for tests at 50 cm). If they pass the eye chart with any of these adds, and meet the other criteria for visual screening (see below) they can participate in the study.
- Checking for far-sightedness.
Worse acuity with a +3 dioptre add over the eye to rule out far- sightedness >3 dioptres. (Children can accommodate up to 9 dioptres and therefore could be very far-sighted and still have 20/20 acuity. If vision is equally good with and without the +3D add, the child must have far-sightedness >3D and therefore fails the screening exam.)
Ask child to hold +3D lens in front of the right eye and to read letters that are on the line representing his acuity threshold (eg, 20/20 line if the child has 20/20 acuity). If he can read those letters, he is too far-sighted and fails our screening. Repeat for the left eye (with the right eye patched).
- Titmus test of stereoacuity.
Child wears polarizing lenses and, for various rows of circles and animals, points to the one that "jumps out." Each symbol is composed of disparte lines and the measure of stereoacuity is the minimum amount of disparity that can be distinguished as separate by the brain and fused into one 3D image.
Child wears polarizing glasses. Ask the child to tickle the wings of the fly. If they do so with their fingers in the air and not touching the picture, they pass. (Some children do this incorrectly even though they have normal stereo vision and so you can proceed with the rest of the stereo test no matter what they do on the fly.) Next point to Row A of the animals and ask the child to point to the one that jumps out. If correct, proceed to Rows B and C. Then do the dots asking the child to point to the one that jumps out. Do this for each diamond. Retest any errors as soon as the child makes one and quit as soon as you are satisfied that the error is real. To verify that correct responses are really caused by stereo vision rather than spurious cues in the test, turn the card upside-down and ask the child if the animals/dots are still "jumping out", or if they are doing something different.
To pass they must say that they are pointing into the page when the card is shown upside-down. To pass, children must have a stereoacuity of at least 40 arc sec (3/3
animals correct and 9/9 circles correct). This is the minimum stereoacuity the test can measure and is considered clinically normal even though it is 40 times worse than maximum performance (1 arc second).
Worth 4 Dot test. This test must be done in the dark. Therefore, take the child to your testing room and do this test with room lights off and only a night light on or the light from a computer screen.
Child wears glasses with red filter over one eye and green filter over the other and is shown the special flash light 33 cm away. The flashlight has 1 red dot, 2 green dots, and 1 white dot. Children are asked to report how many dots they see and their colour. If children fuse, they report 4 (1 red, 2 green and 1 other--usually reported as orange). They see 4 dots because the white dot can be seen by each eye and the brain fuses that into one orange image. Children fail if they give any other response (saying 5 means the child has diplopia or double vision; saying 3 green or 2 red means the child suppresses one eye and saying 3-2-3-2-3-2 alternately means the child alternates eyes). Abnormal responses on this test mean that the child has or had an abnormal early history, usually strabismus (misaligned eyes).
For children less than 8 years old:
For 7- and 7.5-yr-olds, same as for 8-year-olds except:
- 20/25 acuity in each eye on the Lighthouse chart.
For 6-yr-olds, same as for 7-year-olds except:
Method for Cambridge Crowding Cards:
- 20/25 (6/7.5) acuity in each eye on the Lighthouse chart or on the Cambridge Crowding Cards. We prefer to use the Cambridge Crowding Cards at this age because 6-yr-olds often have difficulty coping with so many letters in a line when the going gets tough. The Cambridge Crowding Cards are a matching task where each card contains a letter surrounded by crowding letters. The task is to name or point to the central letter using a matching card.
Children stand 3 metres from the card and either read the central letter or point to the matching letter on a card that they hold. The letters get smaller across cards and the smallest central letter that children identify correctly provides a measure of crowded acuity. Acuities are given in metres and so 6/6 acuity is the same as 3/3 acuity which is the same as 20/20 on the Lighthouse. This test tends to results identical to the Lighthouse chart and hence the criteria are the same for the two tests.
For 5-yr-olds, same as that for 6-year-olds except:
For 4-yr-olds, is the same as that for older children except: