What’s Crowding Got To Do With It?

You’re probably seen this eye chart before, known as the ETDRS, which has nothing to do with the letters on it.  The letters in the name are an acronym for the Early Treatment of Diabetic Retinopathy Study.  The chart was made to take into account a phenomenon known as “crowding”, originally popularized in 1963 by Mert Flom and colleagues under the name “contour interaction”.  Simply put, your ability to identify objects, and in this case letters, is limited not only by the size of the letter, but by its surround.  The chart is most often used for research purposes, but can be very useful when tracking the progress of someone with amblyopia.  Simply put, a regular Snellen Eye Chart biases toward better acuity than what the amblyopic eye can see because there is so much spacing, or so little crowding between the letters.

 

We recognize this concept of crowding when we test younger children.  A child will read a line of letters, let’s say the 20/40 line “F-Z-B-D-4”, but will say the line below that on the chart is too small.  We then box off each letter and the child can read them individually “O”, “F”, “L” and so on.  So crowding or contour interaction has a physiological basis, probably occurring at the cortical level, but also has a developmental component that may be partly rooted in the development of visual attention.  There are charts of individual letters that address the attention component by surrounding the letter with “contour interaction bars”.

 

Pretty boring stuff, ay?  Not at all!  It bears directly on one of the most difficult things we have to do when reading, which is keeping print stable enough to identify the letters.  Alot more goes into that than meets the eye.  One of the earliest activities we do in VT is “Hart Chart Saccadics”.  The reason it’s so powerful is that it trains the crowding function directly.  At the outset when the patient slows considerably toward the middle of the chart, or gets lost, its because they’re into the are of maximum crowding.  We then ask them to move closer to the chart, which effectively photo-enlarges not only the letters, but more importantly the spacing between the letters.  Once they’re able to keep their place, we have them gradually move further away again, effectively increasing the contour interaction between letters.

We’ll have more to say on the subject, but in the interim, follow the crowd.