Before writing about this, I did a cursory search and didn’t come up with anything but have a feeling that someone must have identified this phenomenon before. I bet that one of our colleagues, perhaps Dr. Curt Baxstrom or Dr. Robert Lederman who ponder visual/language conundrums, can come up with a source if there is one. For want of a better phrase, I’ll call the phenomenon “single letter errant phonemic substitution“. It is something observable clinically when measuring visual acuity while children read the Snellen chart, and occurs particularly with children around the ages of six to eight who are struggling with reading.
The substitution typically involves interchangeability of the letters “S”,”K”, and “C”. That is because the “C” can take on either the soft sound of the “S” or the hard sound of the “K”. It is most often occurs on a smaller line near the threshold of acuity, but still within the child’s ability to resolve the letter. So in the case above, the top line might be read as: “N A T C R”. The middle line might be read as: ‘O S D U N”. And the bottom line might be read as “V T C Z A”.
When the letter “C” is substituted for “K” as the child says the letters aloud, I will ask him (using the male pronoun because more boys have early language issues as compared to girls) to read it again. If the substitution is consistent, and occurs even when I zoom up the letter size, or with the letter presented in isolation, then it is likely a phonemic issue. However if there is no substitution or confusion at larger letter sizes, but only smaller ones, a visual component is likely involved.
When I suggest that a visual component may be involved, I don’t mean a resolvability issue. That is what you would expect when for example when uncompensated astigmatism puts a tail on the “V” to turn it into a “Y”. Or when misperceiving the horizontal bar of an “H” turns that letter into an “N”. Or what you would expect if blur confounds a “C” for an “O” because the gap in the “C” isn’t resolvable, or an “S” into a “B” because the loops appear closed rather than open. So the exchange of some letters for others is very understandable due to blur, and I point that out to parents, particularly when lenses result in instantaneous improvement.
My clinical impression, and again this is just an impression, is that these children have been heavily drilled in phonics and have less automaticity in visual identification. I have observed this with children who first master the Hebrew alphabet, which is more phonetically transparent than English. It is also apparent in children transitioning from the Montessori method where individual alphabet letters are identified by their sound. So my theory is that as the letter size becomes smaller there is more visual stress in identification, which errantly flips the switch into the phonetic channel. In that event, when lenses or prism alleviate visual stress and results in more accurate visual performance, the results appear almost magical.