Heather, do your ds' VT sessions include any kind of Brain Gym (up and moving type) exercises, Bal-a-Vis-X, or anything like that? This is just me totally guessing from thin air, but fwiw - here are a few more thoughts -
Our sensory dd had really low tone in her trunk when she was younger (still does but she's not quite as squirmy as she used to be, now she mostly slouches....). Her VT therapists always start off her sessions with a "cross-walk" where you step up high like marching and touch the opposite hand on the knee and follow a type of figure-8 path. There are a few other exercises mixed in between that have her standing up and doing Brain Gym-like activities, and I think that helped her not get overly stressed from eye strain etc.
They also have exercises they do on a mini-tramp - where she jumps and holds out her arms after a pattern that the therapist shows her. Another thing they do which she like is the Bal-a-Vis-X - it's basically just rhythmic and patterned bouncing of small balls. I think having exercises like that tossed in between the other types of exercise (where she is looking at paper up close, trying hard to match letters etc, or staring at the computer screen) helped reduce her overall sensory strain during the therapy.
They also have her sit on a bouncy ball while she does her VT.
Another thought might be to try just one type of VT exercise at home for awhile - just a few minutes each day - if you can find something that just by itself doesn't bother your ds too much. Also maybe consider just giving the VT a rest for now and trying again in 6 months or a year when your ds is a little older and might be more able to communicate what he's feeling and seeing, and also after he's been in OT longer and maybe made more progress with it.
The test I mentioned that discriminates between visual and fine motor issues is called the "Beery VMI" (VMI stands for Visual Motor Integration) and I think that there are OTs who can also administer it as well as psychologists. I wouldn't honestly worry too much about whether or not the person administering the test works with a lot of gifted children - I think in your situation I would start by first thinking through the things you see impacting your ds - does he seem to have difficulty with fine motor tasks? Vision-related tasks? Some of it might not be obvious of course, but think through what you see in everyday life and try to relate that back to the dips in scores on the WISC. If you think there is a possibility of fine motor challenges in the mix, ask his OT if they can do an assessment such as the Beery VMI to tease out what's up.
I'll also add that VT doesn't necessarily cause overnight changes, particularly if it's not a huge issue that is being worked on to begin with. And it's hard work. I think Zen Scanner is right on re the possibility that what you are seeing makes sense as his brain is being retrained to use vision to manage physical coordination.
Best wishes,
polarbear
