It’s been seven years since we did a seven part series on the qualitative nature of stereopsis in real space. I was reminded of this during our case conference yesterday in the office when we were brainstorming about a patient with a TBI who was trying to recapture visual readiness skills for driving. One of his challenges is that he has no reliable perception of stereoscopic localization on our standard binocular targets, and any movement of targets such as vectograms or computerized images disrupts his fragile fusion.
Inspired by the Howard- Dolman depth test, in which the patient has to judge the apparent distance of two movable rods, we discussed using two straws in space as a binocular modification of the pointer-in-straw procedure. The therapist presents the two straws side-by-side, and with variable separation on the z-axis until the patient develops the ability to correctly localize which of the two straws is closer. We purposely don’t want the patient to verbalize it, but to touch down in the hole with a pointer or pencil tip. As relates to readiness skills for spatial judgment when driving, speeding up the patient’s reaction time based on binocular spatial localization is what we’re after.
If you try it yourself, you’ll see that there’s a very noticeable difference in attempting this monocularly as compared to binocularly. You’ll need considerably more z-axis differential in one straw being close than the other, before you can tell which. Patient with compromised binocularity may show little difference in monocular vs. binocular performance at the outset. As they practice binocularly, accuracy should improve. In essence you’re improving your level of JND (just noticeable difference) for real space stereoscopic localization.