Manipulation Errors in Blindfold Pointing Operation for Visual Acuity Screenings

We developed a self-testing device for screening visual acuity, in which patients used a joystick to enter the responses. Since patients kept fixation of the test chart throughout the test, the joystick was operated blindfold. Pointing errors as function of the orientation of the Landolt ring were computed by analyzing records of 457 patients. In 97 % of errors, patients misestimated the orientation of the Landolt ring by 45°, which is the orientation next to the one presented. Mismatches in counter clockwise direction were 3 times more frequent as in clockwise direction. Findings are compared to results recorded in 25 subjects who performed acuity tests by reporting verbally the orientation of presented Landolt rings. In the verbal reporting condition, error rates for orthogonal and diagonal orientations were similar. We suggest considering the limited accuracy of motor response as a major issue in the blindfolded operation of a joystick which is used in combination with a vision screener. Furthermore we suggest a statistical procedure accounting for manipulation errors in an acuity test. The combination of previous findings (Menozzi 2013, 1995) to findings reported here suggests that computerized screening devices enabling self-testing are a reliable and convenient method for large scale vision screenings.