Dynamic visual acuity of normal subjects during vertical optotype and head motion.

PURPOSE To characterize the effect of passive vertical head motion on dynamic visual acuity of young, normally sighted subjects wearing telescopic spectacles, and to relate this to the velocity of images on the retina. METHODS Static visual acuity was measured without motion. Dynamic visual acuity was measured during vertical, sinusoidal motion of either optotypes or of a servo-driven rotating chair in which subjects were seated. Dynamic visual acuity for head motion was measured unaided, as well as with 1.9X, 4X, and 6X telescopic spectacles. Vertical eye movements were recorded using magnetic search coils. RESULTS During optotype motion, acuity declined with increasing velocity to a minimum of approximately 20/200 at 100 degrees/sec. Pursuit gain (eye velocity/optotype velocity) for moving optotypes was low except for optotype velocities of 20 degrees/sec of less. Dynamic visual acuity without telescopic spectacles was not sensitive to head motion. Static visual acuity improved with increasing telescopic spectacle power, but dynamic visual acuity became progressively impaired by head motion as telescopic spectacle power was increased. Compared with static visual acuity, head motion with peak velocity of 40 degrees/sec reduced acuity two-fold for 1.9X telescopic spectacles, fourfold for 4X telescopic spectacles, and eightfold for 6X telescopic spectacles. Visual vestibulo-ocular reflex gain with telescopic spectacles increased to values markedly above 1.0, but was always less than telescopic spectacle magnification. There was visual tolerance of slip velocities of 2 degrees/sec or less, above which acuity declined in proportion to the 0.6 power of retinal slip velocity. Above 2 degrees/sec, retinal slip velocity accounted for 95% of the variance in dynamic visual acuity. CONCLUSIONS These results confirm that acuity is sensitive to retinal image motion in the vertical direction, and extend this finding to indicate that sensitivity of acuity to vertical head motion during wearing of telescopic spectacles is attributable to retinal image slip velocity.