Graded changes in balancing behaviour as a function of visual acuity.

In a dynamic postural task, visual information plays a fundamental role in the selection of the balancing strategy. While standing on a platform oscillating in the antero- posterior direction, subjects almost fix their head in space when vision is allowed and oscillate with the platform with eyes closed. We investigated two competing hypotheses regarding the relationship between visual acuity and balance control strategy. One hypothesis refers to the existence of a threshold value of visual acuity as a turning point between the eyes-open and eyes-closed strategy. The other assumes that the change from eyes-open to eyes-closed behavior is continuous and parallels the progressive worsening of visual acuity. Ten subjects balanced on the mobile platform wearing an examination frame and a facemask occluding peripheral vision. Seven different test lenses were used in different trials to modify visual acuity, from a visus value of 10/10 to severely blurred vision. Head stabilization in space progressively worsened with the decrease in visual acuity and turned toward the eyes-closed behavior when vision was blurred. The increase in head oscillation as a function of visual acuity was best fitted by a logarithmic function. In five of the subjects, additional trials were performed without facemask, to add peripheral vision to each visual acuity level, and with black lenses to allow peripheral vision alone. Addition of peripheral vision gave a significant contribution to head stabilization. With peripheral vision alone, head stabilization was intermediate between the eyes-closed and 10/10 visus value condition. We conclude that, in order to stabilize the head in space, visual information of the environment must be definite and worsening of central vision leads to a graded modification of the ‘head fixed in space’ behavior. Thus, the more conservative hypothesis of two different fundamental balancing strategies is not supported. Instead, the body exhibits a continuous mode of balancing patterns as a function of visual acuity. The findings support the notion that the central mechanism for head stabilization operates through linear integration of the central-field visual input with the general somesthetic feedback.