Development and plasticity of visual and vestibular generated eye movements.

Slow phase horizontal eye movements, elicited by the vestibulo-ocular reflex (VOR) and/or visual pursuit (VP) systems, were examined in normal children (NC, 9-16 years) and normal adults (NA). At slow frequencies of sinusoidal stimulation of the body (with the head immobilized) and/or of a visual target, the NC (9-12 years) exhibited (a) immature VP and VOR Suppression and mature VOR Enhancement in the presence of a visual image; (b) immature VOR Suppression and mature VOR Enhancement in the presence of a non-visual image of the spatial location of a visual target, an extra-retinal signal; (c) inability to augment or depress the VOR gain with a non-visual image; (d) mature VOR Suppression ratios; and (e) adjustment of VOR gain to a high value. The maturational process was featured by (a) parallel development of VP and VOR Suppression; (b) modification of the VOR gain; and (c) an increasing ability to augment or depress the VOR with a non-visual image. The observations in NC (9-12) are ascribed to a maturational lag of an extra-retinal process interacting with mature retinal and vestibular processes and, hence, suggest that both the retinal and vestibular contributions to ocular stability predate the development of an extra-retinal signal. When mature, the central nervous system utilizes both extra-retinal and retinal signals to mediate adaptive regulation of VOR gain and to preserve stable visual pursuit motion and VOR Suppression. Our findings also suggest that during tasks requiring visual-vestibular interaction, VP and VOR mechanisms appear to be indirectly rather than directly coupled.