Age-related modulation of the vestibulo-ocular reflex using real and imaginary targets

In this study, we examined age-related effects upon modulation of the VOR with visual and imaginary targets during sinusoidal harmonic acceleration. Eight young (<30 years) and 5 elderly (>65 years) subjects were tested using sinusoidal whole-body rotation at 0.025 Hz, 0.1 Hz, and 0.5 Hz. Peak velocity for all trials was 50 °/s. Mean VOR gain and phase were calculated under the following paradigms: mental tasking (in darkness), visual enhancement (earth-fixed visual target), visual suppression (chair-fixed visual target), mental enhancement (earth-fixed imaginary target), and mental suppression (chair-fixed imaginary target). Test results from VOR trials in darkness with mental tasking were used as a reference for comparing results obtained from the various paradigms. Our results indicate significant changes in VOR gain for both age groups compared to VOR with mental tasking for the visually enhanced, visually suppressed, and mentally suppressed paradigms at all three frequencies. Neither group showed significant enhancement of VOR gain with an earth-fixed imaginary target at any frequency. Both groups were nearly equal in their abilities to enhance and suppress VOR gain under the visual paradigms. Younger subjects showed a significantly greater degree of gain suppression at 0.5 Hz than the older group for the mental suppression paradigm. VOR phase tended toward zero during the visual paradigms for both groups at all frequencies. For the mental imagery paradigms, VOR phase resembled values for VOR with mental tasking in the younger group at all three frequencies and in the older group at all but 0.1 Hz. We conclude that mental suppression of the VOR (using an imaginary chair-fixed target) appears to be an age-dependent phenomenon. We ascribe the inability of subjects to mentally enhance their VOR to the large amplitude rotations employed in this study. We propose that mental suppression of the VOR in both age groups involves parametric gain changes without predictable changes in phase.