Simple non-invasive measurement of rapid eye vibration

A simple, non-invasive method for the measurement of eye vibrations above 30 Hz is described. The method can be used in either laboratory or natural conditions, and is based on the cancellation of an illusion of motion that occurs when two nearby light sources flickering in counterphase above the flicker fusion limit are observed during eye vibration. In these conditions, the light sources appear to oscillate in space at a frequency equal to the difference between the vibration and flicker frequencies. The frequency of eye vibration can be determined by adjusting the flicker frequency until the illusion disappears (i.e., until the difference frequency becomes zero). The same set-up can also be used to determine the amplitude of eye vibration, by adjusting the spatial separation between the two light sources until the oscillation appears to be the result of their bouncing off each other upon contact. The reliability and sensitivity of this method are illustrated with data from three observers whose eyes were vibrated with a commercial massager applied onto their neck, and using three different settings for the speed of the massager.