A technique has been developed for recording eye movements as large as three or four degrees, with a precision of about ten seconds of arc. The technique is not subject to artifacts when the head moves, or if the eye should shift in its socket, and no attachment to the eye is required. A very small spot of light is focused on the subject’s retina and is made to scan repeatedly across the optic disk. Light reflected back out of the eye is projected on to a photomultiplier tube which drives the vertical amplifier of a cathode-ray oscilloscope. The horizontal sweep of the oscilloscope is triggered in synchrony with the scanning spot. In the optic disk, the blood vessels absorb considerably more light than their whitish background. Therefore, each time the scanning spot passes over a blood vessel, a vertical deflection is registered on the oscilloscope. The distance between the beginning of the scan and this vertical deflection measures the optical position of the retina with respect to any stationary external stimulus. Changes in this distance represent movements of the eye.
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