Ocular tremor
Ocular tremor (ocular microtremor) is a constant, involuntary eye tremor of a low amplitude and high frequency. It is a type of fixational eye movement that occurs in all normal people, even when the eye appears still.[1] The frequency of ocular microtremor has been found to range from 30 Hz to 103 Hz, and the amplitude is approximately four thousandths of a degree.[1][2]
Cause
Human eyes are constantly moving, even if they appear to be focused on an object.[1] These constant oscillations are called fixational eye movements, and they include ocular microtremor, microsaccades, and drift.[1][3] Ocular tremor is the smallest of these movements, and it often overlaps with drift.[1][3] This makes it the most difficult fixational eye movement to measure.[1][3] Due to these difficulties in measurement, fewer studies have been performed on ocular microtremor,[1][3] leading to the phenomenon of ocular tremor not being well-understood.[1][3]
Researchers are not entirely sure of the cause of ocular microtremor.[1] It may be a result from the firing of motor neurons at different times.[1][4] It has also been suggested that ocular tremor is a result of the eye being balanced between opposing muscles.[1]
Size and measurement
Ocular microtremor is smaller than the other fixational eye movements of microsaccades and drift.[1][3] It occurs between microsaccades, during the same time intervals as drift.[1][3] The measured frequency and amplitude of ocular tremor have been found to vary.[1][2][3] The frequency typically falls between 30 Hz and 103 Hz, while the amplitude has been measured to be approximately four thousandths of a degree.[3][1][2]
Specific tools, known as piezoelectric probes or piezoelectric strain gauges, are commonly used to measure ocular microtremor.[1] A local anesthetic is used for both of these invasive methods, because contact with the sclera is required.[1] While non-contact methods of measuring ocular tremor have been found, they are not typically used due to their low resolution.[1]