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]

Ocular microtremor tracing with burst sections underlined

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]

See also

• Eye movements
• Fixational eye movements
• Human eye
• Visual perception
• Piezoelectric sensor

References

1. Alexander RG, Martinez-Conde S (2019). "Fixational Eye Movements". In Klein C, Ettinger U (eds.). Eye Movement Research: An Introduction to its Scientific Foundations and Applications. Studies in Neuroscience, Psychology and Behavioral Economics. Cham: Springer International Publishing. pp. 73–115. doi:10.1007/978-3-030-20085-5_3. ISBN 978-3-030-20085-5.
2. Shakhnovich AR (1977). "Fixation Movements of the Eyes". The Brain and Regulation of Eye Movement. Boston, MA: Springer US. pp. 23–66. doi:10.1007/978-1-4684-6964-6_2. ISBN 978-1-4684-6966-0.
3. Alexander RG, Macknik SL, Martinez-Conde S (2018-03-13). "Microsaccade Characteristics in Neurological and Ophthalmic Disease". Frontiers in Neurology. 9: 144. doi:10.3389/fneur.2018.00144. PMC 5859063. PMID 29593642.
4. Skalicky, Simon E. (2016), "Neural Control of Eye Movements", Ocular and Visual Physiology, Singapore: Springer Singapore, pp. 251–270, doi:10.1007/978-981-287-846-5_18, ISBN 978-981-287-845-8