Ocular tremor (ocular microtremor) is a constant, involuntary eye

tremor A tremor is an involuntary, somewhat rhythmic, muscle contraction and relaxation involving oscillations or twitching movements of one or more body parts. It is the most common of all involuntary movements and can affect the hands, arms, eyes, fa ...

of a low amplitude and high frequency. It is a type of

fixational eye movement Fixation or visual fixation is the maintaining of the gaze on a single location. An animal can exhibit visual fixation if it possess a fovea in the anatomy of their eye. The fovea is typically located at the center of the retina and is the poin ...

that occurs in all normal people, even when the eye appears still. The

frequency Frequency is the number of occurrences of a repeating event per unit of time. It is also occasionally referred to as ''temporal frequency'' for clarity, and is distinct from ''angular frequency''. Frequency is measured in hertz (Hz) which is eq ...

of ocular microtremor has been found to range from 30 Hz to 103 Hz, and the

amplitude The amplitude of a periodic variable is a measure of its change in a single period (such as time or spatial period). The amplitude of a non-periodic signal is its magnitude compared with a reference value. There are various definitions of amplit ...

is approximately four thousandths of a degree.

Cause

Human eyes are constantly moving, even if they appear to be focused on an object. These constant

oscillation Oscillation is the repetitive or periodic variation, typically in time, of some measure about a central value (often a point of equilibrium) or between two or more different states. Familiar examples of oscillation include a swinging pendulum ...

s are called fixational eye movements, and they include ocular microtremor,

microsaccade Microsaccades are a kind of fixational eye movement. They are small, jerk-like, involuntary eye movements, similar to miniature versions of voluntary saccades. They typically occur during prolonged visual fixation (of at least several seconds), ...

s, and drift. Ocular tremor is the smallest of these movements, and it often overlaps with drift. This makes it the most difficult fixational eye movement to measure. Due to these difficulties in measurement, fewer studies have been performed on ocular microtremor, leading to the phenomenon of ocular tremor not being well-understood. Researchers are not entirely sure of the cause of ocular microtremor. It may be a result from the firing of

motor neuron A motor neuron (or motoneuron or efferent neuron) is a neuron whose cell body is located in the motor cortex, brainstem or the spinal cord, and whose axon (fiber) projects to the spinal cord or outside of the spinal cord to directly or indirectl ...

s at different times. It has also been suggested that ocular tremor is a result of the eye being balanced between opposing muscles.

Size and Measurement

Ocular microtremor is smaller than the other fixational eye movements of microsaccades and drift. It occurs between microsaccades, during the same time intervals as drift. The measured frequency and amplitude of ocular tremor have been found to vary. The frequency typically falls between 30 Hz and 103 Hz, while the amplitude has been measured to be approximately four thousandths of a degree. Specific tools, known as

piezoelectric Piezoelectricity (, ) is the electric charge that accumulates in certain solid materials—such as crystals, certain ceramics, and biological matter such as bone, DNA, and various proteins—in response to applied Stress (mechanics), mechanical s ...

probes or piezoelectric strain gauges, are commonly used to measure ocular microtremor. A

local anesthetic A local anesthetic (LA) is a medication that causes absence of pain sensation. In the context of surgery, a local anesthetic creates an absence of pain in a specific location of the body without a loss of consciousness, as opposed to a general an ...

is used for both of these invasive methods, because contact with the

sclera The sclera, also known as the white of the eye or, in older literature, as the tunica albuginea oculi, is the opaque, fibrous, protective, outer layer of the human eye containing mainly collagen and some crucial elastic fiber. In humans, and som ...

is required. While non-contact methods of measuring ocular tremor have been found, they are not typically used due to their low resolution. *

Eye movements Eye movement includes the voluntary or involuntary movement of the eyes. Eye movements are used by a number of organisms (e.g. primates, rodents, flies, birds, fish, cats, crabs, octopus) to fixate, inspect and track visual objects of inte ...

Fixational eye movements Fixation or visual fixation is the maintaining of the gaze on a single location. An animal can exhibit visual fixation if it possess a fovea in the anatomy of their eye. The fovea is typically located at the center of the retina and is the poi ...

Human eye The human eye is a sensory organ, part of the sensory nervous system, that reacts to visible light and allows humans to use visual information for various purposes including seeing things, keeping balance, and maintaining circadian rhythm. ...

Visual perception Visual perception is the ability to interpret the surrounding environment through photopic vision (daytime vision), color vision, scotopic vision (night vision), and mesopic vision (twilight vision), using light in the visible spectrum reflecte ...

Piezoelectric sensor A piezoelectric sensor is a device that uses the piezoelectric effect to measure changes in pressure, acceleration, temperature, strain, or force by converting them to an electrical charge. The prefix ''piezo-'' is Greek for 'press' or 'squeeze'. ...

References

Perception Vision {{eye-stub