User:Hphan1719/NeuroPro

Fixational eye movement
When encounter an object of interest, humans, along with other animals with foveae, can direct their gaze at it. This process is called fixation. However, the visual perception starts fading away as the fixation starts happening. Thus, the nervous system provides a mechanism to restore the loss of vision due to fixation. This mechanism is called “fixational eye movements,” which forces the eyes to continuously moving. In other words, during fixation, the eyes are, by no means, at rest.

There are three types of “fixational eye movements” that today scientists agree upon: microsaccades, ocular drifts, and ocular microtremors.

Microsaccades


Microsaccades, also known as “flicks,” are saccades, involuntarily, produced during the fixation periods. They are the largest and fastest of the fixational eye movements. Like saccades, microsaccades are usually binocular, conjugate movements with comparable amplitudes and directions in both eyes. In the 1960s, scientists suggested the maximum amplitude for microsaccades should be 12 arcminutes to distinguish microsaccades and saccades. However, recent studies have shown that microsaccades can certainly exceed this value. Therefore, amplitude can no longer be used to distinguish microsaccades and saccades. The only way to distinguish microsaccades from saccades is the time they happen: during fixation.

Microsaccades have an ability to carry the retinal image from several dozen to several hundred photoreceptor widths. To maintain the visibility during fixation, they shift the retina image in a straight-line-fashion which overcomes adaption and generate neural responses to stationary stimuli in visual neurons.

Though it is still controversial and highly debated, some neuroscientists believe that microsaccades are potentially important in neurological and ophthalmic diseases since they are strongly related to many features of visual perception, attention and cognition. Research of the function of microsaccades has been controversial and highly debated. Researches in finding the purpose of microsaccades began in the 1990s. From this point on, more research and studies have been making an impact on the study of microsaccades. Research on the function of microsaccades grew at this time because of the development of non-invasive eye-movement-recording devices, the ability to record single-neuron activity in monkeys, and the use of computational processing power in the analysis of dynamic behavior. Today, researching microsaccades is a central and quick-growing topic of interest in the visual, cognitive, and oculomotor neurosciences. The interest varies from investigating the perceptual effects of microsaccades to the neural responses they induce as well as the mechanisms behind their oculomotor generation.

There are studies that use microsaccades as a diagnostic method for ADHD. The researchers measure the microsaccades movement of the participants while they are taking the Test of Variables of Attention. The adults with no medication treatments tend to blink and make more microsaccades. However, there are also trials that fail to confirm this result. Therefore, it is still arguable to state whether measuring microsaccades, though sounds interesting, can be used as a foolproof diagnostic method for ADHD.

Ocular drifts


Ocular drift is the fixational eye movement characterized by a smoother, slower, roaming motion of the eye when fixed on an object. The exact movement of ocular drift is often compared to Brownian motion, which is the random motion of a particle suspended in fluid as a result of its collision with the atoms and molecules that comprise that fluid. It is problematic to detect this type of fixational eye movement. There are studies show that ocular drifts sometimes occur simultaneously with ocular microtremors. Although the frequency of ocular drifts is usually lower than the frequency of ocular microtremors (from 20 to 40 Hz compared to from 40 to 100 Hz), it is still challenging to distinguish them in the range from 30 to 40 Hz. Moreover, there are studies show that ocular drifts occurs between periods of microsaccade movements, which is quite short since microsaccades occur and end quickly. This makes it harder to detect ocular drifts. Thus, some ocular drifts' detection may in fact detect non-drift movements.

The motion of ocular drift is related to the processing and encoding of space and time. It is also related to acquiring minute visual details of objects that are stationary, in order for these details to be further processed.

Ocular drifts were first found to be caused by an instability of the ocular motor system. However, more recent findings have shown that there are actually a number of suggested hypotheses as to why ocular drifts occur. First, ocular drifts can be caused by the uncontrollable random movements driven by neuronal or muscular noise, known as open loop scheme. Second, ocular drifts can occur to counter controlled motor variables, namely a motor control loop. Lastly, ocular drifts can be driven by retinal information, in a retino-motor control loop.

Ocular microtremors


Ocular microtremors (OMTs) are small, quick, and synchronized oscillations of the eyes occurring at a frequency in a range from 40 to 100 Hz, although they typically occur at around 90 Hz in the average healthy individual. They are characterized by their high frequency and minuscule amplitude of just a few arcseconds. Although the function of ocular microtremors is debatable and not fully known, they seem to play a role in processing of high spatial frequencies, which allows for perception of fine detail. Recent studies show that ocular microtremors have become a promising tool for determining the level of consciousness in an individual, as well as the progression of some degenerative diseases including Parkinson's disease and multiple sclerosis.

Although originally thought to stem from spontaneous firing of motor units, the origin of ocular microtremors is now believed to be in the oculomotor nuclei in the reticular formation of the brainstem. This new insight opened the possibility of using ocular tremors as a gauge for neuronal activity in that region of the central nervous system. More research must be done, but recent studies strongly suggest that decreased activity in the brainstem correlates with decreased frequency of OMTs.

Several methods of recording have been developed to observe these minuscule events, the most successful being the piezoelectric strain gauge method, which translates eye movement through a latex probe in contact with the eye leading to piezoelectric strain gauge. This method is practical in research but more practical adaptations of this technology have been developed by companies like Brainstem Biometrics for use in a clinical setting.