Fixation (visual)

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Microsaccades and Ocular Drifts Two Types of Fixational Eye Movement.png
Microsaccades and Ocular Drifts

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 point of clearest vision. The species in which fixational eye movement has been verified thus far include humans, primates, cats, rabbits, turtles, salamanders, and owls. Regular eye movement alternates between saccades and visual fixations, the notable exception being in smooth pursuit, controlled by a different neural substrate that appears to have developed for hunting prey. The term "fixation" can either be used to refer to the point in time and space of focus or the act of fixating. Fixation, in the act of fixating, is the point between any two saccades, during which the eyes are relatively stationary and virtually all visual input occurs. In the absence of retinal jitter, a laboratory condition known as retinal stabilization, perceptions tend to rapidly pass away. [1] [2] To maintain visibility, the nervous system carries out a procedure called fixational eye movement, which continuously stimulates neurons in the early visual areas of the brain responding to transient stimuli. There are three categories of fixational eye movement: microsaccades, ocular drifts, and ocular microtremor. At small amplitudes the boundaries between categories become unclear, particularly between drift and tremor. [3] [4]

Contents

History

In 1738, James Jurin made the first known reference to a "trembling of the eye" that was presumably caused by fixational eye movements. [4] Robert Darwin noted in 1786 that the jiggling of color after-effects was presumably the consequence of small eye movements. Eye tracking with sufficient resolution to record fixational eye movements was developed in the 1950s. Retinal stabilization, the ability to project stabilized images on the retina, showed that retinal motion was necessary for visual perception, also in the 1950s. The field remained quiet until the 2000s, when key neurological properties of fixational eye movement were discovered and a new wave of research began. [5] [6]

Microsaccades

The lines on this image display the saccadic and microsaccadic movements of a person's eye while they looked at this face. The involuntary, micro-saccadic movement is not steady when the person's eyes are concentrated at the eyes of the woman, while the voluntary, saccadic movement goes around the periphery of the face once at any give point. Saccades and Microsaccades.jpg
The lines on this image display the saccadic and microsaccadic movements of a person's eye while they looked at this face. The involuntary, micro-saccadic movement is not steady when the person's eyes are concentrated at the eyes of the woman, while the voluntary, saccadic movement goes around the periphery of the face once at any give point.

A microsaccade, also known as a "flick", is a type of saccade. Microsaccades are the largest and fastest of the fixational eye movements. Like saccades in general, microsaccades are usually binocular, and conjugate movements with comparable amplitudes and directions in both eyes. However, the definition of microsaccade varies from study to study and no common definition has emerged. [7]

In the 1960s, scientists suggested the maximum amplitude for microsaccades should be 12 arcminutes to distinguish microsaccades and saccades. [8] However, further studies have shown that microsaccades can certainly exceed this value. [9] Newer studies have used a threshold of up to 2° to categorize microsaccades, expanding the definition by an order of magnitude. The distribution of saccade amplitudes is unimodal, giving no empirical threshold to distinguish microsaccades and saccades. Poletti et al. propose using a threshold based on the amplitude of sustained fixations and give a cutoff of 30 arcminutes or 0.5 degrees. [7]

Another way to distinguish microsaccades from saccades is by the intention of the subject when they happen. By this definition regular saccades are produced during the active and intentional exploration of the eye, during non-fixation tasks such as free viewing or visual search. Microsaccades are defined as the "involuntary saccades that occur spontaneously during intended fixation". The subjectivity of this definition has drawn criticism. [10]

Mechanism

Moving in a straight-line-fashion, microsaccades have the ability to carry the retinal image from several dozen to several hundred photoreceptor widths. Because they shift the retinal image, microsaccades overcome adaption [8] and generate neural responses to stationary stimuli in visual neurons. [11] These movements might serve the function of maintaining visibility during fixation, [8] or might be related to attentional shifts to objects in the visual field [12] or in memory, [13] might help limit binocular fixation disparity, [14] or may serve some combination of those functions.

Medical application

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. [15] Research aimed at finding the purpose of microsaccades began in the 1990s. [15] 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 led to advancements in microsaccade research. [11] [ non-primary source needed ] Today, there is growing interest in research on microsaccades. Research on microsaccades includes investigating the perceptual effects of microsaccades, recording the neural responses they induce, and tracking the mechanisms behind their oculomotor generation. It has been shown that when fixation is not explicitly enforced as it often occurs in vision research experiments, microsaccades precisely shift gaze to nearby locations of interest. [16] This behavior compensates for non-uniform vision within the foveola. [17]

Some studies suggest the use of microsaccades as a diagnostic method for ADHD. [18] [19] Adults diagnosed with ADHD but with no medication treatments tend to blink more and make more microsaccades. [19] [20] Microsaccades are also being explored as diagnostic measures for Progressive supranuclear palsy, Alzheimer's disease, autism spectrum disorder, acute hypoxia, and other conditions. [20]

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. The movement can also be compared to a random walk, characterized by random and often erratic changes in direction. [21] Ocular drifts occur incessantly during intersaccadic fixation. Although the frequency of ocular drifts is usually lower than the frequency of ocular microtremors (from 0 to 40 Hz compared to from 40 to 100 Hz), it is problematic to distinguish ocular drifts and ocular microtremors. In fact, microtremors might reflect the Brownian engine underlying the drift motion. [22] Resolution of intersaccadic eye movements is technically challenging. [6]

Brownian Motion Brownian motion large.gif
Brownian Motion

Mechanism

The motion of ocular drift is related to the processing and encoding of space and time. [23] It is also related to acquiring minute visual details of objects that are stationary, in order for these details to be further processed. [24] [25] Recent results have shown that ocular drift reformats the input signal to the retina equalizing (whitening) spatial power at non-zero temporal frequencies across a broad spatial frequency range. [26]

Medical application

Ocular drift of one type was first found to be caused by an instability of the ocular motor system.[ citation needed ] However, more recent findings suggest that there are actually a number of hypotheses as to why ocular drifts occur. First, ocular drifts can be caused by the uncontrollable random movements driven by neuronal or muscular noise. [27] Second, ocular drifts can occur to counter controlled motor variables, namely a faulty motor negative feedback loop.[ citation needed ] When the head is not immobilized, as in daily life and as is often true in eye movement recordings in the laboratory, ocular drifts compensate for the natural fixational instability of the head. [21] Ocular drifts are altered by some neurologic conditions [20] including Tourette syndrome [28] and autism spectrum disorder [29]

Ocular microtremors

Ocular microtremors (OMTs) are small, quick, and synchronized oscillations of the eyes occurring at frequencies in a range of 40 to 100 Hz, although they typically occur at around 90 Hz in the average healthy individual.[ citation needed ] 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. [26] [30] [31] Studies show that ocular microtremors have some promise as a tool for determining the level of consciousness in an individual, [32] as well as the progression of some degenerative diseases including Parkinson's disease [33] and multiple sclerosis. [34]

Ocular microtremor tracing with burst sections underlined Tremblement oculaire.jpg
Ocular microtremor tracing with burst sections underlined

Mechanism

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. [35] 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. [36]

Medical application

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 used in research settings; more practical adaptations of this technology have been developed for use in clinical settings to monitor the depth of anesthesia. [37] Despite the availability of these methods, tremor remains more difficult to measure than other fixational eye movements, and studies addressing medical applications of tremor movements are rare as a result. [20] Some studies have, nevertheless, pointed to the possibility that tremor movements may be useful in assessing the progression of degenerative diseases including Parkinson's disease [33] and multiple sclerosis. [34]

See also

Related Research Articles

<span class="mw-page-title-main">Saccade</span> Eye movement

A saccade is a quick, simultaneous movement of both eyes between two or more phases of focal points in the same direction. In contrast, in smooth-pursuit movements, the eyes move smoothly instead of in jumps. it could be associated with a shift in frequency of an emitted signal or a movement of a body part or device. Controlled cortically by the frontal eye fields (FEF), or subcortically by the superior colliculus, saccades serve as a mechanism for focal points, rapid eye movement, and the fast phase of optokinetic nystagmus. The word appears to have been coined in the 1880s by French ophthalmologist Émile Javal, who used a mirror on one side of a page to observe eye movement in silent reading, and found that it involves a succession of discontinuous individual movements.

<span class="mw-page-title-main">Vestibulo–ocular reflex</span> Reflex where rotation of the head causes eye movement to stabilize vision

The vestibulo-ocular reflex (VOR) is a reflex that acts to stabilize gaze during head movement, with eye movement due to activation of the vestibular system, it is also known as the Cervico-ocular reflex. The reflex acts to stabilize images on the retinas of the eye during head movement. Gaze is held steadily on a location by producing eye movements in the direction opposite that of head movement. For example, when the head moves to the right, the eyes move to the left, meaning the image a person sees stays the same even though the head has turned. Since slight head movement is present all the time, VOR is necessary for stabilizing vision: people with an impaired reflex find it difficult to read using print, because the eyes do not stabilise during small head tremors, and also because damage to reflex can cause nystagmus.

<span class="mw-page-title-main">Human eye</span> Sensory organ of vision

The human eye is a sensory organ in the visual system that reacts to visible light allowing eyesight. Other functions include maintaining the circadian rhythm, and keeping balance.

<span class="mw-page-title-main">Superior colliculus</span> Structure in the midbrain

In neuroanatomy, the superior colliculus is a structure lying on the roof of the mammalian midbrain. In non-mammalian vertebrates, the homologous structure is known as the optic tectum or optic lobe. The adjective form tectal is commonly used for both structures.

<span class="mw-page-title-main">Eye tracking</span> Measuring the point of gaze or motion of an eye relative to the head

Eye tracking is the process of measuring either the point of gaze or the motion of an eye relative to the head. An eye tracker is a device for measuring eye positions and eye movement. Eye trackers are used in research on the visual system, in psychology, in psycholinguistics, marketing, as an input device for human-computer interaction, and in product design. In addition, eye trackers are increasingly being used for assistive and rehabilitative applications such as controlling wheelchairs, robotic arms, and prostheses. Recently, eye tracking has been examined as a tool for the early detection of autism spectrum disorder. There are several methods for measuring eye movement, with the most popular variant using video images to extract eye position. Other methods use search coils or are based on the electrooculogram.

<span class="mw-page-title-main">Ocular tremor</span>

Ocular tremor 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. 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.

<span class="mw-page-title-main">Eye movement</span> Movement of the eyes

Eye movement includes the voluntary or involuntary movement of the eyes. Eye movements are used by a number of organisms to fixate, inspect and track visual objects of interests. A special type of eye movement, rapid eye movement, occurs during REM sleep.

Oscillopsia is a visual disturbance in which objects in the visual field appear to oscillate. The severity of the effect may range from a mild blurring to rapid and periodic jumping. Oscillopsia is an incapacitating condition experienced by many patients with neurological disorders. It may be the result of ocular instability occurring after the oculomotor system is affected, no longer holding images steady on the retina. A change in the magnitude of the vestibulo-ocular reflex due to vestibular disease can also lead to oscillopsia during rapid head movements. Oscillopsia may also be caused by involuntary eye movements such as nystagmus, or impaired coordination in the visual cortex and is one of the symptoms of superior canal dehiscence syndrome. Those affected may experience dizziness and nausea. Oscillopsia can also be used as a quantitative test to document aminoglycoside toxicity. Permanent oscillopsia can arise from an impairment of the ocular system that serves to maintain ocular stability. Paroxysmal oscillopsia can be due to an abnormal hyperactivity in the peripheral ocular or vestibular system.

Dysmetria is a lack of coordination of movement typified by the undershoot or overshoot of intended position with the hand, arm, leg, or eye. It is a type of ataxia. It can also include an inability to judge distance or scale.

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, not only in humans, but also in animals with foveal vision. Microsaccade amplitudes vary from 2 to 120 arcminutes. The first empirical evidence for their existence was provided by Robert Darwin, the father of Charles Darwin.

<span class="mw-page-title-main">Smooth pursuit</span> Type of eye movement used for closely following a moving object

In the scientific study of vision, smooth pursuit describes a type of eye movement in which the eyes remain fixated on a moving object. It is one of two ways that visual animals can voluntarily shift gaze, the other being saccadic eye movements. Pursuit differs from the vestibulo-ocular reflex, which only occurs during movements of the head and serves to stabilize gaze on a stationary object. Most people are unable to initiate pursuit without a moving visual signal. The pursuit of targets moving with velocities of greater than 30°/s tends to require catch-up saccades. Smooth pursuit is asymmetric: most humans and primates tend to be better at horizontal than vertical smooth pursuit, as defined by their ability to pursue smoothly without making catch-up saccades. Most humans are also better at downward than upward pursuit. Pursuit is modified by ongoing visual feedback.

<span class="mw-page-title-main">Frontal eye fields</span> Region of the frontal cortex of the brain

The frontal eye fields (FEF) are a region located in the frontal cortex, more specifically in Brodmann area 8 or BA8, of the primate brain. In humans, it can be more accurately said to lie in a region around the intersection of the middle frontal gyrus with the precentral gyrus, consisting of a frontal and parietal portion. The FEF is responsible for saccadic eye movements for the purpose of visual field perception and awareness, as well as for voluntary eye movement. The FEF communicates with extraocular muscles indirectly via the paramedian pontine reticular formation. Destruction of the FEF causes deviation of the eyes to the ipsilateral side.

Within computer technology, the gaze-contingency paradigm is a general term for techniques allowing a computer screen display to change in function depending on where the viewer is looking. Gaze-contingent techniques are part of the eye movement field of study in psychology.

Eye–hand coordination is the coordinated motor control of eye movement with hand movement and the processing of visual input to guide reaching and grasping along with the use of proprioception of the hands to guide the eyes, a modality of multisensory integration. Eye–hand coordination has been studied in activities as diverse as the movement of solid objects such as wooden blocks, archery, sporting performance, music reading, computer gaming, copy-typing, and even tea-making. It is part of the mechanisms of performing everyday tasks; in its absence, most people would not be able to carry out even the simplest of actions such as picking up a book from a table.

A visual prosthesis, often referred to as a bionic eye, is an experimental visual device intended to restore functional vision in those with partial or total blindness. Many devices have been developed, usually modeled on the cochlear implant or bionic ear devices, a type of neural prosthesis in use since the mid-1980s. The idea of using electrical current to provide sight dates back to the 18th century, discussed by Benjamin Franklin, Tiberius Cavallo, and Charles LeRoy.

In vision science, stabilized Images are images that remain immobile on the retina. Under natural viewing conditions, the eyes are always in motion. Small eye movements continually occur even when attempting fixation. Experiments in the early 1950s established that stabilized images result in the fading and disappearance of the visual percept, possibly due to retinal adaptation to a stationary field. In 2007, studies indicated that stabilizing vision between saccades selectively impairs vision of fine spatial detail.

Chronostasis is a type of temporal illusion in which the first impression following the introduction of a new event or task-demand to the brain can appear to be extended in time. For example, chronostasis temporarily occurs when fixating on a target stimulus, immediately following a saccade. This elicits an overestimation in the temporal duration for which that target stimulus was perceived. This effect can extend apparent durations by up to half a second and is consistent with the idea that the visual system models events prior to perception.

<span class="mw-page-title-main">Parafovea</span>

Parafovea or the parafoveal belt is a region in the retina that circumscribes the fovea and is part of the macula lutea. It is circumscribed by the perifovea.

Michele Rucci is an Italian born neuroscientist and biomedical engineer who studies visual perception. He is a Professor of Brain and Cognitive Sciences and member of the Center for Visual Science at the University of Rochester.

Binocular switch suppression (BSS) is a technique to suppress usually salient images from an individual's awareness, a type of experimental manipulation used in visual perception and cognitive neuroscience. In BSS, two images of differing signal strengths are repetitively switched between the left and right eye at a constant rate of 1 Hertz. During this process of switching, the image of lower contrast and signal strength is perceptually suppressed for a period of time.

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