Microsaccades: small steps on a long way

Suppressive interactions underlying visually evoked fixational saccades

Small saccades occur frequently during fixation, and are coupled to changes in visual stimulation and cognitive state. Neurophysiologically, fixational saccades reflect neural activity near the foveal region of a continuous visuomotor map. It is well known that competitive interactions between neurons within visuomotor maps contribute to target selection for large saccades. Here we asked how such interactions in visuomotor maps shape the rate and direction of small fixational saccades. We measured fixational saccades during periods of prolonged fixation while presenting pairs of visual stimuli (parafoveal: 0.8° eccentricity; peripheral: 5° eccentricity) of various contrasts. Fixational saccade direction was biased toward locations of parafoveal stimuli but not peripheral stimuli, ∼100-250ms following stimulus onset. The rate of fixational saccades toward parafoveal stimuli (congruent saccades) increased systematically with parafoveal stimulus contrast, and was suppressed by the simultaneous presentation of a peripheral stimulus. The suppression was best characterized as a combination of two processes: a subtractive suppression of the overall fixational saccade rate and a divisive suppression of the direction bias. These results reveal the nature of suppressive interactions within visuomotor maps and constrain models of the population code for fixational saccades.

On the possible roles of microsaccades and drifts in visual perception

During natural viewing large saccades shift the visual gaze from one target to another every few hundreds of milliseconds. The role of microsaccades (MSs), small saccades that show up during long fixations, is still debated. A major debate is whether MSs are used to redirect the visual gaze to a new location or to encode visual information through their movement. We argue that these two functions cannot be optimized simultaneously and present several pieces of evidence suggesting that MSs redirect the visual gaze and that the visual details are sampled and encoded by ocular drifts. We show that drift movements are indeed suitable for visual encoding. Yet, it is not clear to what extent drift movements are controlled by the visual system, and to what extent they interact with saccadic movements. We analyze several possible control schemes for saccadic and drift movements and propose experiments that can discriminate between them. We present the results of preliminary analyses of existing data as a sanity check to the testability of our predictions.

Why have microsaccades become larger? Investigating eye deformations and detection algorithms

The reported size of microsaccades is considerably larger today compared to the initial era of microsaccade studies during the 1950s and 1960s. We investigate whether this increase in size is related to the fact that the eye-trackers of today measure different ocular structures than the older techniques, and that the movements of these structures may differ during a microsaccade. In addition, we explore the impact such differences have on subsequent analyzes of the eye-tracker signals. In Experiment I, the movement of the pupil as well as the first and fourth Purkinje reflections were extracted from series of eye images recorded during a fixation task. Results show that the different ocular structures produce different microsaccade signatures. In Experiment II, we found that microsaccade amplitudes computed with a common detection algorithm were larger compared to those reported by two human experts. The main reason was that the overshoots were not systematically detected by the algorithm and therefore not accurately accounted for. We conclude that one reason to why the reported size of microsaccades has increased is due to the larger overshoots produced by the modern pupil-based eye-trackers compared to the systems used in the classical studies, in combination with the lack of a systematic algorithmic treatment of the overshoot. We hope that awareness of these discrepancies in microsaccade dynamics across eye structures will lead to more generally accepted definitions of microsaccades.

Fixational eye movement correction of blink-induced gaze position errors

Our eyes move continuously. Even when we attempt to fix our gaze, we produce “fixational” eye movements including microsaccades, drift and tremor. The potential role of microsaccades versus drifts in the control of eye position has been debated for decades and remains in question today. Here we set out to determine the corrective functions of microsaccades and drifts on gaze-position errors due to blinks in non-human primates (Macaca mulatta) and humans. Our results show that blinks contribute to the instability of gaze during fixation, and that microsaccades, but not drifts, correct fixation errors introduced by blinks. These findings provide new insights about eye position control during fixation, and indicate a more general role of microsaccades in fixation correction than thought previously.

Fine-scale plasticity of microscopic saccades

When asked to maintain their gaze steady on a given location, humans continually perform microscopic eye movements, including fast gaze shifts known as microsaccades. It has long been speculated that these movements may contribute to the maintenance of fixation, but evidence has remained contradictory. We used a miniaturized version of saccadic adaptation, an experimental procedure by which motor control of saccades is modified through intrasaccadic displacements of the target. We found that the statistical distribution of microsaccade amplitudes changes after brief exposure to systematic shifts of the fixation point during microsaccade occurrence. Shifts in the same directions as microsaccades produce movements with larger amplitudes, whereas shifts against microsaccade directions result in smaller movements. Our findings show that microsaccades are precisely monitored during fixation and that their motor program is modified if the postsaccadic target position is not at the expected retinal location. These results demonstrate that saccadic adaptation occurs even when the stimulus is already close to the foveal center and precise execution of the movement may not be critical. They support the proposal that adaptation is necessary to maintain a consistent relationship between motor control and its visual consequences and that the representation of space is intrinsically multimodal, even during fixation.

Closed-loop optical stabilization and digital image registration in adaptive optics scanning light ophthalmoscopy

Eye motion is a major impediment to the efficient acquisition of high resolution retinal images with the adaptive optics (AO) scanning light ophthalmoscope (AOSLO). Here we demonstrate a solution to this problem by implementing both optical stabilization and digital image registration in an AOSLO. We replaced the slow scanning mirror with a two-axis tip/tilt mirror for the dual functions of slow scanning and optical stabilization. Closed-loop optical stabilization reduced the amplitude of eye-movement related-image motion by a factor of 10-15. The residual RMS error after optical stabilization alone was on the order of the size of foveal cones: ~1.66-2.56 μm or ~0.34-0.53 arcmin with typical fixational eye motion for normal observers. The full implementation, with real-time digital image registration, corrected the residual eye motion after optical stabilization with an accuracy of ~0.20-0.25 μm or ~0.04-0.05 arcmin RMS, which to our knowledge is more accurate than any method previously reported.

Characteristics of fixational eye movements in people with macular disease

PURPOSE

Fixation stability is known to be poor for people with macular disease and has been suggested as a contributing factor for the poor visual performance of these individuals. In this study, we examined the characteristics of the different components of fixational eye movements and determined the component that plays a major role in limiting fixation stability in people with macular disease.

METHODS

Sixteen observers with macular disease and 14 older adults with normal vision (control observers) monocularly fixated a small cross presented using a Rodenstock scanning laser ophthalmoscope, for trials of 30 seconds. The retinal image and the position of the cross on the retina were recorded digitally. Eye movements were extracted from the recorded videos at a sampling rate of 540 Hz using a cross-correlation technique. A velocity criterion of 8°/s was used to differentiate between slow drifts and microsaccades.

RESULTS

Observers with macular disease demonstrated higher fixation instability, larger amplitudes of slow drifts and microsaccades, and lower drift velocities, when compared with older adults with normal vision. The velocity and the rate of microsaccades were comparable between the two groups of observers. Multiple linear regression analysis showed that the amplitude of microsaccades, and to a smaller extent, the amplitude of slow drifts, play a major role in limiting fixation stability.

CONCLUSIONS

Fixation stability in people with macular disease is primarily limited by the amplitude of microsaccades, implying that rehabilitative strategies targeted at reducing the amplitude of microsaccades should improve fixation stability, and may lead to improved visual functions.

Fixational eye movements and binocular vision

During attempted visual fixation, small involuntary eye movements-called fixational eye movements-continuously change of our gaze's position. Disagreement between the left and right eye positions during such motions can produce diplopia (double vision). Thus, the ability to properly coordinate the two eyes during gaze fixation is critical for stable perception. For the last 50 years, researchers have studied the binocular characteristics of fixational eye movements. Here we review classical and recent studies on the binocular coordination (i.e., degree of conjugacy) of each fixational eye movement type: microsaccades, drift and tremor, and its perceptual contribution to increasing or reducing binocular disparity. We also discuss how amblyopia and other visual pathologies affect the binocular coordination of fixational eye movements.

Mapping the perceptual grain of the human retina

In humans, experimental access to single sensory receptors is difficult to achieve, yet it is crucial for learning how the signals arising from each receptor are transformed into perception. By combining adaptive optics microstimulation with high-speed eye tracking, we show that retinal function can be probed at the level of the individual cone photoreceptor in living eyes. Classical psychometric functions were obtained from cone-sized microstimuli targeted to single photoreceptors. Revealed psychophysically, the cone mosaic also manifests a variable sensitivity to light across its surface that accords with a simple model of cone light capture. Because this microscopic grain of vision could be detected on the perceptual level, it suggests that photoreceptors can act individually to shape perception, if the normally suboptimal relay of light by the eye's optics is corrected. Thus the precise arrangement of cones and the exact placement of stimuli onto those cones create the initial retinal limits on signals mediating spatial vision.

ADHD subjects fail to suppress eye blinks and microsaccades while anticipating visual stimuli but recover with medication

Oculomotor behavior and parameters are known to be affected by the allocation of attention and could potentially be used to investigate attention disorders. We explored the oculomotor markers of Attention-deficit/hyperactivity disorder (ADHD) that are involuntary and quantitative and that could be used to reveal the core-affected mechanisms, as well as be used for differential diagnosis. We recorded eye movements in a group of 22 ADHD-diagnosed patients with and without medication (methylphenidate) and in 22 control observers while performing the test of variables of attention (t.o.v.a.). We found that the average microsaccade and blink rates were higher in the ADHD group, especially in the time interval around stimulus onset. These rates increased monotonically over session time for both groups, but with significantly faster increments in the unmedicated ADHD group. With medication, the level and time course of the microsaccade rate were fully normalized to the control level, regardless of the time interval within trials. In contrast, the pupil diameter decreased over time within sessions and significantly increased above the control level with medication. We interpreted the suppression of microsaccades and eye blinks around the stimulus onset as reflecting a temporal anticipation mechanism for the transient allocation of attention, and their overall rates as inversely reflecting the level of arousal. We suggest that ADHD subjects fail to maintain sufficient levels of arousal during a simple and prolonged task, which limits their ability to dynamically allocate attention while anticipating visual stimuli. This impairment normalizes with medication and its oculomotor quantification could potentially be used for differential diagnosis.

Active vision: microsaccades direct the eye to where it matters most

Even in the most sensitive part of human retina, the fovea, perception is not uniform. To compensate for such non-uniformity, tiny fixational microsaccades direct the optimal foveal locus to relevant parts of the fixated scene, similarly to larger exploratory saccades but on a miniature scale.

Vibrating Makes for Better Seeing: From the Fly's Micro-Eye Movements to Hyperacute Visual Sensors

Active vision means that visual perception not only depends closely on the subject’s own movements, but that these movements actually contribute to the visual perceptual processes. Vertebrates’ and invertebrates’ eye movements are probably part of an active visual process, but their exact role still remains to be determined. In this paper, studies on the retinal micro-movements occurring in the compound eye of the fly are reviewed. Several authors have located and identified the muscles involved in these small retinal movements. Others have established that these retinal micro-movements occur in walking and flying flies, but their exact functional role still remains to be determined. Many robotic studies have been performed in which animals’ (flies’ and spiders’) miniature eye movements have been modeled, simulated, and even implemented mechanically. Several robotic platforms have been endowed with artificial visual sensors performing periodic micro-scanning movements. Artificial eyes performing these active retinal micro-movements have some extremely interesting properties, such as hyperacuity and the ability to detect very slow movements (motion hyperacuity). The fundamental role of miniature eye movements still remains to be described in detail, but several studies on natural and artificial eyes have advanced considerably toward this goal.

Highly informative natural scene regions increase microsaccade production during visual scanning

Classical image statistics, such as contrast, entropy, and the correlation between central and nearby pixel intensities, are thought to guide ocular fixation targeting. However, these statistics are not necessarily task relevant and therefore do not provide a complete picture of the relationship between informativeness and ocular targeting. Moreover, it is not known whether either informativeness or classical image statistics affect microsaccade production; thus, the role of microsaccades in information acquisition is also unknown. The objective quantification of the informativeness of a scene region is a major challenge, because it can vary with both image features and the task of the viewer. Thus, previous definitions of informativeness suffered from subjectivity and inconsistency across studies. Here we developed an objective measure of informativeness based on fixation consistency across human observers, which accounts for both bottom-up and top-down influences in ocular targeting. We then analyzed fixations in more versus less informative image regions in relation to classical statistics. Observers generated more microsaccades on more informative than less informative image regions, and such regions also exhibited low redundancy in their classical statistics. Increased microsaccade production was not explained by increased fixation duration, suggesting that the visual system specifically uses microsaccades to heighten information acquisition from informative regions.

The neurophysiological index of visual working memory maintenance is not due to load dependent eye movements

The Contralateral Delayed Activity (CDA) is slow negative potential found during a variety of tasks, providing an important measure of the representation of information in visual working memory. However, it is studied using stimulus arrays in which the to-be-remembered objects are shown in the periphery of the left or the right visual field. Our goal was to determine whether fixational eye movements in the direction of the memoranda might underlie the CDA. We found that subjects' gaze was shifted toward the visual field of the memoranda during the retention interval, with its magnitude increasing with the set size. However, the CDA was clearly observed even when the subjects' gaze shifts were absent. In addition, the magnitude of the subjects' gaze shifts was unrelated to their visual working memory capacity measured with behavioral data, unlike the CDA. Finally, the onset latency of the set size dependent eye movements followed the onset of the set size dependent CDA. Thus, our findings clearly show that the CDA does not represent a simple inability to maintain fixation during visual working memory maintenance, but that this neural index of representation in working memory appears to induce eye movements toward the locations of the objects being remembered.

Distinctive features of microsaccades in Alzheimer's disease and in mild cognitive impairment

During visual fixation, the eyes are never completely still, but produce small involuntary movements, called "fixational eye movements," including microsaccades, drift, and tremor. In certain neurological disorders, attempted fixation results in abnormal fixational eye movements with distinctive characteristics. Thus, determining how normal fixation differs from pathological fixation has the potential to aid early and differential noninvasive diagnosis of neurological disease as well as the quantification of its progression and response to treatment. Here, we recorded the eye movements produced by patients with Alzheimer's disease, patients with mild cognitive impairment, and healthy age-matched individuals during attempted fixation. We found that microsaccade magnitudes, velocities, durations, and intersaccadic intervals were comparable in the three subject groups, but microsaccade direction differed in patients versus healthy subjects. Our results indicate that microsaccades are more prevalently oblique in patients with Alzheimer's disease or mild cognitive impairment than in healthy subjects. These findings extended to those microsaccades paired in square-wave jerks, supporting the hypothesis that microsaccades and square-wave jerks form a continuum, both in healthy subjects and in neurological patients.

Figure-ground processing during fixational saccades in V1: indication for higher-order stability

In a typical visual scene we continuously perceive a "figure" that is segregated from the surrounding "background" despite ongoing microsaccades and small saccades that are performed when attempting fixation (fixational saccades [FSs]). Previously reported neuronal correlates of figure-ground (FG) segregation in the primary visual cortex (V1) showed enhanced activity in the "figure" along with suppressed activity in the noisy "background." However, it is unknown how this FG modulation in V1 is affected by FSs. To investigate this question, we trained two monkeys to detect a contour embedded in a noisy background while simultaneously imaging V1 using voltage-sensitive dyes. During stimulus presentation, the monkeys typically performed 1-3 FSs, which displaced the contour over the retina. Using eye position and a 2D analytical model to map the stimulus onto V1, we were able to compute FG modulation before and after each FS. On the spatial cortical scale, we found that, after each FS, FG modulation follows the stimulus retinal displacement and "hops" within the V1 retinotopic map, suggesting visual instability. On the temporal scale, FG modulation is initiated in the new retinotopic position before it disappeared from the old retinotopic position. Moreover, the FG modulation developed faster after an FS, compared with after stimulus onset, which may contribute to visual stability of FG segregation, along the timeline of stimulus presentation. Therefore, despite spatial discontinuity of FG modulation in V1, the higher-order stability of FG modulation along time may enable our stable and continuous perception.

Capture of fixation by rotational flow; a deterministic hypothesis regarding scaling and stochasticity in fixational eye movements

Visual scan paths exhibit complex, stochastic dynamics. Even during visual fixation, the eye is in constant motion. Fixational drift and tremor are thought to reflect fluctuations in the persistent neural activity of neural integrators in the oculomotor brainstem, which integrate sequences of transient saccadic velocity signals into a short term memory of eye position. Despite intensive research and much progress, the precise mechanisms by which oculomotor posture is maintained remain elusive. Drift exhibits a stochastic statistical profile which has been modeled using random walk formalisms. Tremor is widely dismissed as noise. Here we focus on the dynamical profile of fixational tremor, and argue that tremor may be a signal which usefully reflects the workings of oculomotor postural control. We identify signatures reminiscent of a certain flavor of transient neurodynamics; toric traveling waves which rotate around a central phase singularity. Spiral waves play an organizational role in dynamical systems at many scales throughout nature, though their potential functional role in brain activity remains a matter of educated speculation. Spiral waves have a repertoire of functionally interesting dynamical properties, including persistence, which suggest that they could in theory contribute to persistent neural activity in the oculomotor postural control system. Whilst speculative, the singularity hypothesis of oculomotor postural control implies testable predictions, and could provide the beginnings of an integrated dynamical framework for eye movements across scales.

Low-level and high-level modulations of fixational saccades and high frequency oscillatory brain activity in a visual object classification task

Until recently induced gamma-band activity (GBA) was considered a neural marker of cortical object representation. However, induced GBA in the electroencephalogram (EEG) is susceptible to artifacts caused by miniature fixational saccades. Recent studies have demonstrated that fixational saccades also reflect high-level representational processes. Do high-level as opposed to low-level factors influence fixational saccades? What is the effect of these factors on artifact-free GBA? To investigate this, we conducted separate eye tracking and EEG experiments using identical designs. Participants classified line drawings as objects or non-objects. To introduce low-level differences, contours were defined along different directions in cardinal color space: S-cone-isolating, intermediate isoluminant, or a full-color stimulus, the latter containing an additional achromatic component. Prior to the classification task, object discrimination thresholds were measured and stimuli were scaled to matching suprathreshold levels for each participant. In both experiments, behavioral performance was best for full-color stimuli and worst for S-cone isolating stimuli. Saccade rates 200-700 ms after stimulus onset were modulated independently by low and high-level factors, being higher for full-color stimuli than for S-cone isolating stimuli and higher for objects. Low-amplitude evoked GBA and total GBA were observed in very few conditions, showing that paradigms with isoluminant stimuli may not be ideal for eliciting such responses. We conclude that cortical loops involved in the processing of objects are preferentially excited by stimuli that contain achromatic information. Their activation can lead to relatively early exploratory eye movements even for foveally-presented stimuli.

Analysis of microsaccades and pupil dilation reveals a common decisional origin during visual search

During free viewing visual search, observers often refixate the same locations several times before and after target detection is reported with a button press. We analyzed the rate of microsaccades in the sequence of refixations made during visual search and found two important components. One related to the visual content of the region being fixated; fixations on targets generate more microsaccades and more microsaccades are generated for those targets that are more difficult to disambiguate. The other empathizes non-visual decisional processes; fixations containing the button press generate more microsaccades than those made on the same target but without the button press. Pupil dilation during the same refixations reveals a similar modulation. We inferred that generic sympathetic arousal mechanisms are part of the articulated complex of perceptual processes governing fixational eye movements.

Simulation of drug distribution in the vitreous body after local drug application into intact vitreous body and in progress of posterior vitreous detachment

Intravitreal injections and drug-loaded implants are current approaches to treat diseases of the posterior eye. To investigate the release of active agents and their distribution in the vitreous body, a new test system was developed that enables a realistic simulation of eye motions. It is called the eye movement system (EyeMoS). In combination with a previously developed model containing a polyacrylamide gel as a substitute for the vitreous body, this new system enables the characterization of the influence of eye motions on drug distribution within the vitreous body. In the presented work, the distribution of fluorescence-tagged model drugs of different molecular weight within the simulated vitreous was examined under movement with the EyeMoS and without movement. By replacing a part of the gel in the simulated vitreous body with buffer, the influence of the progress of posterior vitreous detachment (PVD) on the distribution of these model substances was also studied. The results indicate that convective forces may be of predominate influence on initial drug distribution. The impact of these forces on drug transport increases with simulated progression of PVD. Using the EyeMoS, the investigation of release and distribution from intravitreal drug delivery systems becomes feasible under biorelevant conditions.

On the dissociation between microsaccade rate and direction after peripheral cues: microsaccadic inhibition revisited

Microsaccades during fixation exhibit distinct time courses of frequency and direction modulations after stimulus onsets, but the mechanisms for these modulations are unresolved. On the one hand, microsaccade rate drops within <100 ms after stimulus onset, a phenomenon described as microsaccadic inhibition. On the other, the directions of the rare microsaccades that do occur during inhibition are, surprisingly, the most highly correlated with stimulus location. Here we show, using a combined computational and experimental approach, that these apparently dichotomous observations can simply result from a single mechanism: the phase resetting by stimulus onsets of ongoing microsaccadic oscillatory rhythms during fixation. Using experiments on monkeys and model simulations, we show that stimulus onsets act as countermanding stimuli, such as those in large saccadic countermanding tasks: they cancel an upcoming movement program and start a competing one, thus implementing phase resetting. We also show that the rare microsaccades occurring during microsaccadic inhibition are simply noncanceled movements in the countermanding framework and that they reflect the instantaneous state of visual representations expected in spatial maps representing stimuli. Remarkably, a dynamic interaction between the efficacy of the countermanding process and the metrics of the microsaccade being countermanded not only explains microsaccade rate changes, but it also predicts the time course patterns of microsaccade directions and amplitudes. Our parsimonious framework for understanding microsaccadic modulations around stimulus onsets allows analyzing microsaccades (and larger saccades) using the extensive toolkit of oscillatory dynamical systems often used for modeling spiking neurons, and it constrains neural models of microsaccade triggering.

Fixational eye movements during viewing of dynamic natural scenes

Even during periods of fixation our eyes undergo small amplitude movements. These movements are thought to be essential to the visual system because neural responses rapidly fade when images are stabilized on the retina. The considerable recent interest in fixational eye movements (FEMs) has thus far concentrated on idealized experimental conditions with artificial stimuli and restrained head movements, which are not necessarily a suitable model for natural vision. Natural dynamic stimuli, such as movies, offer the potential to move beyond restrictive experimental settings to probe the visual system with greater ecological validity. Here, we study FEMs recorded in humans during the unconstrained viewing of a dynamic and realistic visual environment, revealing that drift trajectories exhibit the properties of a random walk with memory. Drifts are correlated at short time scales such that the gaze position diverges from the initial fixation more quickly than would be expected for an uncorrelated random walk. We propose a simple model based on the premise that the eye tends to avoid retracing its recent steps to prevent photoreceptor adaptation. The model reproduces key features of the observed dynamics and enables estimation of parameters from data. Our findings show that FEM correlations thought to prevent perceptual fading exist even in highly dynamic real-world conditions.

Comparing thoracic and intra-nasal pressure transients to monitor active odor sampling during odor-guided decision making in the mouse

BACKGROUND

Recording of physiological parameters in behaving mice has seen an immense increase over recent years driven by, for example, increased miniaturization of recording devices. One parameter particularly important for odorant-driven behaviors is the breathing frequency, since the latter dictates the rate of odorant delivery to the nasal cavity and the olfactory receptor neurons located therein.

NEW METHOD

Typically, breathing patterns are monitored by either measuring the breathing-induced temperature or pressure changes in the nasal cavity. Both require the implantation of a nasal cannula and tethering of the mouse to either a cable or tubing. To avoid these limitations we used an implanted pressure sensor which reads the thoracic pressure and transmits the data telemetrically, thus making it suitable for experiments which require a freely moving animal.

RESULTS

Mice performed a Go/NoGo odorant-driven behavioral task with the implanted pressure sensor, which proved to work reliably to allow recording of breathing signals over several weeks from a given animal.

COMPARISON TO EXISTING METHOD(S)

We simultaneously recorded the thoracic and nasal pressure changes and found that measuring the thoracic pressure change yielded similar results compared to measurements of nasal pressure changes.

CONCLUSION

Telemetrically recorded breathing signals are a feasible method to monitor odorant-guided behavioral changes in breathing rates. Its advantages are most significant when recording from a freely moving animal over several weeks. The advantages and disadvantages of different methods to record breathing patterns are discussed.

Microscopic eye movements compensate for nonhomogeneous vision within the fovea

Humans rely on the fovea, the small region of the retina where receptors are most densely packed, for seeing fine spatial detail. Outside the fovea, it is well established that a variety of visual functions progressively decline with eccentricity. In contrast, little is known about how vision varies within the central fovea, as incessant microscopic eye movements prevent isolation of adjacent foveal locations. Using a new method for restricting visual stimulation to a selected retinal region, we examined the discrimination of fine patterns at different eccentricities within the foveola. We show that high-acuity judgments are impaired when stimuli are presented just a few arcminutes away from the preferred retinal locus of fixation. Furthermore, we show that this dependence on eccentricity is normally counterbalanced by the occurrence of precisely directed microsaccades, which bring the preferred fixation locus onto the stimulus. Thus, contrary to common assumptions, vision is not uniform within the foveola, but targeted microscopic eye movements compensate for this lack of homogeneity. Our results reveal that microsaccades, like larger saccades, enable examination of the stimulus at a finer level of detail and suggest that a reduced precision in oculomotor control may be responsible for the visual acuity impairments observed in various disorders.

Postmicrosaccadic enhancement of slow eye movements

Active sensation poses unique challenges to sensory systems because moving the sensor necessarily alters the input sensory stream. Sensory input quality is additionally compromised if the sensor moves rapidly, as during rapid eye movements, making the period immediately after the movement critical for recovering reliable sensation. Here, we studied this immediate postmovement interval for the case of microsaccades during fixation, which rapidly jitter the "sensor" exactly when it is being voluntarily stabilized to maintain clear vision. We characterized retinal-image slip in monkeys immediately after microsaccades by analyzing postmovement ocular drifts. We observed enhanced ocular drifts by up to ~28% relative to premicrosaccade levels, and for up to ~50 ms after movement end. Moreover, we used a technique to trigger full-field image motion contingent on real-time microsaccade detection, and we used the initial ocular following response to this motion as a proxy for changes in early visual motion processing caused by microsaccades. When the full-field image motion started during microsaccades, ocular following was strongly suppressed, consistent with detrimental retinal effects of the movements. However, when the motion started after microsaccades, there was up to ~73% increase in ocular following speed, suggesting an enhanced motion sensitivity. These results suggest that the interface between even the smallest possible saccades and "fixation" includes a period of faster than usual image slip, as well as an enhanced responsiveness to image motion, and that both of these phenomena need to be considered when interpreting the pervasive neural and perceptual modulations frequently observed around the time of microsaccades.

Small Saccades and Image Complexity during Free Viewing of Natural Images in Schizophrenia

In schizophrenia, patients display dysfunctions during the execution of simple visual tasks such as antisaccade or smooth pursuit. In more ecological scenarios, such as free viewing of natural images, patients appear to make fewer and longer visual fixations and display shorter scanpaths. It is not clear whether these measurements reflect alterations in their proficiency to perform basic eye movements, such as saccades and fixations, or are related to high-level mechanisms, such as exploration or attention. We utilized free exploration of natural images of different complexities as a model of an ecological context where normally operative mechanisms of visual control can be accurately measured. We quantified visual exploration as Euclidean distance, scanpaths, saccades, and visual fixation, using the standard SR-Research eye tracker algorithm (SR). We then compared this result with a computation that includes microsaccades (EM). We evaluated eight schizophrenia patients and corresponding healthy controls (HC). Next, we tested whether the decrement in the number of saccades and fixations, as well as their increment in duration reported previously in schizophrenia patients, resulted from the increasing occurrence of undetected microsaccades. We found that when utilizing the standard SR algorithm, patients displayed shorter scanpaths as well as fewer and shorter saccades and fixations. When we employed the EM algorithm, the differences in these parameters between patients and HC were no longer significant. On the other hand, we found that image complexity plays an important role in exploratory behaviors, demonstrating that this factor explains most of differences between eye-movement behaviors in schizophrenia patients. These results help elucidate the mechanisms of visual motor control that are affected in schizophrenia and contribute to the finding of adequate markers for diagnosis and treatment for this condition.

Microsaccade and drift dynamics reflect mental fatigue

Our eyes are always in motion. Even during periods of relative fixation we produce so-called 'fixational eye movements', which include microsaccades, drift and tremor. Mental fatigue can modulate saccade dynamics, but its effects on microsaccades and drift are unknown. Here we asked human subjects to perform a prolonged and demanding visual search task (a simplified air traffic control task), with two difficulty levels, under both free-viewing and fixation conditions. Saccadic and microsaccadic velocity decreased with time-on-task whereas drift velocity increased, suggesting that ocular instability increases with mental fatigue. Task difficulty did not influence eye movements despite affecting reaction times, performance errors and subjective complexity ratings. We propose that variations in eye movement dynamics with time-on-task are consistent with the activation of the brain's sleep centers in correlation with mental fatigue. Covariation of saccadic and microsaccadic parameters moreover supports the hypothesis of a common generator for microsaccades and saccades. We conclude that changes in fixational and saccadic dynamics can indicate mental fatigue due to time-on-task, irrespective of task complexity. These findings suggest that fixational eye movement dynamics have the potential to signal the nervous system's activation state.

A model of microsaccade-related neural responses induced by short-term depression in thalamocortical synapses

Microsaccades during fixation have been suggested to counteract visual fading. Recent experiments have also observed microsaccade-related neural responses from cellular record, scalp electroencephalogram (EEG), and functional magnetic resonance imaging (fMRI). The underlying mechanism, however, is not yet understood and highly debated. It has been proposed that the neural activity of primary visual cortex (V1) is a crucial component for counteracting visual adaptation. In this paper, we use computational modeling to investigate how short-term depression (STD) in thalamocortical synapses might affect the neural responses of V1 in the presence of microsaccades. Our model not only gives a possible synaptic explanation for microsaccades in counteracting visual fading, but also reproduces several features in experimental findings. These modeling results suggest that STD in thalamocortical synapses plays an important role in microsaccade-related neural responses and the model may be useful for further investigation of behavioral properties and functional roles of microsaccades.

Two stages of programming eye gaze shifts in 3-D space

Accurate saccadic and vergence eye movements towards selected visual targets are fundamental to perceive the 3-D environment. Despite this importance, shifts in eye gaze are not always perfect given that they are frequently followed by small corrective eye movements. The oculomotor system receives distinct information from various visual cues that may cause incongruity in the planning of a gaze shift. To test this idea, we analyzed eye movements in humans performing a saccade task in a 3-D setting. We show that saccades and vergence movements towards peripheral targets are guided by monocular (perceptual) cues. Approximately 200ms after the start of fixation at the perceived target, a fixational saccade corrected the eye positions to the physical target location. Our findings suggest that shifts in eye gaze occur in two phases; a large eye movement toward the perceived target location followed by a corrective saccade that directs the eyes to the physical target location.

An oculomotor continuum from exploration to fixation

During visual exploration, saccadic eye movements scan the scene for objects of interest. During attempted fixation, the eyes are relatively still but often produce microsaccades. Saccadic rates during exploration are higher than those of microsaccades during fixation, reinforcing the classic view that exploration and fixation are two distinct oculomotor behaviors. An alternative model is that fixation and exploration are not dichotomous, but are instead two extremes of a functional continuum. Here, we measured the eye movements of human observers as they either fixed their gaze on a small spot or scanned natural scenes of varying sizes. As scene size diminished, so did saccade rates, until they were continuous with microsaccadic rates during fixation. Other saccadic properties varied as function of image size as well, forming a continuum with microsaccadic parameters during fixation. This saccadic continuum extended to nonrestrictive, ecological viewing conditions that allowed all types of saccades and fixation positions. Eye movement simulations moreover showed that a single model of oculomotor behavior can explain the saccadic continuum from exploration to fixation, for images of all sizes. These findings challenge the view that exploration and fixation are dichotomous, suggesting instead that visual fixation is functionally equivalent to visual exploration on a spatially focused scale.

Simultaneous recordings of ocular microtremor and microsaccades with a piezoelectric sensor and a video-oculography system

Our eyes are in continuous motion. Even when we attempt to fix our gaze, we produce so called "fixational eye movements", which include microsaccades, drift, and ocular microtremor (OMT). Microsaccades, the largest and fastest type of fixational eye movement, shift the retinal image from several dozen to several hundred photoreceptors and have equivalent physical characteristics to saccades, only on a smaller scale (Martinez-Conde, Otero-Millan & Macknik, 2013). OMT occurs simultaneously with drift and is the smallest of the fixational eye movements (∼1 photoreceptor width, >0.5 arcmin), with dominant frequencies ranging from 70 Hz to 103 Hz (Martinez-Conde, Macknik & Hubel, 2004). Due to OMT's small amplitude and high frequency, the most accurate and stringent way to record it is the piezoelectric transduction method. Thus, OMT studies are far rarer than those focusing on microsaccades or drift. Here we conducted simultaneous recordings of OMT and microsaccades with a piezoelectric device and a commercial infrared video tracking system. We set out to determine whether OMT could help to restore perceptually faded targets during attempted fixation, and we also wondered whether the piezoelectric sensor could affect the characteristics of microsaccades. Our results showed that microsaccades, but not OMT, counteracted perceptual fading. We moreover found that the piezoelectric sensor affected microsaccades in a complex way, and that the oculomotor system adjusted to the stress brought on by the sensor by adjusting the magnitudes of microsaccades.

The effects of fixation target size and luminance on microsaccades and square-wave jerks

A large amount of classic and contemporary vision studies require subjects to fixate a target. Target fixation serves as a normalizing factor across studies, promoting the field's ability to compare and contrast experiments. Yet, fixation target parameters, including luminance, contrast, size, shape and color, vary across studies, potentially affecting the interpretation of results. Previous research on the effects of fixation target size and luminance on the control of fixation position rendered conflicting results, and no study has examined the effects of fixation target characteristics on square-wave jerks, the most common type of saccadic intrusion. Here we set out to determine the effects of fixation target size and luminance on the characteristics of microsaccades and square-wave jerks, over a large range of stimulus parameters. Human subjects fixated a circular target with varying luminance and size while we recorded their eye movements with an infrared video tracker (EyeLink 1000, SR Research). We detected microsaccades and SWJs automatically with objective algorithms developed previously. Microsaccade rates decreased linearly and microsaccade magnitudes increased linearly with target size. The percent of microsaccades forming part of SWJs decreased, and the time from the end of the initial SWJ saccade to the beginning of the second SWJ saccade (SWJ inter-saccadic interval; ISI) increased with target size. The microsaccadic preference for horizontal direction also decreased moderately with target size . Target luminance did not affect significantly microsaccades or SWJs, however. In the absence of a fixation target, microsaccades became scarcer and larger, while SWJ prevalence decreased and SWJ ISIs increased. Thus, the choice of fixation target can affect experimental outcomes, especially in human factors and in visual and oculomotor studies. These results have implications for previous and future research conducted under fixation conditions, and should encourage forthcoming studies to report the size of fixation targets to aid the interpretation and replication of their results.

Superior colliculus inactivation alters the relationship between covert visual attention and microsaccades

Microsaccades are tiny saccades that occur during gaze fixation. Whereas these movements have traditionally been viewed as random, it was recently discovered that microsaccade directions can be significantly biased by covertly attended visual stimuli. The detailed mechanisms mediating such a bias are neither known nor immediately obvious, especially because the amplitudes of the movements influenced by attentional cueing could be up to two orders of magnitude smaller than the eccentricity of the attended location. Here, we tested whether activity in the peripheral superior colliculus (SC) is necessary for this correlation between attentional cueing and microsaccades. We reversibly and focally inactivated SC neurons representing peripheral regions of visual space while rhesus monkeys performed a demanding covert visual attention task. The normal bias of microsaccade directions observed in each monkey before SC inactivation was eliminated when a cue was placed in the visual region affected by the inactivation; microsaccades were, instead, biased away from the affected visual space. When the cue was placed at another location unaffected by SC inactivation, the baseline cue-induced bias of microsaccade directions remained mostly intact, because the cue was in unaffected visual space, and any remaining changes were again explained by a repulsion of microsaccades away from the inactivated region. Our results indicate that peripheral SC activity is required for the link between microsaccades and the cueing of covert visual attention, and that it could do so by altering the probability of triggering microsaccades without necessarily affecting the motor generation of these movements.

Quantitative evaluation of arousal level based on the analyses of microsaccade rates and pupil fluctuations

In this study, we proposed an objective estimation of decline of arousal level by analyzing microsaccade rate and pupil fluctuation while subjects were continuously gazing a fixation target. Previous studies show that the slow eye movements (SEMs) could be a candidate for an indicator of decline of arousal. However, it is not sufficient to evaluate transition of arousal states since SEMs appear just prior to sleep onset. To establish more objective assessment of arousal, we examined the effects of the transition of arousal on microsaccade rate and pupil fluctuation. The subjects were instructed to indicate by mouse clicks when they were aware of having slept. We have analyzed the eye movement and pupil fluctuation data in advance of the occurrence of SEMs which were detected just before the mouse clicks. In the results, longitudinal pupil diameter shrinking and gradual rise of microsaccade rate were observed prior to SEMs. These results suggest that the arousal level could be evaluated by monitoring eye movements and pupil fluctuations.

Statistical patterns of visual search for hidden objects

The movement of the eyes has been the subject of intensive research as a way to elucidate inner mechanisms of cognitive processes. A cognitive task that is rather frequent in our daily life is the visual search for hidden objects. Here we investigate through eye-tracking experiments the statistical properties associated with the search of target images embedded in a landscape of distractors. Specifically, our results show that the twofold process of eye movement, composed of sequences of fixations (small steps) intercalated by saccades (longer jumps), displays characteristic statistical signatures. While the saccadic jumps follow a log-normal distribution of distances, which is typical of multiplicative processes, the lengths of the smaller steps in the fixation trajectories are consistent with a power-law distribution. Moreover, the present analysis reveals a clear transition between a directional serial search to an isotropic random movement as the difficulty level of the searching task is increased.

Maxwellian eye fixation during natural scene perception

When we explore a visual scene, our eyes make saccades to jump rapidly from one area to another and fixate regions of interest to extract useful information. While the role of fixation eye movements in vision has been widely studied, their random nature has been a hitherto neglected issue. Here we conducted two experiments to examine the Maxwellian nature of eye movements during fixation. In Experiment 1, eight participants were asked to perform free viewing of natural scenes displayed on a computer screen while their eye movements were recorded. For each participant, the probability density function (PDF) of eye movement amplitude during fixation obeyed the law established by Maxwell for describing molecule velocity in gas. Only the mean amplitude of eye movements varied with expertise, which was lower in experts than novice participants. In Experiment 2, two participants underwent fixed time, free viewing of natural scenes and of their scrambled version while their eye movements were recorded. Again, the PDF of eye movement amplitude during fixation obeyed Maxwell's law for each participant and for each scene condition (normal or scrambled). The results suggest that eye fixation during natural scene perception describes a random motion regardless of top-down or of bottom-up processes.

Attentional shifts by gaze direction in voluntary orienting: evidence from a microsaccade study

Shifts in spatial attention can be induced by the gaze direction of another. However, it is unclear whether gaze direction influences the allocation of attention by reflexive or voluntary orienting. The present study was designed to examine which type of attentional orienting is elicited by gaze direction. We conducted two experiments to answer this question. In Experiment 1, we used a modified Posner paradigm with gaze cues and measured microsaccades to index the allocation of attention. We found that microsaccade direction followed cue direction between 200 and 400 ms after gaze cues were presented. This is consistent with the latencies observed in other microsaccade studies in which voluntary orienting is manipulated, suggesting that gaze direction elicits voluntary orienting. However, Experiment 1 did not separate voluntary and reflexive orienting directionally, so in Experiment 2, we used an anticue task in which cue direction (direction to allocate attention) was the opposite of gaze direction (direction of gaze in depicted face). The results in Experiment 2 were consistent with those from Experiment 1. Microsaccade direction followed the cue direction, not gaze direction. Taken together, these results indicate that the shift in spatial attention elicited by gaze direction is voluntary orienting.

Microsaccadic efficacy and contribution to foveal and peripheral vision

Our eyes move constantly, even when we try to fixate our gaze. Fixational eye movements prevent and restore visual loss during fixation, yet the relative impact of each type of fixational eye movement remains controversial. For over five decades, the debate has focused on microsaccades, the fastest and largest fixational eye movements. Some recent studies have concluded that microsaccades counteract visual fading during fixation. Other studies have disputed this idea, contending that microsaccades play no significant role in vision. The disagreement stems from the lack of methods to determine the precise effects of microsaccades on vision versus those of other eye movements, as well as a lack of evidence that microsaccades are relevant to foveal vision. Here we developed a novel generalized method to determine the precise quantified contribution and efficacy of human microsaccades to restoring visibility compared with other eye movements. Our results indicate that microsaccades are the greatest eye movement contributor to the restoration of both foveal and peripheral vision during fixation. Our method to calculate the efficacy and contribution of microsaccades to perception can determine the strength of connection between any two physiological and/or perceptual events, providing a novel and powerful estimate of causal influence; thus, we anticipate wide-ranging applications in neuroscience and beyond.

Bayesian selection of Markov models for symbol sequences: application to microsaccadic eye movements

Complex biological dynamics often generate sequences of discrete events which can be described as a Markov process. The order of the underlying Markovian stochastic process is fundamental for characterizing statistical dependencies within sequences. As an example for this class of biological systems, we investigate the Markov order of sequences of microsaccadic eye movements from human observers. We calculate the integrated likelihood of a given sequence for various orders of the Markov process and use this in a Bayesian framework for statistical inference on the Markov order. Our analysis shows that data from most participants are best explained by a first-order Markov process. This is compatible with recent findings of a statistical coupling of subsequent microsaccade orientations. Our method might prove to be useful for a broad class of biological systems.

Motion parallax from microscopic head movements during visual fixation

Under normal viewing conditions, adjustments in body posture and involuntary head movements continually shift the eyes in space. Like all translations, these movements may yield depth information in the form of motion parallax, the differential motion on the retina of objects at different distances from the observer. However, studies on depth perception rarely consider the possible contribution of this cue, as the resulting changes in viewpoint appear too small to be of perceptual significance. Here, we quantified the parallax present during fixation in normally standing observers. We measured the trajectories followed by the eyes in space by means of a high-resolution head-tracking system and used an optical model of the eye to reconstruct the stimulus on the observer's retina. We show that, within several meters from the observer, relatively small changes in depth yield changes in the velocity of the retinal stimulus that are well above perceivable thresholds. Furthermore, relative velocities are little influenced by fixation distance, target eccentricity, and the precise oculomotor strategy followed by the observer to maintain fixation. These results demonstrate that the parallax available during normal head-free fixation is a reliable source of depth information, which the visual system may use in a variety of tasks.

Active vision: fixational eye movements help seeing space in time

The significance of the miniature eye movements that we make during visual fixation has been intensely debated for the last 80 years. Recent studies have revealed that these motions of the eyes fulfill an important functional role: helping to extract useful information from natural scenes.

Quantifying Eye Stability During a Fixation Task: A Review of Definitions and Methods

Several definitions, measurements, and implicit meanings of ‘fixation stability’ have been used in clinical vision research, leading to some confusion. One definition concerns eye movements observed within fixations (i.e., within periods separated by saccades) when observing a point target: drift, microsaccades and physiological tremor all lead to some degree of within-fixation instability. A second definition relates to eye position during multiple fixations (and saccades) when patients fixate a point target. Increased between-fixation variability, combined with within-fixation instability, is known to be associated with poorer visual function in people with retinal disease such as age-related macular degeneration. In this review article, methods of eye stability measurement and quantification are summarised. Two common measures are described in detail: the bivariate contour ellipse area (BCEA) and the within-isolines area. The first measure assumes normality of the underlying positions distribution whereas the second does not. Each of these measures can be applied to two fundamentally different kinds of eye position data collected during a period of target observation. In the first case, mean positions of eye fixations are used to obtain an estimate of between-fixation variability. In the second case, often used in clinical vision research, eye position samples recorded by the eyetracker are used to obtain an estimate that confounds within- and between-fixation variability.

We show that these two methods can produce significantly different values of eye stability, especially when reported as BCEA values. Statistical techniques for describing eye stability when the distribution of eye positions is multimodal and not normally distributed are also reviewed.