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Trace of saccades of the human eye on a face while scanning
Saccades during observation of a picture on a computer screen

In vision science, a saccade (/səˈkɑːd/ sə-KAHD; French: [sakad]; French for 'jerk') is a quick, simultaneous movement of both eyes between two or more phases of focal points in the same direction.[1] In contrast, in smooth-pursuit movements, the eyes move smoothly instead of in jumps.[2] 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.[1] The word appears to have been coined in the 1880s by the 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.[3] These movements quickly scan objects of attention and aid the brain in grasping a scene visually.[4]

Function

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Humans and many organisms do not look at a scene in steadiness; instead, the eyes move around, locating interesting parts of the scene and building up a three-dimensional 'map' corresponding to the scene (as opposed to the graphical map of avians, which often relies upon detection of angular movement on the retina).[citation needed]

When scanning immediate surroundings or reading, human eyes make saccadic movements and stop several times, moving very quickly between each stop. The speed of movement during each saccade cannot be controlled; the eyes move as fast as they are able.[5] One reason for the saccadic movement of the human eye is that the central part of the retina—known as the fovea—which provides the high-resolution portion of vision is very small in humans, only about 1–2 degrees of vision, but it plays a critical role in resolving objects.[6] Saccades allow the eyes to sense small parts of a scene with greater resolution and the brain thereby to assemble them into a coherent, continuous mental representation.[7]

Timing and kinematics

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Saccades are one of the fastest movements produced by the human eye (blinks may reach even higher peak velocities). The peak angular speed of the eye during a saccade reaches up to 700°/s in humans for great saccades (25° of visual angle); in some monkeys, peak speed can reach 1000°/s.[8] Saccades to an unexpected stimulus normally take about 200 milliseconds (ms) to initiate, and then last from about 20–200 ms, depending on their amplitude (20–30 ms is typical in language reading). Under certain laboratory circumstances, the latency of, or reaction time to, saccade production can be cut nearly in half (express saccades). These saccades are generated by a neuronal mechanism that bypasses time-consuming circuits and activates the eye muscles more directly.[9][10] Specific pre-target oscillatory (alpha rhythms) and transient activities occurring in posterior-lateral parietal cortex and occipital cortex also characterize express saccades.[11]

To achieve such high speeds, there are specialized oculomotor burst neurons in the brainstem that wire into the ocular motor neuron. The burst neurons implement bang-bang control: they are either completely inhibited, or firing at its full rate of ~1000 Hz.[12] Since the motion of the eye is essentially a linear system, bang-bang control minimizes travel time.[13] After a saccade, a constant force is required to hold the position against elastic force, thus resulting in a pulse-step control.[14]

Saccadic main sequence, showing single saccades from a participant performing a visually-guided saccade task. It is called "main sequence" because it looks like the main sequence in astrophysics.

The amplitude of a saccade is the angular distance the eye travels during the movement. For amplitudes up to 15 or 20°, the velocity of a saccade linearly depends on the amplitude (the so-called saccadic main sequence,[15] a term borrowed from astrophysics; see Figure). For amplitudes larger than 20°, the peak velocity starts to plateau[15] (nonlinearly) toward the maximum velocity attainable by the eye at around 60°. For instance, a 10° amplitude is associated with a velocity of 300°/s, and 30° is associated with 500°/s.[16] Therefore, for larger amplitude ranges, the main sequence can best be modeled by an inverse power law function.[17]

The high peak velocities and the main sequence relationship can also be used to distinguish micro-/saccades from other eye movements (like ocular tremor, ocular drift, and smooth pursuit). Velocity-based algorithms are a common approach for saccade detection in eye tracking.[18][19][20] Although, depending on the demands on timing accuracy, acceleration-based methods are more precise.[21]

Saccades may rotate the eyes in any direction to relocate gaze direction (the direction of sight that corresponds to the fovea), but normally saccades do not rotate the eyes torsionally. (Torsion is clockwise or counterclockwise rotation around the line of sight when the eye is at its central primary position; defined this way, Listing's law says that, when the head is motionless, torsion is kept at zero.)

Head-fixed saccades can have amplitudes of up to 90° (from one edge of the oculomotor range to the other), but in normal conditions saccades are far smaller, and any shift of gaze larger than about 20° is accompanied by a head movement. During such gaze saccades, first, the eye produces a saccade to get gaze on target, whereas the head follows more slowly and the vestibulo-ocular reflex (VOR) causes the eyes to roll back in the head to keep gaze on the target. Since the VOR can actually rotate the eyes around the line of sight, combined eye and head movements do not always obey Listing's law.[22]

The rotational inertia of the eye is negligible compared to the elastic and viscous force.

Types

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Saccades can be categorized by intended goal in four ways:[23]

  1. In a visually guided saccade, the eyes move toward a visual transient, or stimulus. The parameters of visually guided saccades (amplitude, latency, peak velocity, and duration) are frequently measured as a baseline when measuring other types of saccades. Visually guided saccades can be further subcategorized:
    • A reflexive saccade is triggered exogenously by the appearance of a peripheral stimulus, or by the disappearance of a fixation stimulus.
    • A scanning saccade is triggered endogenously for the purpose of exploring the visual environment.
  2. In an antisaccade, the eyes move away from the visual onset. They are more delayed than visually guided saccades, and observers often make erroneous saccades in the wrong direction. A successful antisaccade requires inhibiting a reflexive saccade to the onset location, and voluntarily moving the eye in the other direction.
  3. In a memory guided saccade, the eyes move toward a remembered point, with no visual stimulus.
  4. In a sequence of predictive saccades, the eyes are kept on an object moving in a temporally and/or spatially predictive manner. In this instance, saccades often coincide with (or anticipate) the predictable movement of an object.

As referenced to above, it is also useful to categorize saccades by latency (time between go-signal and movement onset). In this case the categorization is binary: Either a given saccade is an express saccade or it is not. The latency cut-off is approximately ~200 ms; any longer than this is outside the express saccade range.[9][10]

Microsaccades are a related type of fixational eye movement that are small, jerk-like, involuntary eye movements, similar to miniature versions of voluntary saccades. They typically occur during visual fixation, not only in humans, but also in other animals with foveal vision, such as haplorhine primates in general and hawks.[24] Microsaccade amplitudes vary from 2 to 120 arcminutes.

In depth

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When exploring the visual environment with the gaze, humans make two to three fixations a second. Each fixation involves binocularly coordinated movements of the eyes to acquire the new target in three dimensions: horizontal and vertical, but also in-depth. In literature it has been shown how an upward or a vertical saccade is generally accompanied by a divergence of the eyes, while a downward saccade is accompanied by a convergence.[25] The amount of this intra-saccadic vergence has a strong functional significance for the effectiveness of binocular vision.[26] When making an upward saccade, the eyes diverged to be aligned with the most probable uncrossed disparity in that part of the visual field. On the other way around, when making a downward saccade, the eyes converged to enable alignment with crossed disparity in that part of the field. The phenomenon can be interpreted as an adaptation of rapid binocular eye movements to the statistics of the 3D environment, in order to minimize the need for corrective vergence movements at the end of saccades.

Pathophysiologic saccades

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Saccadic oscillations not fitting the normal function are a deviation from a healthy or normal condition. Nystagmus is characterized by the combination of 'slow phases', which usually take the eye off the point of regard, interspersed with saccade-like "quick phases" that serve to bring the eye back on target. Pathological slow phases may be due to either an imbalance in the vestibular system or damage to the brainstem "neural integrator" that normally holds the eyes in place.[citation needed] On the other hand, opsoclonus or ocular flutter are composed purely of fast-phase saccadic eye movements. Without the use of objective recording techniques, it may be very difficult to distinguish between these conditions.[medical citation needed]

Eye movement measurements are also used to investigate psychiatric disorders. For example, ADHD is characterized by an increase of antisaccade errors and an increase in delays for visually guided saccade.[23] Various pathological conditions also alter microsaccades and other fixational eye movements.[27][28]

Paroxysmal eye–head movements, termed aberrant gaze saccades, are an early symptom of GLUT1 deficiency syndrome in infancy.[29][non-primary source needed]

Saccade adaptation

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When the brain is led to believe that the saccades it is generating are too large or too small (by an experimental manipulation in which a saccade-target steps backward or forward contingent on the eye movement made to acquire it), saccade amplitude gradually decreases (or increases), an adaptation (also termed gain adaptation) widely seen as a simple form of motor learning, possibly driven by an effort to correct visual error. This effect was first observed in humans with ocular muscle palsy.[30] In these cases, it was noticed that the patients would make hypometric (small) saccades with the affected eye, and that they were able to correct these errors over time. This led to the realization that visual or retinal error (the difference between the post-saccadic point of regard and the target position) played a role in the homeostatic regulation of saccade amplitude. Since then, much scientific research has been devoted to various experiments employing saccade adaptation.[31]

Reading

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Saccadic eye movement allows the mind to read quickly, but it comes with its disadvantages. It can cause the mind to skip over words because it does not see them as important to the sentence, and the mind completely leaves it from the sentence or it replaces it with the wrong word. This can be seen in "Paris in the the Spring". This is a common psychological test, where the mind will often skip the second "the", especially when there is a line break in between the two.

When speaking, the mind plans what will be said before it is said. Sometimes the mind is not able to plan in advance and the speech is rushed out. This is why there are errors like mispronunciation, stuttering, and unplanned pauses. The same thing happens when reading. The mind does not always know what will come next. This is another reason that the second "the" can be missed.[32]

Vision

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Saccadic masking

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Main article: Saccadic masking

It is a common but false belief that during the saccade, no information is passed through the optic nerve to the brain. Whereas low spatial frequencies (the 'fuzzier' parts) are attenuated, higher spatial frequencies (an image's fine details) that would otherwise be blurred by the eye movement remain unaffected. This phenomenon, known as saccadic masking or saccadic suppression, is known to begin prior to saccadic eye movements in every primate species studied, implying neurological reasons for the effect rather than simply the image's motion blur.[33] This phenomenon leads to the so-called stopped-clock illusion, or chronostasis.

A person may observe the saccadic masking effect by standing in front of a mirror and looking from one eye to the next (and vice versa). The subject will not experience any movement of the eyes or any evidence that the optic nerve has momentarily ceased transmitting. Through saccadic masking, the eye/brain system not only hides the eye movements from the individual but also hides the evidence that anything has been hidden. Of course, a second observer watching the experiment will see the subject's eyes moving back and forth. The function's main purpose is to prevent an otherwise significant smearing of the image.[16] (One can experience one’s eye saccade movements by using a cellphone's front-facing camera as a delayed mirror; if the screen is held a couple of inches away as the viewer looks from one eye to the other, the signal-processing delay allows them to see the end of their own saccade movement.)

Spatial updating

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When a visual stimulus is seen before a saccade, subjects are still able to make another saccade back to that image, even if it is no longer visible. This shows that the brain is somehow able to take into account the intervening eye movement. It is thought that the brain does this by temporarily recording a copy of the command for the eye movement, and comparing this to the remembered image of the target. This is called spatial updating. Neurophysiologists, having recorded from cortical areas for saccades during spatial updating, have found that memory-related signals get remapped during each saccade.[citation needed]

Trans-saccadic perception

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It is also thought that perceptual memory is updated during saccades so that information gathered across fixations can be compared and synthesized. However, the entire visual image is not updated during each saccade. Some scientists believe that this is the same as visual working memory, but as in spatial updating the eye movement has to be accounted for. The process of retaining information across a saccade is called trans-saccadic memory, and the process of integrating information from more than one fixation is called trans-saccadic integration.

Comparative physiology

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Saccades are a widespread phenomenon across animals with image-forming visual systems. They have been observed in animals across three phyla, including animals that do not have a fovea (most vertebrates do) and animals that cannot move their eyes independently of their head (such as insects).[34] Therefore, while saccades serve in humans and other primates to increase the effective visual resolution of a scene, there must be additional reasons for the behavior. The most frequently suggested of these reasons is to avoid blurring of the image, which would occur if the response time of a photoreceptor cell is longer than the time a given portion of the image is stimulating that photoreceptor as the image drifts across the eye.

In birds, saccadic eye movements serve a further function. The avian retina is highly developed. It is thicker than the mammalian retina, has a higher metabolic activity, and has less vasculature obstruction, for greater visual acuity.[35] Because of this, the retinal cells must obtain nutrients via diffusion through the choroid and from the vitreous humor.[36] The pecten is a specialised structure in the avian retina. It is a highly vascular structure that projects into the vitreous humor. Experiments show that, during saccadic eye oscillations (which occupy up to 12% of avian viewing time), the pecten oculi acts as an agitator, propelling perfusate (natural lubricants) toward the retina. Thus, in birds, saccadic eye movements appear to be important in retinal nutrition and cellular respiration.[37]

See also

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Wikimedia Commons has media related to Saccade.
Look up saccade in Wiktionary, the free dictionary.

References

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Saccade

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A saccade is a rapid, ballistic that shifts the point of fixation from one location to another in the , enabling high-acuity vision by aligning the fovea with objects of interest. These movements are conjugate, meaning both eyes move together in the same direction, and they occur multiple times per second during normal visual scanning, with durations typically ranging from 20 to 100 milliseconds and peak velocities up to 700 degrees per second. Unlike , vision is suppressed during the saccade itself to prevent perceptual blurring, a phenomenon known as . Saccades are essential for exploring the visual environment and are classified into several types based on their triggers and purposes. Reflexive saccades are involuntary responses to sudden stimuli, such as a flashing , with latencies around 150-250 milliseconds. Voluntary saccades, in contrast, are goal-directed and include prosaccades (direct shifts to a target), antisaccades (shifts in the opposite direction to suppress reflexive responses), memory-guided saccades (to recalled locations), and predictive saccades (anticipating a target's movement). Smaller variants, known as microsaccades (less than 0.5 degrees), occur during fixation to counteract visual fading and maintain perceptual stability, happening about once per second. The neural control of saccades involves a distributed network in the , integrating sensory input with motor output for precise execution. High-level planning occurs in cortical areas like the (Brodmann's area 8), which initiate voluntary saccades, while the in the serves as a key integrator for both reflexive and voluntary movements, mapping sensory stimuli to motor commands. Burst neurons in structures, such as the for horizontal saccades and the rostral interstitial nucleus of the for vertical ones, generate the high-frequency signals needed for rapid eye acceleration. The amplitude and direction are encoded by the duration and pattern of activity in oculomotor nuclei, ensuring conjugate movement without mid-flight corrections due to the ballistic nature of the process. Clinically, saccade abnormalities provide diagnostic insights into neurological disorders, as their metrics like latency, , and accuracy reflect integrity of oculomotor pathways. Hypometric (shortened) saccades are common in , while slow or absent vertical saccades characterize progressive supranuclear palsy. Impairments also appear in neuropsychiatric conditions, such as increased saccade latencies and elevated anti-saccade error rates in attention-deficit/hyperactivity disorder or erratic patterns in , underscoring saccades' role as a for function.

Fundamentals

Definition and Function

A saccade is a rapid, ballistic movement of both eyes that abruptly shifts the point of fixation from one location to another in the . These movements are conjugate, meaning the eyes move together in the same direction, and they typically last between 20 and 200 milliseconds, depending on their size. Saccades range in from less than 1 degree for small shifts, such as during reading, to up to 90 degrees for large changes when scanning a broad environment. The primary function of saccades is to direct the fovea—the central, high-acuity region of the —toward objects or features of interest, enabling detailed visual processing where peripheral vision lacks resolution. This compensates for the 's nonuniform sensitivity, allowing efficient sampling of the visual scene without relying solely on head or body movements. Evolutionarily, saccades facilitate active exploration of the visual environment, supporting survival by quickly orienting to potential threats, rewards, or relevant stimuli in a dynamic world. Key characteristics distinguish saccades from other eye movements, such as or vestibular reflexes. Saccades exhibit peak velocities reaching up to 700 degrees per second for larger amplitudes, with acceleration and deceleration phases that follow a stereotyped, nonlinear profile known as the . This ballistic nature ensures precise, high-speed repositioning, executed approximately three to four times per second during active vision.

Types of Saccades

Saccades are categorized into distinct types based on their triggers, cognitive involvement, and functional roles, reflecting the diversity of oculomotor control mechanisms. Reflexive saccades, also known as exogenous saccades, are rapid, automatic responses elicited by the sudden onset of a peripheral visual stimulus, such as a flashing or abrupt target appearance. These movements orient the eyes toward novel or salient environmental cues with relatively short latencies, typically around 150-250 ms, facilitating immediate visual exploration. In contrast, voluntary or endogenous saccades are goal-directed movements initiated by internal cognitive intent, such as shifting to a remembered or following an instructed direction, without direct sensory prompting. These saccades involve higher-level planning and exhibit longer latencies, generally 200-250 ms, due to the integration of top-down attentional processes. Microsaccades represent small-amplitude, involuntary eye movements, typically less than 1 degree of visual angle, that occur during attempted visual fixation. Functioning to counteract retinal image adaptation and prevent perceptual fading, they occur at a rate of about 1-2 per second and help maintain visual stability by subtly repositioning the gaze. Express saccades are a specialized subset of reflexive saccades characterized by ultra-fast latencies of 70-120 ms, often triggered in paradigms involving a temporal gap between the offset of a central fixation point and the onset of a peripheral target. This rapid response is facilitated by direct collicular pathways, bypassing some cortical processing, and is more prevalent with repeated exposure or predictable stimuli.90760-6) Other notable variants include memory-guided saccades, which direct the eyes to a previously viewed but no longer visible target based on representations, often showing reduced accuracy compared to visually guided ones; anti-saccades, which require voluntary suppression of a reflexive response to a stimulus and instead shifting to the opposite direction, testing with latencies around 250-350 ms and error rates of 10-20%; and predictive saccades, anticipatory movements made in advance of an expected target appearance, such as in rhythmic or learned sequences, where timing relies on temporal expectations rather than immediate visual input. These latency differences among saccade types are tied to distinct neural processing pathways.90218-3)

Physiological Mechanisms

Timing and Kinematics

Saccadic eye movements exhibit characteristic temporal and dynamic properties that ensure rapid reorientation of . The duration of a saccade, defined as the time from to completion, typically ranges from 30 to 120 milliseconds, depending on the of the movement. This duration increases approximately linearly with saccade , following the relationship: duration ≈ 2.2 + 2.6 × (in degrees). Larger saccades thus take longer to execute, reflecting the ballistic nature of the movement where the eye accelerates and decelerates without mid-flight corrections. The velocity profile of a saccade forms a triangular waveform, characterized by an initial rapid acceleration phase followed by deceleration to zero velocity. Acceleration can reach up to 10,000 degrees per second squared, enabling quick attainment of peak velocity, which occurs roughly midway through the movement. Peak velocity (V_max) also adheres to the main sequence, increasing with amplitude but saturating for larger movements. More generally, this relationship can be modeled logarithmically as log(V_max) = log(k) + b × log(amplitude), where k and b are empirical constants derived from experimental data, capturing the nonlinear saturation observed in human saccades. Saccadic latency, the interval from stimulus onset to movement initiation, varies by task demands and typically measures 150-250 milliseconds for voluntary saccades. This delay encompasses visual processing and motor planning. The gap effect, where latency shortens when the central fixation point disappears before target onset, can reduce this time by 50-100 milliseconds, facilitating faster responses in certain paradigms. Saccades are not always perfectly accurate, often undershooting the target by 10-20% of the intended , which is subsequently corrected by smaller corrective saccades. In some contexts, dynamic overshoot may occur, where the eye briefly exceeds the target before settling. These errors highlight the between speed and precision inherent in saccadic control. These kinematic parameters—duration, , latency, and accuracy—are measured using high-resolution eye-tracking devices, such as video-based systems or scleral search coils, which record eye position over time to compute (via differentiation) and acceleration (). Such techniques allow precise quantification of saccade dynamics in both laboratory and clinical settings.

Neural Control

The neural control of saccades involves a distributed network of cortical and subcortical structures that , select, and execute rapid eye movements, ensuring precise foveation of visual targets. This operates hierarchically, with higher cortical areas sensory inputs and cognitive demands to generate saccade commands, which are then relayed through subcortical pathways for motor . Feedback mechanisms within this network refine accuracy and adapt to ongoing visual updates, integrating voluntary intention with reflexive responses. At the core of reflexive saccades lies the (SC), a structure that integrates multisensory inputs, including visual, auditory, and somatosensory signals, to trigger orienting movements. Neurons in the intermediate layers of the SC encode target locations in retinotopic coordinates and generate burst activity that directly projects to motor circuits, facilitating quick, stimulus-driven saccades. Electrical stimulation or lesion studies in confirm the SC's essential role, as its inactivation impairs reflexive but spares voluntary saccades. Voluntary saccades and their planning are primarily orchestrated by cortical regions, including the (FEF) and supplementary eye fields (SEF). The FEF, located in the , plays a pivotal role in target selection during tasks, where visuomotor neurons accumulate evidence about stimulus salience until reaching a threshold that initiates the saccade command. Projections from the FEF descend to the SC and brainstem, influencing both reflexive and goal-directed movements. In contrast, the SEF, situated on the , contributes to sequencing multiple saccades and monitoring performance, with neurons showing sustained activity during planned chains. The parietal eye fields, particularly the lateral intraparietal area (), support spatial and perceptual stability by remapping visual representations ahead of impending saccades. LIP neurons shift their receptive fields to anticipate the postsaccadic gaze position, enabling trans-saccadic continuity of object features and attention allocation to potential . This remapping integrates with FEF and SC activity, ensuring that attentional priorities guide saccade metrics without disrupting visual perception. Execution of saccades relies on brainstem circuits that translate cortical and collicular commands into coordinated ocular motor output. The (PPRF) contains excitatory burst neurons that generate the high-velocity pulse for horizontal saccades, while the integrates velocity and position signals to control eye trajectory. The (MLF) interconnects these nuclei with the oculomotor and abducens nuclei, synchronizing contractions of for conjugate gaze shifts. Omnipause neurons in the raphe interpositus pause their tonic inhibition during saccades, gating the burst to prevent unwanted movements. Inhibitory control prevents reflexive saccades in tasks requiring suppression, such as the anti-saccade paradigm, where subjects look away from a sudden stimulus. The , via the pars reticulata (SNr), exert tonic inhibition on the SC; reduced SNr firing disinhibits SC neurons to permit saccades, while sustained activity blocks erroneous responses. Fixation-related neurons in the SC further enhance suppression by increasing discharge during anti-saccade preparation, countering visuomotor bursts. This dual mechanism—prestimulus top-down gating and postsaccade override—ensures flexible behavioral control. The overall hierarchical model posits that cortical planning in the FEF, SEF, and feeds descending signals to the SC for target selection and amplitude specification, which then converge on circuits for final motor burst generation. Feedback loops, including corollary discharge from the to the SC and cortex, allow real-time corrections for accuracy, as evidenced by adaptive adjustments in neuronal thresholds during error trials. This architecture balances speed and precision, with disruptions in any level altering saccade dynamics.

Perceptual Integration

Role in Visual Perception

Saccades play a central role in visual scanning by sequencing brief periods of fixation to construct a coherent representation of the visual environment. During natural viewing, humans typically execute three to four saccades per second, each redirecting to points of interest and allowing the accumulation of detailed information across successive fixations. A key benefit of saccades lies in their ability to shift high-resolution foveal vision toward selected targets, thereby enhancing beyond the coarse resolution provided by peripheral detection. This foveation process enables precise examination of objects or features initially detected in the low-acuity periphery, optimizing the use of the retina's central high-density photoreceptor region. Through saccades, the integrates disparate fragments of information acquired during separate fixations into a stable and unified of the scene, compensating for the discontinuous nature of eye movements. This integration supports the perception of a continuous despite the rapid shifts in retinal input that occur with each saccade. Saccades frequently align with shifts in covert attention, where attentional focus precedes and guides the eye movement to improve target detection and processing efficiency. This linkage, rooted in shared neural mechanisms, ensures that saccades are directed toward attended locations, enhancing overall perceptual selectivity. Recent studies from the 2020s highlight how saccades facilitate perception in dynamic environments, such as driving, by dynamically prioritizing salient features like moving vehicles or road hazards through modulated saccadic patterns.

Saccadic Masking

Saccadic masking, also known as saccadic suppression, refers to the temporary reduction in visual sensitivity that occurs during the execution of saccadic eye movements, effectively preventing the of motion blur resulting from the high-velocity sweep of images across the . This phenomenon ensures perceptual stability by inhibiting the processing of perisaccadic visual input, which would otherwise produce a disorienting smear due to retinal slip speeds exceeding 500 degrees per second in larger saccades. The suppression begins approximately 50-100 ms before saccade onset and persists through the movement, aligning closely with typical saccade durations of 30-120 ms. The underlying mechanism involves neural inhibition of visual processing pathways, primarily through corollary discharge signals from oculomotor centers that modulate activity in the , reducing neuronal firing rates and contrast sensitivity by 50-80% in regions affected by the . This creates a transient functional —a blind spot—centered on the saccadic , where sensitivity to , motion, and other stimuli is markedly diminished. Suppression occurs at multiple levels, starting in the with reduced ganglion cell responses to sequential stimuli and extending to cortical areas like V1 and V4, where inhibitory and feedback from higher centers amplify the effect. The adaptive purpose is to maintain a coherent visual world despite constant refixations, avoiding conflicts between pre- and post-saccadic scenes. Classic experimental evidence demonstrates elevated detection thresholds for brief visual flashes presented during saccades, with sensitivity dropping to as low as 10-20% of baseline levels, as thresholds can rise by 6-10 times compared to fixation conditions. More recent investigations using electrophysiological recordings have tied this suppression to enhanced oscillatory activity in the ; for instance, 2023 studies show increased alpha-band (7-13 Hz) power in V4 during saccades, which correlates with reduced visual responsiveness and contributes to the inhibitory gating.

Trans-saccadic Perception

Trans-saccadic perception encompasses the neural processes that preserve visual continuity across saccadic eye movements, ensuring the world appears stable despite the eyes' rapid shifts. During natural viewing, humans make approximately 3 saccades per second, displacing the retinal image by several degrees each time, yet compensatory mechanisms like corollary discharge signals from oculomotor commands counteract this instability by updating visual representations in advance. These signals, originating from pathways involving the and , shift neuronal receptive fields to maintain a consistent perceptual map. A core component is the integration of visual features, where pre-saccadic —such as object identity and color—is briefly stored in a limited trans-saccadic buffer with a capacity of about 3–4 items, akin to visual . This stored is then matched and fused with post-saccadic input through efference-copy-based remapping in cortical regions like the parietal and , allowing synthesis of features from ventral and dorsal streams. Such integration relies on egocentric saccade metrics to align features spatially, preventing perceptual fragmentation. Perceptual continuity is further aided by a subjective compression of time during saccades, where visual intervals around saccade onset are underestimated by up to 50%—for instance, a 100 ms gap perceived as roughly 50 ms—peaking at saccade initiation and spanning a 300 ms window. This temporal distortion, specific to visual stimuli and independent of saccade amplitude, minimizes disruptions in the flow of without inverting order entirely. Despite these mechanisms, trans-saccadic exhibits clear limits, with poor retention of fine details like precise object positions; fidelity declines sharply as memory load increases from 1 to 4 items, evidenced by rising response variability (from ~22° to ~37°) and bias toward post-saccadic cues. The system compensates via heuristics, such as prioritizing recent sensory input or attentional cues to allocate limited resources efficiently, rather than maintaining high-resolution storage across all features. Recent advances highlight dynamic influences on these processes: a 2025 study demonstrated that short-term priors (from immediate prior stimuli) induce behavioral oscillations in orientation judgments during saccades, at ~9–10 Hz and synchronized to saccade onset, with amplitudes up to 1°. These oscillations, absent for long-term priors, align with frameworks, where alpha-range neural rhythms facilitate Bayesian integration of pre- and post-saccadic signals for enhanced stability.

Spatial Updating

Spatial updating during saccades involves the brain's predictive remapping of visual receptive fields to compensate for the impending shift in . Neurons in the parietal cortex, particularly the , and adjust their receptive fields prior to saccade onset, shifting them by the vector of the planned . This remapping ensures that neural representations of space remain stable despite the retinal displacement caused by the saccade. A key mechanism underlying this process is the corollary discharge, or , which originates from oculomotor commands in the and . This internal signal is transmitted via pathways such as the projection from the to the and then to parietal areas, informing sensory regions about the expected change in eye position. The corollary discharge allows for anticipatory adjustments in visual processing, preventing perceptual disruptions from self-generated eye movements. The primary purpose of spatial updating is to maintain coherent spatial representations across gaze shifts, supporting behaviors like selective attention and visuomotor actions such as grasping objects. For instance, it enables the tracking of during sequential saccades, ensuring that attentional resources and motor plans align with the updated world coordinates. In parietal regions, this integration facilitates the remapping of attentional maps, minimizing disruptions to ongoing tasks. Evidence from single-cell recordings in monkeys demonstrates predictive remapping in neurons, where responses to stimuli shift to future receptive fields before the saccade executes. Human (fMRI) studies confirm similar processes in the posterior parietal cortex, showing gaze-centered updating during double-step saccade tasks. Errors in spatial updating can lead to perceptual mislocalization, particularly when multiple objects are present, as the system struggles to remap all locations accurately. Such inaccuracies manifest as systematic shifts in perceived positions, especially in complex scenes with competing stimuli, highlighting the limits of predictive mechanisms under high . Computational models suggest that these errors arise from incomplete integration of remapping signals across neural populations.

Applications

In Reading

During reading, the eyes make rapid saccadic movements interspersed with brief fixations, allowing the extraction of visual from text. Forward saccades typically span 7-9 character spaces, advancing the to the next word or within a word, while fixations last 200-250 milliseconds, during which most linguistic processing occurs. Regressions, which are backward saccades comprising 10-15% of all eye movements, return the to previously fixated text to resolve comprehension difficulties or integrate . The perceptual span—the region from which useful information is acquired during a fixation—is asymmetrical in left-to-right languages, extending approximately 7-8 characters to the right of the fixation point but only 3-4 characters to the left. This rightward bias facilitates previewing upcoming words, aiding in word identification and sentence prediction without disrupting the forward flow of reading. Saccade length and the frequency of regressions are influenced by linguistic properties of the text. Longer words elicit shorter forward saccades and more fixations within them, while high-frequency and predictable words promote longer saccades and fewer regressions by enabling efficient parafoveal processing. Low predictability, in contrast, increases regression rates as readers seek clarification from earlier material. Developmentally, children's reading eye movements differ markedly from those of adults. Young readers produce more regressions—often exceeding 20% of saccades—and shorter forward saccades due to immature linguistic and oculomotor control, leading to less efficient text processing. With reading skill acquisition, individuals make fewer regressions and longer, more targeted saccades, optimizing the balance between information uptake and . Eye-tracking research has linked saccade efficiency to , where affected individuals exhibit prolonged fixations, shorter saccades, and higher regression rates, reflecting disrupted text integration.

In Neurological Assessment

Saccades serve as valuable biomarkers in neurological assessment, where deviations from normal parameters—such as latencies typically ranging from 150-250 ms, peak velocities up to 500 degrees per second, and absence of intrusions—can signal underlying dysfunction in key brain regions. Abnormal saccadic latencies, velocities, or the presence of intrusions often indicate cerebellar involvement, as seen in slow or hypometric saccades due to impaired coordination; disorders, which may prolong latencies through disrupted initiation; or cortical dysfunction, leading to inaccurate targeting or inhibitory errors. These metrics, measured via eye-tracking, enable clinicians to localize lesions and monitor progression non-invasively. In (PD), saccades reveal characteristic impairments that aid early diagnosis and treatment evaluation. Patients commonly exhibit prolonged saccadic latencies and hypometric saccades, where eye movements undershoot targets, reflecting dopaminergic deficits in the . Eye-tracking studies from 2022-2025 have demonstrated that pro-saccade deficits, including increased latency and reduced velocity, can aid in early PD detection. For (), saccadic intrusions—unintended small eye movements during fixation—emerge as a reliable progression . A 2024 of 28 ALS patients found that intrusion frequency and increased over 12 months, correlating with declines in the ALS Functional Rating Scale-Revised (ALSFRS-R) bulbar subscale (r ≈ -0.45), highlighting their utility in tracking bulbar-onset disease. This non-invasive measure outperforms subjective scales for early detection of involvement, as intrusions reflect progressive loss of oculomotor control. Beyond neurodegenerative conditions, anti-saccade tasks, which require suppressing reflexive gazes to look away from a stimulus, quantify in psychiatric disorders. In attention-deficit/hyperactivity disorder (ADHD), individuals exhibit oculomotor deficits including elevated anti-saccade error rates indicating impaired , increased intrusive saccades during fixation, and difficulties in saccade inhibition, with children showing more anticipatory saccades and adults displaying a higher number of saccades overall. Eye-tracking studies have demonstrated increased saccade latency, shorter fixation times, and more intrusive saccades in ADHD compared to controls. These findings are validated by 2024 eye-tracking protocols using portable devices for screening. Similarly, in , anti-saccade error rates are elevated in patients, serving as a of prefrontal cortex deficits. Automated analysis algorithms, incorporating on eye-tracking data, enable non-invasive assessment of reading issues in children. Video-oculography (VOG) remains the gold standard for quantitative saccade assessment, using infrared cameras to capture high-resolution metrics like latency and velocity with sub-degree precision. Emerging devices, such as the 2025 EYE ROLL system, facilitate targeted saccadic training by delivering controlled visual stimuli, improving symmetry and speed.

Abnormalities and Adaptation

Pathophysiologic Saccades

Pathophysiologic saccades refer to abnormal eye movements arising from disruptions in the neural pathways controlling saccadic function, often manifesting in various neurological disorders. These abnormalities can include slowed velocities, hypometria (undershooting the target), intrusions (unwanted saccades interrupting fixation), or impaired conjugacy (coordinated movement of both eyes), reflecting underlying in , cerebellar, or cortical structures. Unlike normal saccades, which are rapid and precise, pathophysiologic variants impair visual stability and contribute to symptoms like or gaze instability. In ocular motor disorders, is associated with -like saccadic intrusions, where involuntary saccades mimic the fast phases of , disrupting steady fixation due to cerebellar dysfunction in modulating saccade accuracy. (PSP), a affecting structures, characteristically features slow saccades, particularly vertical ones, with peak velocities significantly reduced compared to normals, stemming from degeneration of the rostral interstitial nucleus of the . Neurodegenerative conditions further exemplify saccade pathologies. In (PD), hypometric saccades often require multiple corrective steps to reach the target, a pattern linked to basal ganglia dopamine depletion and common in PD patients, contrasting with the single-step saccades in healthy individuals. (MSA), involving olivopontocerebellar atrophy, presents with frequent square-wave jerks—small horizontal saccadic intrusions (1-5 degrees) that interrupt fixation every few seconds—correlating with cerebellar involvement and observed in 64% of MSA patients versus 15% in PD, helping to distinguish MSA from PD. Vascular and traumatic insults commonly produce gaze palsies that abolish or impair saccades. Post-stroke lesions in the pontine paramedian reticular formation or result in conjugate gaze palsies, preventing ipsilesional saccades and leading to a persistent deviation of eyes to the contralesional side, as seen in 20-30% of hemispheric strokes. (INO), often from demyelination or ischemia in the , disrupts conjugate horizontal saccades, causing adduction failure in the ipsilesional eye with abducting in the fellow eye. Recent longitudinal studies from 2023-2025 highlight saccade metrics as biomarkers for early disease detection and progression. In PD, reductions in saccade velocity—particularly prosaccades dropping below 300 degrees/second—emerge as an early indicator, detectable up to two years before motor symptoms and outperforming traditional imaging in sensitivity. For (ALS), increasing saccadic intrusions, such as square-wave jerks, track bulbar progression over 12-24 months, correlating with ALSFRS-R scores (r=0.65) and offering a noninvasive marker independent of respiratory decline. Genetic conditions also yield distinct saccade deficits. Congenital nystagmus, arising from in FRMD7 or other genes, features impaired saccadic inhibition, with quick phases showing altered latencies (around 80-140 ms) and reduced amplitudes, perpetuating the oscillatory cycle from infancy. In Niemann-Pick disease type C, a lysosomal storage disorder, patients exhibit slowed vertical saccades and hypometric horizontal ones, reflecting accumulation in nuclei and correlating with cognitive severity across age groups.

Saccade Adaptation

Saccade adaptation refers to the brain's capacity to modify the and direction of saccades through experience-dependent plasticity, ensuring precise shifts despite changes in the oculomotor system. This process maintains saccadic accuracy, typically landing within 0.5–1 degree of the target in healthy individuals, by adjusting motor commands based on post-saccadic visual . The primary mechanism involves error signals generated from retinal-target misalignment at saccade endpoint, which drive gain adjustments—scaling the saccade up or down—primarily through cerebellar climbing fibers. These fibers convey sensory error information from the inferior olive to Purkinje cells in the cerebellar cortex, triggering long-term depression or potentiation of synaptic weights to refine the saccadic command. Adaptation manifests in two main types: outward adaptation, which increases saccade when the target steps away from the fovea during the saccade, and inward adaptation, which decreases when the target steps closer. These are commonly induced using the double-step , where the target initially appears at one location and then jumps to a second position either during or immediately after the saccade onset, allowing selective modification of primary saccade metrics without altering secondary corrective movements. The neural basis centers on the oculomotor vermis of the posterior (lobules VI-VII) and interconnected brainstem nuclei, such as the , where error-driven signals modulate burst activity. Studies in show that lesions or inactivation of the abolish , while electrical stimulation can induce it, with healthy adaptation achieving 70–80% correction of imposed errors over repeated trials in a single session. Functionally, saccade adaptation compensates for transient perturbations like muscle fatigue during prolonged gaze shifts or prism-induced visual displacements, restoring accuracy without conscious effort. This plasticity is incomplete in cerebellar disorders, highlighting its reliance on intact vermal circuits for ongoing calibration. Recent research in the 2020s has explored (VR)-based protocols for enhancing saccade in rehabilitation, demonstrating improved oculomotor plasticity in patients with neurological impairments through immersive error-feedback . These approaches, including integration with sports vision devices for dynamic target tracking, show promise for , with studies from 2020-2022 reporting improved rates compared to traditional methods.

Comparative Physiology

In Non-Human Primates

Saccades in non-human primates, particularly rhesus macaques, exhibit kinematic properties closely resembling those in humans, establishing them as premier model organisms for oculomotor research. Visually guided saccades in macaques typically have latencies ranging from 150 to 250 ms, aligning with human values and reflecting shared visuomotor processing timelines. They adhere to the —a nonlinear relationship between amplitude and velocity—observed across primate species. These similarities extend to neural circuits, where the (FEF) and lateral intraparietal area () encode saccade-related activity, as demonstrated by single-unit recordings in behaving monkeys that mirror human findings. Invasive electrophysiological techniques, feasible in non-human primates due to ethical and technical advantages over human studies, have elucidated key mechanisms underlying saccade generation. Recordings from the reveal burst neurons that discharge at high frequencies during saccades, providing the motor command signals absent in non-invasive human data. Additionally, laboratory training enables the elicitation of express saccades in macaques, with latencies as short as 100-120 ms, which depend on target luminance and size and offer a window into preparatory neural states not easily studied in humans. Subtle differences in saccade characteristics exist between s and non- . models also display more robust to oculomotor s; for example, following dorsal ablation, macaques recover saccade accuracy through compensatory mechanisms faster than observed in cases. Non- research has been pivotal in validating trans-saccadic , confirming that mechanisms like perisaccadic suppression operate similarly across species. In the , optogenetic tools have advanced causal understanding, such as by selectively activating corticotectal pathways in macaques to dissect their role in saccade targeting and perisaccadic visual stability.

In Other Animals

In birds, eye movements are typically limited in range, often to about 20 degrees, with the primary role of stabilizing during larger head saccades rather than independent ocular shifts. This configuration arises from the relatively fixed position of the eyes in the , necessitating coordinated head movements to redirect toward targets. For instance, in chickens, saccadic eye movements occur predominantly during the thrust phases of head bobbing while walking, ensuring across a wide . In species like barn owls, which possess high despite these constraints, microsaccades—small, involuntary flicks—contribute to maintaining sharp focus by counteracting drift and enhancing resolution during fixation, particularly in low-light scenarios. Reptiles and amphibians exhibit saccades that are generally constrained in amplitude, with eye rotations limited to 3–6 degrees in frogs, complemented by broader head movements up to 30–40 degrees for overall redirection. These movements facilitate prey detection but are slower and less frequent than in mammals, prioritizing stability over rapid scanning. In frogs, saccades play a critical role in ballistic tongue projection during prey capture, where eye and head positioning establish a shared with the trajectory, allowing precise of moving targets' paths in milliseconds. Fish and invertebrates rely heavily on optokinetic saccades to stabilize retinal images during locomotion or environmental motion. In zebrafish, these saccades form the fast phase of the optokinetic response, resetting eye position to counteract slow-phase drifts induced by visual stimuli like rotating gratings, thereby maintaining a stable view of the surroundings. Invertebrates such as flies demonstrate foveation-like scanning through stereotyped head saccades that interrupt smooth optokinetic tracking, resetting gaze to high-resolution regions of the compound eye for targeted inspection, akin to foveal shifts in vertebrates. This strategy supports rapid flight navigation and prey pursuit in species like robber flies. Evolutionary trends in saccades reflect adaptations to ecological niches, with mammals developing quick, high-velocity foveal shifts to enable the "saccade-and-fixate" strategy for detailed visual sampling. In contrast, non-mammalian vertebrates emphasize reflexive stabilization over voluntary exploration. Recent comparative studies, such as those on predatory , link saccade speed and predictability to predation success; for example, faster predictive saccades in robber flies allow interception of evasive prey by estimating wingbeat frequencies from visual cues. Functional divergence across phylogeny shows reduced voluntary control in lower vertebrates, where saccades are predominantly reflexive and often disconjugate between eyes, driven by sensory triggers rather than cognitive intent, unlike the integrated voluntary-reflexive systems in higher taxa.

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