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Saccade
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. In contrast, in smooth-pursuit movements, the eyes move smoothly instead of in jumps. Controlled cortically by the frontal eye fields (FEF), or subcortically by the superior colliculus, saccades serve as a mechanism for focal points, rapid eye movement, and the fast phase of optokinetic nystagmus. The word appears to have been coined in the 1880s by 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. These movements quickly scan objects of attention and aid the brain in grasping a scene visually.
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. 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. 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.
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. 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. Specific pre-target oscillatory (alpha rhythms) and transient activities occurring in posterior-lateral parietal cortex and occipital cortex also characterize express saccades.
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. Since the motion of the eye is essentially a linear system, bang-bang control minimizes travel time. After a saccade, a constant force is required to hold the position against elastic force, thus resulting in a pulse-step control.
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, a term borrowed from astrophysics; see Figure). For amplitudes larger than 20°, the peak velocity starts to plateau (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. Therefore, for larger amplitude ranges, the main sequence can best be modeled by an inverse power law function.
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. Although, depending on the demands on timing accuracy, acceleration-based methods are more precise.
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.)
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Saccade
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. In contrast, in smooth-pursuit movements, the eyes move smoothly instead of in jumps. Controlled cortically by the frontal eye fields (FEF), or subcortically by the superior colliculus, saccades serve as a mechanism for focal points, rapid eye movement, and the fast phase of optokinetic nystagmus. The word appears to have been coined in the 1880s by 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. These movements quickly scan objects of attention and aid the brain in grasping a scene visually.
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. 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. 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.
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. 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. Specific pre-target oscillatory (alpha rhythms) and transient activities occurring in posterior-lateral parietal cortex and occipital cortex also characterize express saccades.
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. Since the motion of the eye is essentially a linear system, bang-bang control minimizes travel time. After a saccade, a constant force is required to hold the position against elastic force, thus resulting in a pulse-step control.
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, a term borrowed from astrophysics; see Figure). For amplitudes larger than 20°, the peak velocity starts to plateau (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. Therefore, for larger amplitude ranges, the main sequence can best be modeled by an inverse power law function.
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. Although, depending on the demands on timing accuracy, acceleration-based methods are more precise.
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.)
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