Pursuit movement
Encyclopedia
Smooth pursuit eye movements allow the eye
s to closely follow a moving object. It is one of two ways that visual animals can voluntarily shift gaze
, the other being saccadic
eye movements. Pursuit differs from the vestibulo-ocular reflex
, which only occurs during movements of the head and serves to stabilize gaze on a stationary object. Most people are unable to initiate pursuit without a moving visual signal. The pursuit of targets moving with velocities of greater than 30°/s tend to require catch-up saccades. Smooth pursuit is asymmetric: most humans and primates tend to be better at horizontal than vertical smooth pursuit, as defined by their ability to pursue smoothly without making catch-up saccades. Most humans are also better at downward than upward pursuit. Pursuit is modified by ongoing visual feedback.
. This technique is most common in primate
research, and is extremely accurate. An eye movement shifts the orientation of the coil to induce an electrical current, which is translated into horizontal and vertical eye position. The second technique is an eye tracker. This device, while somewhat more noisy, is non-invasive and is often used in human psychophysics
and recently also in instructional psychology. It relies on the infrared illumination of the pupil to track eye position with a camera.
During oculomotor experiments, it is often important to ensure that no saccades occurred when the subject was supposed to be smoothly pursuing a target. Such eye movements are called catch-up saccades and are more common when pursuing at high speeds. Researchers are able to discard portions of eye movement recordings that contain saccades, in order to analyze the two components separately. Saccadic eye movements differ from the smooth pursuit component by their very high initial acceleration and deceleration, and peak velocity.
in the frontal lobe
, known as the frontal pursuit area, responds to particular vectors of pursuit, and can be electrically stimulated to induce pursuit movements. Recent evidence suggests that the superior colliculus
also responds during smooth pursuit eye movement. These two areas are likely involved in providing the GO signal to initiate pursuit, as well as selecting which target to track. The GO signal from the cortex and the superior colliculus is relayed to several pontine nuclei, including the dorsolateral pontine nuclei and the nucleus reticularis tegmenti pontis The neurons of the pons
are tuned to eye velocity and are directionally selective, and can be stimulated to change the velocity of pursuit. The pontine nuclei project to the cerebellum, specifically the vermis and the paraflocculus. These neurons code for the target velocity and are responsible for the particular velocity profile of pursuit. The cerebellum
, especially the vestibulo-cerebellum, is also involved in the online correction of velocity during pursuit. The cerebellum
then projects to optic motoneurons, which control the eye muscles and cause the eye to move.
If you know which way a target will move, or know the target trajectory (because it is periodic for instance), you can initiate pursuit before the target motion actually starts, especially if you know exactly when the motion will start..
It is also possible to maintain pursuit if a target momentarily disappears, especially if the target appears to be occluded by a larger object .
Under conditions in which there is no visual stimulation (in total darkness), we can still perform smooth pursuit eye movements with the help of a proprioceptive motion signal (e.g. your moving finger) .
or Ketamine
).
Human eye
The human eye is an organ which reacts to light for several purposes. As a conscious sense organ, the eye allows vision. Rod and cone cells in the retina allow conscious light perception and vision including color differentiation and the perception of depth...
s to closely follow a moving object. It is one of two ways that visual animals can voluntarily shift gaze
Gaze
Gaze is a psychoanalytical term brought into popular usage by Jacques Lacan to describe the anxious state that comes with the awareness that one can be viewed. The psychological effect, Lacan argues, is that the subject loses some sense of autonomy upon realizing that he or she is a visible object...
, the other being saccadic
Saccade
A saccade is a fast movement of an eye, head or other part of an animal's body or device. It can also be a fast shift in frequency of an emitted signal or other quick change. Saccades are quick, simultaneous movements of both eyes in the same direction...
eye movements. Pursuit differs from the vestibulo-ocular reflex
Vestibulo-ocular reflex
The vestibulo-ocular reflex is a reflex eye movement that stabilizes images on the retina during head movement by producing an eye movement in the direction opposite to head movement, thus preserving the image on the center of the visual field. For example, when the head moves to the right, the...
, which only occurs during movements of the head and serves to stabilize gaze on a stationary object. Most people are unable to initiate pursuit without a moving visual signal. The pursuit of targets moving with velocities of greater than 30°/s tend to require catch-up saccades. Smooth pursuit is asymmetric: most humans and primates tend to be better at horizontal than vertical smooth pursuit, as defined by their ability to pursue smoothly without making catch-up saccades. Most humans are also better at downward than upward pursuit. Pursuit is modified by ongoing visual feedback.
Measurement
There are two basic methods for recording smooth pursuit eye movements, and eye movement in general. The first is with a search coilSearch coil
A search coil is a device for measuring eye movement using coils that are embedded into a tightly-fitting contact lens or a rubber ring that adheres to the eye. In animal studies, the search coil may be surgically implanted into the sclera of the eye. Alternating magnetic fields are generated by...
. This technique is most common in primate
Primate
A primate is a mammal of the order Primates , which contains prosimians and simians. Primates arose from ancestors that lived in the trees of tropical forests; many primate characteristics represent adaptations to life in this challenging three-dimensional environment...
research, and is extremely accurate. An eye movement shifts the orientation of the coil to induce an electrical current, which is translated into horizontal and vertical eye position. The second technique is an eye tracker. This device, while somewhat more noisy, is non-invasive and is often used in human psychophysics
Psychophysics
Psychophysics quantitatively investigates the relationship between physical stimuli and the sensations and perceptions they effect. Psychophysics has been described as "the scientific study of the relation between stimulus and sensation" or, more completely, as "the analysis of perceptual...
and recently also in instructional psychology. It relies on the infrared illumination of the pupil to track eye position with a camera.
During oculomotor experiments, it is often important to ensure that no saccades occurred when the subject was supposed to be smoothly pursuing a target. Such eye movements are called catch-up saccades and are more common when pursuing at high speeds. Researchers are able to discard portions of eye movement recordings that contain saccades, in order to analyze the two components separately. Saccadic eye movements differ from the smooth pursuit component by their very high initial acceleration and deceleration, and peak velocity.
Neural circuitry
The neural circuitry underlying smooth pursuit is an object of debate. The first step towards the initiation of pursuit is to see a moving target. Signals from the retina ascend through the lateral geniculate nucleus and activate neurons in primary visual cortex. Primary visual cortex sends the information about the target to the middle temporal visual cortex, which responds very selectively to directions of movement. The processing of motion in this area is necessary for smooth pursuit responses. This sensory area provides the motion signal, which may or may not be smoothly pursued. A region of cortexCerebral cortex
The cerebral cortex is a sheet of neural tissue that is outermost to the cerebrum of the mammalian brain. It plays a key role in memory, attention, perceptual awareness, thought, language, and consciousness. It is constituted of up to six horizontal layers, each of which has a different...
in the frontal lobe
Frontal lobe
The frontal lobe is an area in the brain of humans and other mammals, located at the front of each cerebral hemisphere and positioned anterior to the parietal lobe and superior and anterior to the temporal lobes...
, known as the frontal pursuit area, responds to particular vectors of pursuit, and can be electrically stimulated to induce pursuit movements. Recent evidence suggests that the superior colliculus
Superior colliculus
The optic tectum or simply tectum is a paired structure that forms a major component of the vertebrate midbrain. In mammals this structure is more commonly called the superior colliculus , but, even in mammals, the adjective tectal is commonly used. The tectum is a layered structure, with a...
also responds during smooth pursuit eye movement. These two areas are likely involved in providing the GO signal to initiate pursuit, as well as selecting which target to track. The GO signal from the cortex and the superior colliculus is relayed to several pontine nuclei, including the dorsolateral pontine nuclei and the nucleus reticularis tegmenti pontis The neurons of the pons
Pons
The pons is a structure located on the brain stem, named after the Latin word for "bridge" or the 16th-century Italian anatomist and surgeon Costanzo Varolio . It is superior to the medulla oblongata, inferior to the midbrain, and ventral to the cerebellum. In humans and other bipeds this means it...
are tuned to eye velocity and are directionally selective, and can be stimulated to change the velocity of pursuit. The pontine nuclei project to the cerebellum, specifically the vermis and the paraflocculus. These neurons code for the target velocity and are responsible for the particular velocity profile of pursuit. The cerebellum
Cerebellum
The cerebellum is a region of the brain that plays an important role in motor control. It may also be involved in some cognitive functions such as attention and language, and in regulating fear and pleasure responses, but its movement-related functions are the most solidly established...
, especially the vestibulo-cerebellum, is also involved in the online correction of velocity during pursuit. The cerebellum
Cerebellum
The cerebellum is a region of the brain that plays an important role in motor control. It may also be involved in some cognitive functions such as attention and language, and in regulating fear and pleasure responses, but its movement-related functions are the most solidly established...
then projects to optic motoneurons, which control the eye muscles and cause the eye to move.
Stages of smooth pursuit
Pursuit eye movement can be divided into two stages: open loop pursuit and closed loop pursuit. Open loop pursuit is the visual system's first response to a moving object we want to track and typically lasts ~100 ms. Meaning that this stage is ballistic: visual signals have not yet had time to correct the ongoing pursuit velocity or direction . The second stage of pursuit is called closed-loop pursuit. This stage lasts from 100 ms after the initiation of pursuit until the pursuit movement has ceased. This stage is characterized by the online correction of pursuit velocity to compensate for retinal slip. In other words, the pursuit system tries to null retinal velocity of the object of interest. This is achieved at the end of the open-loop phase. Pursuit gain in the closed-loop phase, i.e. the ratio of eye angular velocity and target angular velocity, is near to 1.Smooth pursuit and spatial attention
Various lines of research suggests a tight coupling for closed loop pursuit and spatial attention. For instance, during the close loop phase selective attention is coupled to the pursuit target such that untracked targets which move in the same direction with the target are poorly processed by the visual system . Recently, a loose coupling of open loop pursuit and attention was suggested, when there is only one possible moving target.. This difference between pursuit and saccades may be accounted for by the differences in latency. Pursuit eye movements are initiated within 90-150 ms, while typical latencies for voluntary saccades are in the order of 200-250 msSmooth pursuit in the absence of a visual target
Performing smooth pursuit without a moving visual stimulus is difficult , and typically results in a series of saccades. However, pursuit without a visible target is possible under some particular conditions, that show the importance of high-level functions in smooth pursuit maintenance.If you know which way a target will move, or know the target trajectory (because it is periodic for instance), you can initiate pursuit before the target motion actually starts, especially if you know exactly when the motion will start..
It is also possible to maintain pursuit if a target momentarily disappears, especially if the target appears to be occluded by a larger object .
Under conditions in which there is no visual stimulation (in total darkness), we can still perform smooth pursuit eye movements with the help of a proprioceptive motion signal (e.g. your moving finger) .
Following stimuli from peripheral gaze
When a bright light appears in the periphery, the fastest it can achieve a smooth pursuit is 30°/second. It first fixes the gaze to the peripheral light, and if not in excess of 30°/second, will follow the target equally with the movement. At higher velocities, the eye will not move smoothly, and requires corrective saccades. Unlike saccades, this process uses a continuous feedback system, which is based strictly on error.Distinction between smooth pursuit, optokinetic nystagmus, and ocular following response
Although we can clearly separate smooth pursuit from the vestibulo-ocular reflex, we can not always draw a clear separation between smooth pursuit and other tracking eye movements like the slow phase of the optokinetic nystagmus and the ocular following response (OFR), recently discovered by Miles, Kawano and Optican , which is a transient ocular traking response to full-field motion. The latter are both slow eye movements in response to extended targets, with the purpose of stabilizing the image. Therefore, some processing stages are shared with the smooth pursuit system . Those different kinds of eye movements may not be simply differentiated by the stimulus that is appropriate to generate them, as smooth pursuit eye movements can be generated to track extended targets as well. The main difference may lie in the voluntary nature of pursuit eye movements .Smooth-pursuit deficits
Smooth pursuit requires the coordination of many brain regions that are far away from each other. This makes it particularly susceptible to impairment from a variety of disorders and conditions.Schizophrenia
There is significant evidence that smooth pursuit is deficient in schizophrenic patients and their relatives. Schizophrenic patients tend to have trouble pursuing very fast targets. This impairment is correlated with less activation in areas known to play a role in pursuit, such as the frontal eye field However, other studies have shown that schizophrenic patients show relatively normal pursuit, compared to controls, when tracking objects that move unexpectedly. The greatest deficits are when the patients track objects of a predictable velocity which begin moving at a predictable time. This study speculates that smooth pursuit deficits in schizophrenia are a function of the patients' inability to store motion vectors.Autism
Autistic patients show a plethora of visual deficits. One such deficit is to smooth pursuit. Children with autism show reduced velocity of smooth pursuit compared to controls during ongoing tracking However, the latency of the pursuit response is similar to controls. This deficit appears to only emerge after middle adolescence.Trauma
Patients with post traumatic stress disorder, with secondary psychotic symptoms, show pursuit deficits. These patients tend to have trouble maintaining pursuit velocity above 30 degree/second. A correlation has also been found between performance on tracking tasks and a childhood history of physical and emotional abuse.Drugs and Alcohol
"Lack of Smooth Pursuit" is a scorable clue on the NHTSA's standardized field sobriety tests. The clue, in combination with others, may be used to determine if a person is impaired by alcohol and/or drugs. Drugs causing lack of smooth pursuit include depressants, some inhalants, and dissociative anisthetics (such as PhencyclidinePhencyclidine
Phencyclidine , commonly initialized as PCP and known colloquially as angel dust, is a recreational dissociative drug...
or Ketamine
Ketamine
Ketamine is a drug used in human and veterinary medicine. Its hydrochloride salt is sold as Ketanest, Ketaset, and Ketalar. Pharmacologically, ketamine is classified as an NMDA receptor antagonist...
).
See also
- Eye movement
- Eye trackingEye trackingEye tracking is the process of measuring either the point of gaze or the motion of an eye relative to the head. An eye tracker is a device for measuring eye positions and eye movement. Eye trackers are used in research on the visual system, in psychology, in cognitive linguistics and in product...
- Frontal eye fieldsFrontal eye fieldsThe frontal eye fields is a region located in the premotor cortex, which is part of the frontal cortex of the primate brain.-Function:...
- SaccadeSaccadeA saccade is a fast movement of an eye, head or other part of an animal's body or device. It can also be a fast shift in frequency of an emitted signal or other quick change. Saccades are quick, simultaneous movements of both eyes in the same direction...
- Superior colliculusSuperior colliculusThe optic tectum or simply tectum is a paired structure that forms a major component of the vertebrate midbrain. In mammals this structure is more commonly called the superior colliculus , but, even in mammals, the adjective tectal is commonly used. The tectum is a layered structure, with a...
- EndophenotypeEndophenotypeEndophenotype is a psychiatric concept and a special kind of biomarker. The purpose of the concept is to divide behavioral symptoms into more stable phenotypes with a clear genetic connection...