Pursuit movement is the ability of the eyes to smoothly 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 find pursuit extremely difficult, if not impossible, to initiate without a moving visual signal.
There are two basic methods for recording smooth pursuit eye movements, and eye movements in general. The first is with a search coil. This technique is most common in primate research, and is extremely accurate. Eye movements shift 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. Although, no algorithm exists so far to analyze smooth pursuit with eye-tracking data.
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 mistakes are called catch-up saccades and are more common when pursuing at high speeds. Tools are available which allow researchers to look at traces of eye movement and discard sections of the data that contain saccades, which differ qualitatively from smooth pursuit because of their very high acceleration and decceleration.
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 cortex 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 movements. 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.
Stages of Smooth Pursuit
Pursuit eye movements 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 it wishes to track and typically lasts ~100ms. This stage of pursuit is ballistic in the sense that visual signals have not yet had time to travel through the visual system and correct the ongoing pursuit velocity. The second stage of pursuit is called closed-loop pursuit. This stage lasts from 100ms 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, if you are trying to pursue a target, but that target is getting farther and farther away from your fovea, during closed loop pursuit you will increase the gain of pursuit until you stabilize the image.
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.
Smooth Pursuit Deficits
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.
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.
Patients with post traumatic stress disorder, with secondary psychotic symptoms, show pursuit deficits. These patients tend to have trouble maintaining pursuit velocity above 30 degrees/second. A correlation has also been found between performance on tracking tasks and a childhood history of physical and emotional abuse.
Smooth pursuit trivia
- Smooth pursuit is asymmetric- most people and primates tend to be much better at horizontal smooth pursuit than vertical smooth pursuit, as defined by their ability to pursue smoothly without making catch-up saccades. Most people are also better at downward pursuit than upward pursuit.
- Usually, pursuit is impossible without a moving target. But there are a few exceptions:
- It is possible to pursue an imaginary target (eg. your moving finger) in total darkness. 
- It is possible to maintain pursuit even if a target momentarily disappears, especially if the target appears to be occluded by a larger object. 
- If you know which way a target will move, or how quickly it will move, you can initiate pursuit before the movement actually begins, especially if you know exactly when the motion will start.
- Latencies for smooth pursuit are actually shorter than latencies for saccades.
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- Overview at Brown University
- Overview at Université catholique de Louvain
- Pursuit,+Smooth at the US National Library of Medicine Medical Subject Headings (MeSH)