Smooth pursuit

Smooth pursuit basics used for pursuing or following moving targets smooth if the target movements are smooth match the angular velocity of the eye to the velocity of a moving target as long as the target velocity is not too high!

Saccades and pursuit We can also “follow” with saccades discontinuously We mostly follow a slow continuously moving target mostly with smooth pursuit (SP) movements SP is continuous often some saccades are superimposed on the SP

Vision remains clear through smooth pursuit Vision is suppressed briefly during saccades SP is an attempt to match eye velocity to the velocity of the target Saccades occur to correct foveation when there is pursuit error SP is complementary to saccades

Stimulus for SP: a slowly moving target. SP inaccurate above 20-30° per second. As target velocity increases, saccadic movements occur more frequently. SP movements can be altered in progress

Latency Latency = interval from initiation of stimulus to start of movement Latency of SP: 100-150 milliseconds (ms) considerably longer than VOR (~10 ms) about the same as OKN considerably shorter than the latency for saccades (200-250 ms.)

Alteration “in progress” if the direction or velocity of the target changes, SP is correspondingly altered after one latency period SP movements are under continuous control Saccades are under discontinuous (sampled) control

SP: stimulus is small Function : to keep maintained foveation of moving objects Vs. OKN: optimally stimulus is full-field rotation (OKN drum) Need a moving stimulus for SP cannot voluntarily generate SP without a target

Models of SP: stimulus is retinal slip velocity

Retinal slip velocity RSV = target velocity - eye velocity Gain = eye velocity/target velocity Usually, the eye doesn’t move as fast as the target gain of the response is less than 1.0 we make a saccade to “catch up” If our eyes move faster than the target, the gain is greater than 1.0 we make a saccade in the direction opposite to the target’s direction

SP and OKN Many SP targets also elicit some OKN if a small target moves on a stationary background, the background will move in the opposite direction on the retina and this can stimulate OKN best indication that the optokinetic system has been activated: presence of optokinetic after-nystagmus (OKAN)

SP pathways Retinal image motion LGN Striate cortex Extrastriate cortex (MT, MST, posterior parietal Pontine nuclei and Cerebellar cortex Ocular motoneurons

Cortical damage Hemispherectomy impairs pursuit of ipsilaterally moving targets deficit for targets moving toward the side of the lesion Unilateral lesions of parieto-temporal cortex : reduction in pursuit speed consequent increase in the frequency of “catch up” saccades

Unilateral MST damage deficit present for any pursuit eye movement toward the lesioned hemisphere independent of point of origin of the target not retinotopic.

Unilateral MT damage difficulty in initiating smooth pursuit worst in a relatively small portion of the visual field a scotoma for moving stimuli may have difficulty in estimating target speed thought to provide sensory inputs to the parieto-temporal areas involved in motor aspects of smooth pursuit.

Cerbellum important for normal smooth pursuit gain and for combined eye/head tracking (cancellation of the VOR bilateral damage to the flocculus and paraflocculus leads to enduring deficits in smooth pursuit OKN relatively preserved.

MLF Medial longitudinal fasiculus: signals necessary for vertical smooth pursuit vertical vestibular eye movements maintenance of vertical eye position MLF fibers signal eye velocity during vertical smooth pursuit

Motor neurons Motor neuron behavior slow movements of smooth pursuit = vestibular slow phases same final common path All muscle fibers seem to participate in all types of eye movements.