2 Smooth pursuit basics used for pursuing or following moving targets smooth if the target movements are smoothmatch the angular velocity of the eye to the velocity of a moving targetas long as the target velocity is not too high!
3 Saccades and pursuit We can also “follow” with saccades discontinuouslyWe mostly follow a slow continuously moving target mostly with smooth pursuit (SP) movementsSP is continuousoften some saccades are superimposed on the SP
4 Vision remains clear through smooth pursuit Vision is suppressed briefly during saccadesSP is an attempt to match eye velocity to the velocity of the targetSaccades occur to correct foveation when there is pursuit errorSP is complementary to saccades
5 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
6 LatencyLatency = interval from initiation of stimulus to start of movementLatency of SP: milliseconds (ms)considerably longer than VOR (~10 ms)about the same as OKNconsiderably shorter than the latency for saccades ( ms.)
7 Alteration “in progress” if the direction or velocity of the target changes, SP is correspondingly alteredafter one latency periodSP movements are under continuous controlSaccades are under discontinuous (sampled) control
8 SP: stimulus is smallFunction : to keep maintained foveation of moving objectsVs. OKN: optimally stimulus is full-field rotation (OKN drum)Need a moving stimulus for SPcannot voluntarily generate SP without a target
10 Retinal slip velocity RSV = target velocity - eye velocity Gain = eye velocity/target velocityUsually, the eye doesn’t move as fast as the targetgain of the response is less than 1.0we make a saccade to “catch up”If our eyes move faster than the target, the gain is greater than 1.0we make a saccade in the direction opposite to the target’s direction
11 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 OKNbest indication that the optokinetic system has been activated: presence of optokinetic after-nystagmus (OKAN)
13 Cortical damageHemispherectomy impairs pursuit of ipsilaterally moving targetsdeficit for targets moving toward the side of the lesionUnilateral lesions of parieto-temporal cortex :reduction in pursuit speedconsequent increase in the frequency of “catch up” saccades
14 Unilateral MST damagedeficit present for any pursuit eye movement toward the lesioned hemisphereindependent of point of origin of the targetnot retinotopic.
15 Unilateral MT damage difficulty in initiating smooth pursuit worst in a relatively small portion of the visual fielda scotoma for moving stimulimay have difficulty in estimating target speedthought to provide sensory inputs to the parieto-temporal areas involved in motor aspects of smooth pursuit.
16 Cerbellumimportant for normal smooth pursuit gain and for combined eye/head tracking (cancellation of the VORbilateral damage to the flocculus and paraflocculus leads to enduring deficits in smooth pursuitOKN relatively preserved.
17 MLF Medial longitudinal fasiculus: signals necessary for vertical smooth pursuitvertical vestibular eye movementsmaintenance of vertical eye positionMLF fibers signal eye velocity during vertical smooth pursuit
18 Motor neurons Motor neuron behavior slow movements of smooth pursuit = vestibular slow phasessame final common pathAll muscle fibers seem to participate in all types of eye movements.