Smooth pursuit

1. Smooth pursuit

2. 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!

3. 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

4. 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

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. 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.)

7. 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

8. 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

9. Models of SP: stimulus is retinal slip velocity

10. 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

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 OKN • best indication that the optokinetic system has been activated: presence of optokinetic after-nystagmus (OKAN)

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

13. 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

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

15. 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.

16. 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.

17. 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

18. 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.