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: milliseconds (ms)
considerably longer than VOR (~10 ms)
about the same as OKN
considerably shorter than the latency for saccades ( 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. SP pathways Retinal image motion LGN Striate cortex
Extrastriate cortex (MT, MST, posterior parietal
Pontine nuclei and Cerebellar cortex
Ocular motoneurons

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.