Lecture # 25

1. Lecture #25 Motion detection and eye movement 4/30/13

2. Online course evaluations • If you like small class • If you like interdisciplinary class • If you have suggestions to make the course better • Please do the course evaluation

3. Final exam Q • For next Tuesday, please write 1 final exam question • Based on 1 lecture or concept • Should be a different topic from your wiki • List critical points • Write study question to distill key point • Write synthetic question for exam and answer it • Study questions will get posted • Best 4 synthetic Q will be on exam : choose 1 of 4 to answer

4. Final exam format • Part 1 • I will post 10 questions on web ahead of exam. You will pick 3 of 5 to answer on exam • Part 2 • I will post your study questions on web. The best 4 synthetic questions will be on exam as pick 1 of 4 • Part 3 • One question which will cover eye development and evolution lectures

5. Wiki project • Final wiki project is due next Thursday • 3 major sections • Try to avoid or at least explain medical jargon • Make it readable – do not sound like encyclopedia • Include references in the text • Have someone else read it • Or finish it and then wait a day to proof it

6. Wiki grading rubric • Introduction Total 40 pts • Section 1 • Section 2 • Section 3 • Facts correct • Use of literature • Figures cited • Writing style • Interesting presentation • Extra effort

7. Eye movements • Eyes are in constant motion • Keeps receptors from being saturated with same signal • If stare at same thing for long time - scene would fade

8. Eye movements

9. Eye movements • Saccades • Rapid eye movements to sample different parts of a scene • Smooth eye pursuits • Slower smooth motions that follow slow moving objects • Not possible for static image • Vergence movement • Eyes move together as focus in on object

10. Eye movements Saccades Smooth pursuit Vergence

11. Two other reflexes • Optokineticreflex • Eyes sense motion of object and move to keep object centered • Object moves clockwise - Eyes move clockwise • If no moving object - hard to make eyes move smoothly • Eyes want to saccade

12. Two other reflexes • Vestibular-ocular reflex • Sense motion of head and move eyes in opposite direction to compensate • Use semi-circular canals to detect motion and feed to eye muscles • Move head while fixated • on object

13. Eye movements 2 Optokinetic reflex Vestibulo-ocular reflex

14. Saccades are not random - take eyes to regions of interest Fig 7.15

15. Making a cup o’ tea

16. Stabilize gaze Eyes compensate for head movement so that gaze can remain stable or fixate on certain objects

17. Other animals also use eye saccades • Coordinate eye and head movements • Saccade and then fixate

18. Birds can’t move eyes as much so use head motion = head saccade • Spend 50% of time with head fixed while rest of body moves • Creates bobbing motion of head Cronin et al 2005

19. Cranes in motion

20. Head angle fixates and then changes rapidly Cronin et al 2005 Supp Fig 1

21. Eye movements • Constant small jerky movements • Refresh image so retinal image doesn’t saturate • Larger saccades • Up to 1000°/s per motion • Number of saccades • 3 per second

22. Eye movements • How do we deal with rapid saccades? • You don’t notice them

23. Eye movements • How do we deal with rapid saccades? • You don’t notice them • Saccadic suppression • Suspend visual activity during saccade • Suppress input to brain • But no suppression during smooth pursuit • Spend 10% of time in saccade • 1.5 hrs per day being blind!!

24. Why use saccades and fixation? • Avoid loss of resolution due to motion blur • Blur occurs if object moves > 1 receptor diameter / response time • Human - • Receptor 0.016 deg acceptance angle • Response time = 20 ms = 0.02 s • Blur if motion > 1°/s

25. Why use saccades and fixation? • Detect motion of objects against steady background • Detect motion of self against background

26. Eye movements 6 muscles move the eye, work in pairs

27. Cranial nerves send control to eye muscles Oculomotor nucleus sends output to four eye muscles (cranial nerve III) Trochlear nucleus to superior oblique (cranial nerve IV) Abducens nucleus to lateral rectus (cranial nerve VI)

28. Brain regions controlling eye movement The motor nuclei get input from superior colliculus through gaze centers: Vertical / horizontal SC gets input from retinal ganglion cells FEEDBACK If stimulate superior colliculus, eyes will move

29. Motion perception question? • Place  on paper in front of you • Fixate on dot • Move pen from left to right as stare at dot • See pen move • Follow motion of pen with eyes • See pen move • Image of dot moves across retina • Do you perceive it as moving?

30. Distinguish eye movement and object movement

31. Distinguish eye movement and object movement

32. Comparator • Motor systems send signal to eyes to move and message to comparator that eyes are moving • Compensate for eye movement so other parts of brain do not interpret retinal image change as motion Wolfe Fig 7.16

33. Motion perception • Needed to detect motion • Direction • Speed • Special circuits • Peripheral retina (M ganglion cells)  • Magnocellular layers of LGN  • primary visual cortex (V1)  • middle temporal lobe (MT)

34. Wiring - motion detector A B W Use two cells to detect object first at point A and then point B

35. A B W Motion neuron, M stimulated by large stationary object Wiring - motion detector A B W A detects object before B. Motion neuron, M doesn’t get signal from both at same time

36. Wiring - motion detector Detects motion from left to right Not sensitive to motion right to left A B A B A B D D D X X X W W W Cell X signals only if signal from both D and B Cell D delays signal

37. Q. How could a neuron be a delay cell? Cell D delays signal Cell X signals only if signal from both D and B A B A B A B D D D X X X W W W Adding a 2nd cell makes for an additional synapse. This will add a few ms delay. Having a longer axon can also build in a delay time. These delays are known in fly eyes and auditory systems.

38. Simulation

39. A B D X W Motion circuitry • Can detect: • Motion direction • Speed • Circuitry works for actual moving object or object that • Appears at A • Disappears • Reappears at B • = Apparent motion

40. Apparent motion

41. Perception of apparent motion • Apparent motion changes with speed at which alternates • Optimal frequency • Too fast - flickers • Too slow - hops • Just right - motion appears smooth

42. Apparent motion

43. Aperture problem • Viewing moving image through an aperture • How do we know which parts correspond through time? • Mask or aperture makes correspondence uncertain

44. Correspondence problem Which dot does the blue one correspond to in the next frame? A or B? A B

45. Global motion detection solves correspondence problem

46. Global motion detection By comparing output of all 4 motion sensors, the global sensor can only come to one conclusion Motion is down and to right

47. Motion correspondence

48. Other ways to test for motion sensors • Motion detection is in middle temporal lobe (MT) • Train monkeys to detect dots moving • Can detect even if only a few % are in motion • Lesion parts of MT • Monkeys then need many more dots to detect the motion

49. Dot motion

50. Using motion to navigate - optic flow • Motion of visual field tells if moving in space • Focus of expansion • Locate non-moving part of scene - where you are headed • Use motion vs lack of motion to set direction