Why do we move our eyes?

1. Why do we move our eyes? - Image stabilization in the presence of body movements. - Information acquisition - bring objects of interest onto high acuity region in fovea.

2. Retinal structure

3. Cone Photoreceptors are densely packed in the central fovea

4. Visual Acuity matches photoreceptor density

5. Oculomotor Muscles Muscles innervated by oculomotor, trochlear, and abducens (cranial) nerves from the oculomotor nuclei in the brainstem. Oculo-motor neurons: 100-600Hz vs spinal motor Neurons: 50-100Hz

6. Types of Eye Movement Information GatheringStabilizing Voluntary (attention) Reflexive Saccades vestibular ocular reflex (vor) new location, high velocity (700 deg/sec), body movements ballistic(?) Smooth pursuit optokineticnystagmus (okn) object moves, velocity, slow(ish) – typically whole field image motion up to 35 deg/sec Vergence change point of fixation in depth slow, disjunctive (eyes rotate in opposite directions) (all others are conjunctive) Note: link between accommodation and vergence Fixation: period when eye is relatively stationary between saccades.

7. Acuity – babies Acceleration Depth-dept gain, Precision in natural vision Velocity

8. https://www.youtube.com/watch?v=KSJksSA6Q-A

9. Latency of vestibular-ocular reflex=10msec

10. Demonstration of VOR and its precision – sitting vs standing Miniature eye movements Slow drift Micro-saccades tremor It is almost impossible to hold the eyes still.

12. Step-ramp allows separation of pursuit (slip)and saccade (displacement) Saccade latency approx 200 msec, pursuit approx 100 – smaller when there is a context that allows prediction.

14. “main sequence”: duration = c Amplitude + b Min saccade duration approx 25 msec, max approx 200msec

15. Factors That Control Gaze. - TASKDefines behavioral goals, what information is relevant. - REWARDSOculomotor circuitry sensitive to reward/subjective value of those goals. - UNCERTAINTY Get information. Peripheral resolution/ working memory REDUCTION decay etc - PRIORS/ Memory Gaze targeting reflects stored knowledge. - IMAGE Salient properties eg high contrast/ spatial outliers

16. Brain Circuitry for Saccades 1. Neural activity related to saccade 2. Microstimulation generates saccade 3. Lesions impair saccade

17. Brain Circuitry for Pursuit

18. Eye Tracking Methods

19. Developments in Eye Tracking Difficulty: optical power of eye + observer movement Head fixed /restricted:Contact lenses: mirror / magnetic coils Early infra-red systems Dual Purkinje Image tracker Head Free:Head mounted IR video-based systems Remote systems with head tracking Scene camera

20. Visual Angle x a d tan(a/2) = x/d a = 2 tan-1 x/d Why eye movements are hard to measure. A small eye rotation translates into a big change in visual angle 18mm 1 diopter = 1/focal length in meters 55 diopters = 1/.018 0.3mm = 1 deg visual angle

21. Measuring Eye Movements Early Methods: “Barlow photographed a droplet of mercury placed on the limbus. Translations of the head were minimized by having subjects lie on a stone slab with their heads wedged tightly inside a rigid iron frame” Kowler, 1990

22. Early methods: “The eye is first cocainized, then the lids should be propped apart by some form of eye-lid fastener, of which the best is probably that in form of a wide-opening spring with tortoise-shell grooves for the lids.” Delabarre, 1898

23. Monitoring Eye Movements; Yarbus Mirror mounted on eye using suction. Light bounces off mirror and is recorded on film

24. Non image-based Eye Trackers • Non image-based eye trackers • Electrical/analog • Limbus • Magnetic search coil

25. EOG The eye is a ‘dipole’ with ~millivolts voltage difference between the retina and the cornea. EOG

26. ElectroOculoGram(EOG) Use in clinic – head not fixed

27. Limbus Trackers By monitoring the ‘whites of the eye’ below the iris, it is possible to determine eye position. Vertical eye movements cause both signals to increase (up) or decrease (down). Horizontal eye movements cause differential illumination between the right and left sensors.

28. Limbus Trackers

29. Limbus

30. EOG and Limbus trackers Good temporal resolution. Lousy spatial resolution High noise, drift Mostly useful in clinic

31. Magnetic Search Coils Used for much animal work, though less so recently. Very high precision and accuracy (few minutes of arc). Used in older human em literature. Can use similar methodology for head and hand (see Hayhoe lab) Skalar search coils

32. Image-based Eye Trackers • Image-based eye trackers • Dual Purkinje • Video based

33. Dual Purkinje Trackers The ‘gold standard’ in eye trackers Multiple reflections from the cornea and lens vary in a very well-defined way as the eye moves. By tracking the 1st and 4th reflections, the tracker can determine eye position with very high precision. Bill Geisler lab has a binocular tracker.

34. Dual Purkinje Trackers Precision: < 1’ (~1/100 deg) Accuracy: a few min arc Update rate: > 500 Hz

35. Dual Purkinje Trackers Usually requires bite bar but theoretically can get away with head rest.

36. Video-based Eye Trackers • Video-based eye trackers: • Head mounted • Remote

37. Head mounted Camera on head views scene, another camera views eye.

38. Video-based Eye Trackers Infra-red video camera finds center of pupil and corneal reflection. Advantages: unconstrained viewing. Disadvantages: temporal resolution may be as low as 30 Hz Accuracy never better than 0.5 deg.

39. RIT Wearable Eyetracker

40. RIT Wearable Eyetracker

43. Build-up neurons in the intermediate layers of the SC are active prior to a saccade. Cell in the superifical layers get input from the retina. This may mediate Very fast saccades – sometimes called “express saccades” Extent of buildup neuron activity reflects stimulus probability. Express saccades might also reflect activity in buildup neurons.

44. Posterior Parietal Cortex Intra-Parietal Sulcus: area of multi-sensory convergence reaching LIP: Lateral Intra-parietal Area Target selection for saccades: cells fire before saccade to attended object grasping Visual stability

45. Model of saccade generation: target selection depends on expected value Area LIP contains a reward expectation signal which modulates the gain of visual neurons in LIP. Reward modulation of saccadic eye movementsoriginates from dopaminergic input to caudate nucleus. Trommershauser, Glimcher, Gegenfurtner, 2009

46. Relation between saccades and attention. Saccade is always preceded by an attentional shift However, attention can be allocated covertly to the peripheral retina without a saccade. Pursuit movements also require attention.

47. Visual Stability Figure 8.18 The comparator

49. A cross seen through an aperture that moves clockwise around the boundary. Alternatively, the aperture may be stationary, and the cross move behind it. Individual views, shown on the right, are ambiguous. Observers have no trouble with this if they have an “internal model” or schema that readily allows interpretation of the sequence.