Eye movements: Lab # 1 - Catching a ball Simple tutorial on Eye Movements: http://www.med.uwo.ca/physiology/courses/sensesweb/L11EyeMovements/L11EyeMovements.swf

What can be learnt from natural tasks?

Gaze exclusively on task-relevant objects (see Land chapter) Eyes deal with one object at a time, corresponding to the duration of the manipulation.(Land: object-related actions) 3. Tight linkage between location of gaze and information needed at that moment. (Just-in-time strategy)

Why do we move our eyes? - Image stabilization - Information acquisition

The Eye and Retina A reason that it helps to move the eyes is the construction of the retina. Humans and many animals have foveated visual system - what this means is that spatial resolution varies across the eye and that the best sensitivity (ie detail) is concentrated In a one degree of visual agle in an area called the fovea – additionally – RGC’s pool over receptors - in the fovea have 1:1 input So there is no spatial summation

Cone Photoreceptors are densely packed in the central fovea 120 million rods, and are more sensitive (low light) than the cones. However, they are not sensitive to color. The 6 to 7 million cones provide the eye's color sensitivity and they are much more concentrated in fovea

Visual Acuity matches photoreceptor density Relative visual acuity Receptor density This is basically the same graph but also demonstrates that the level of acuity (Ie the sharpness of objects) Is best and falls off drastically in the periphery Peak acuity - can distinguish lines separated by a photoreceptor width.

Why do we move our eyes? 1. To bring objects of interest onto high acuity region in fovea.

Why eye movements are hard to measure. A small eye rotation translates into a big change in visual angle x a tan(a/2) = x/d a = 2 tan-1 x/d Visual Angle d 18mm For example, a 3 dioptre lens brings parallel rays of light to focus at 1/3 metre 1 diopter = 1/focal length in meters 55 diopters = 1/.018 0.3mm = 1 deg visual angle

Types of Eye Movement Information Gathering Stabilizing Voluntary (attention) Reflexive Saccades vestibular ocular reflex (vor) new location, high velocity, ballistic body movements Smooth pursuit optokinetic nystagmus (okn) object moves, velocity, slow whole field image motion Vergence change point of fixation in depth slow, disjunctive (eyes rotate in opposite directions) (all others are conjunctive) Now we should review the major types of eye movements Mediated by different brain areas which we’ll review on thursday Fixation: period when eye is relatively stationary between saccades.

Primary Cortical Sub-divisions

Visual Projections – optic nerve exits the eye – hemifields decussate at the optic chiasm so left brain gets left hemifield - right brain gets right hemifield. These projections go to several different areas - 90% of retinal projections go to LGN. (20% of lgn’s inputs are from retina) - this is the major stop before cortex – superior colliculus also receives direct retinal projections. Mid-brain projections: pupil dilation, accomodation, eye movements/head movements.

Brain Circuitry for Saccades 1. Neural activity related to saccade 2. Microstimulation generates saccade 3. Lesions impair saccade the frontal eye field, parietal eye field, supplementary eye field, prefrontal eye field, and area 7m In each of these regions: (1) there is neural activity closely related to eye movements; (2) electrical microstimulation produces or modifies eye movements; (3) surgical lesions or chemical inactivation impairs eye movements; (4) V1: striate cortex Basal ganglia Oculomotor nuclei

Function of Different Areas monitor/plan movements target selection saccade decision saccade command inhibits SC signals to muscles

Posterior Parietal Cortex Intra-Parietal Sulcus: area of multi-sensory convergence reaching grasping PPC contains areas LIP, VIP, MST, and areas 7a and 7b. Sensory motor interface Many types of representations in space have been found in the PPC. It is an area of high convergence where vision, audition, eye & head position, eye velocity, vestibular, and proprioceptive signals combine. LIP: Lateral Intra-parietal Area Target selection for saccades: cells fire before saccade to attended object

Brain Circuitry for Pursuit Smooth pursuit & Supplementary Smooth pursuit frontal eye fields appears to exert the most direct influence. The traditional pathways through the cerebellum are important, but there are also newly identified routes involving structures previously associated with the control of saccades, including the basal ganglia, the superior colliculus, and nuclei in the brain stem reticular formation.

Brain Circuitry for Pursuit Smooth pursuit & Supplementary Velocity signal Early motion analysis Smooth pursuit frontal eye fields appears to exert the most direct influence. The traditional pathways through the cerebellum are important, but there are also newly identified routes involving structures previously associated with the control of saccades, including the basal ganglia, the superior colliculus, and nuclei in the brain stem reticular formation.

How do we use our eyes to catch balls? What information the the brain need? Neurophysiological experiments look at single movements in response to flashes of light.

Eye movements in cricket: Batsman anticipate bounce point Better batsman arrive earlier saccade pursuit Show - Not confined to expert cricketers- general aspect of behav Land & MacLeod, 2001

Why are eye movements predictive? Analysis of visual signals takes a lot of time! Photoreceptors ganglion cells LGN Primary visual cortex other cortical areas mid-brain brain stem muscles Round trip from eye to brain to muscles takes a minumum of 200 msec. Cricket ball only takes about 600 msec. Prediction gets around the problem of sensory delays.

Is prediction seen in cricket a general property of behavior, or only seen in skilled performance like cricket or baseball?

Catching: Gaze Patterns smooth pursuit X X saccade X Thrower Catcher

Unexpected bounce leads to poor performance, particularly in the pursuit movement after the bounce. Implications of this?

After three trials, pursuit has improved a lot. Implications of this?

Different pattern of eye movements when watching (earlier, no pursuit). Implications of this?

Gaze Patterns Different when Watching saccade X X X Sometimes watcher, sometimes catcher - vision used differently Thrower Catcher

Lab groups

1. What are the questions? Is the behavior observed by Land in cricket also true for a simple task like catching a ball? What eye movements are made in this case? Do subjects anticipate the bounce point? By how much? Does it correlate with performance? Do Subjects look at floor or above the bounce point? What happens after bounce? How do subjects adjust to different balls? ….. Similarity between individuals? When do the hands start to move? 2. Choice of task: Catching and throwing a ball. 3. Procedure: Select subject and calibrate eye tracker. Three people stand at equal distances apart and throw the ball back and forth, with a bounce in the trajectory. Need to measure this distance. First throw in a predictable manner, about10 times. Then use a different ball,10 trials. Other balls… Compare one versus two eyes???

2. Data analysis Label your tape. Play it frame-by-frame on the VCR in the lab. …. What to look for: Describe eye movements sequence for each trial eg Trial 1: fixate near hands/saccade to bounce point/fixate/track portion of trajectory/fixate for last part of trajectory (??) Trial 2: fixate near hands/saccade to bounce point/fixate/track portion of trajectory/fixate for last part of trajectory (??) B How regular is the sequence of movements? C What is the timing of the saccades/fixations/tracking relative to movement of the ball. How much do subjects anticipate the bounce point, if at all? D. How accurate are fixations near the bounce point? (Need to measure visual angle.) Compare different conditions. What happens with the different balls? Do the eye movements change with additional experience? How quickly do they adjust? Other Aspects: Compare timing of eye and head movements? When do hands start to move, relative to release of ball? How similar are different individuals? Where would we expect similarities/ differences? What is the role of the pursuit movement? If pursuit is made only on final bounce, implies pursuit is used to guide hands. Maybe position of eye in head. Binocular information versus monocular (looming)