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

Visual Acuity matches photoreceptor 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 do we move our eyes? 1. To bring objects of interest onto high acuity region in fovea. 2. Cortical magnification suggests enhanced processing of image in the central visual field.

Muscles that Move the Eye Since I’ve talked about cortex and eye movements I should back up a bit and address what moves the eye – on Thursday we’ll talk about the brain structures involved in sending different eye movement commands but here I just want to emphasize the Type of eye movements that we can make and we’ll ignore what is the actual source of the motor command 3 pairs that operate antagonistically (this is implemented in brain so when one contracts the other relaxes) Left/Right – Lateral/Medial Rectus Up/Down,Superior/Inferior rectus- Superior/inferior oblique – torsion – almost totally ignored (what is the range of torsion?? Under voluntary control)

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 (700 deg/sec), body movements ballistic(?) Smooth pursuit optokinetic nystagmus (okn) object moves, velocity, slow(ish) 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.

Demonstration of “miniature” eye movements Drift Micro-saccades Micro-nystagmus Keeping you’re eye still may seem rather trivial but it fact is basically impossible = try this test- It is almost impossible to hold the eyes still.

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

What’s involved in making a saccadic eye movement? Behavioral goal: make a sandwich Sub-goal: get peanut butter Visual search for pb: requires memory for eg color of pb or location Visual search provides saccade goal - attend to target location Plan saccade to location (sensory-motor transformation) Coordinate with hands/head Calculate velocity/position signal Execute saccade/

Brain Circuitry for Saccades 1. Neural activity related to saccade 2. Microstimulation generates saccade 3. Lesions impair saccade Dorso-lateral pre-frontal 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 (where to go) inhibits SC V H signals to muscles (forces)

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

Frontal eye fields Voluntary control of saccades. Selection from multiple targets Relates to behavioral goals. The frontal eye field (FEF) of monkeys has been repeatedly implicated in the generation of saccadic eye movements by various experimental approaches. Electrical stimulation of most of the FEF produces saccadic eye movements, many cells have activities related to saccades, and it has anatomical connections with many other oculomotor areas. Surprisingly, complete lesions of the FEF have remarkably little effect on oculomotor behavior. The frontal eye fields form an executive center that can selectively activate superior colliculus neurons, playing a role in the selection and production of voluntary saccades. The activity of frontal eye fields neurons reflects the selection of the visual target for a saccadic eye movement when several potential goals for movements are available. The frontal eye fields is also involved in suppressing reflexive saccades and generating voluntary, non-visualsaccades. FEF receives information concerning the auditory, tactual, and visual environment and is multimodally responsive.

Supplementary eye fields -Saccades/Smooth Pursuit -Planning/ Error Checking -relates to behavioral goals “It appears that the neurons in the secondary eye field are monitoring eye movement, not controlling it, Activity when saccade to wrong target Also implicated in smooth pursuit

Brain areas involved in making a saccadic eye movement Behavioral goal: make a sandwich (learn how to make sandwiches) Frontal cortex. Sub-goal: get peanut butter (secondary reward signal - dopamine - basal ganglia) Visual search for pb: requires memory for eg color of pb or location (memory for visual properties - Inferotemporal cortex; activation of color - V1, V4) Visual search provides saccade goal. LIP - target selection, also FEF Plan saccade - FEF, SEF Coordinate with hands/head Execute saccade/ control time of execution: basal ganglia (substantia nigra pars reticulata, caudate) Calculate velocity/position signal oculomotor nuclei Cerebellum?

Superior colliculus Motor Responses     * Microstimulation produces conjugate, contralateral saccades         -- Amplitude and direction of the saccade induced depends on the site of stimulation within superior colliculus         -- Motor map - eye plus head     * Chronic recording reveals movement fields         --Neurons in intermediate and deep superior colliculus discharge maximally prior to saccades with particular amplitudes and directions.                

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.