Presentation on theme: "Why do we move our eyes? - Image stabilization"— Presentation transcript:
1 Why do we move our eyes? - Image stabilization - Information acquisition
2 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 peripheryPeak acuity - can distinguish lines separated by a photoreceptor width.
3 Why do we move our eyes?1. To bring objects of interest onto high acuity region in fovea.
4 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 imagein the central visual field.
5 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 theType of eye movements that we can make and we’ll ignore what is the actual source of the motor command3 pairs that operate antagonistically (this is implemented in brain so when one contracts the other relaxes)Left/Right – Lateral/Medial RectusUp/Down,Superior/Inferior rectus-Superior/inferior oblique – torsion – almost totally ignored (what is the range of torsion?? Under voluntary control)
6 Why eye movements are hard to measure. A small eye rotation translates into a big change in visual anglexatan(a/2) = x/da = 2 tan-1 x/dVisual Angled18mmFor example, a 3 dioptre lens brings parallel rays of light to focus at 1/3 metre1 diopter = 1/focal length in meters55 diopters = 1/.0180.3mm = 1 deg visual angle
7 Types of Eye Movement Information Gathering Stabilizing Voluntary (attention) ReflexiveSaccades vestibular ocular reflex (vor)new location, high velocity (700 deg/sec), body movementsballistic(?)Smooth pursuit optokinetic nystagmus (okn)object moves, velocity, slow(ish) whole field image motionVergencechange point of fixation in depthslow, disjunctive (eyes rotate in opposite directions)(all others are conjunctive)Now we should review the major types of eye movementsMediated by different brain areas which we’ll review on thursdayFixation: period when eye is relatively stationary between saccades.
8 Demonstration of “miniature” eye movements DriftMicro-saccadesMicro-nystagmusKeeping 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.
9 “main sequence”: duration = c Amplitude + b Min saccade duration approx 25 msec, max approx 200msec
10 What’s involved in making a saccadic eye movement? Behavioral goal: make a sandwichSub-goal: get peanut butterVisual search for pb: requires memory for eg color of pb or locationVisual search provides saccade goal - attend to target locationPlan saccade to location (sensory-motor transformation)Coordinate with hands/headCalculate velocity/position signalExecute saccade/
11 Brain Circuitry for Saccades 1. Neural activity related to saccade2. Microstimulation generates saccade3. Lesions impair saccadeDorso-lateralpre-frontalthe frontal eye field, parietal eye field, supplementary eye field, prefrontal eye field, and area 7mIn 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 cortexBasal gangliaOculomotor nuclei
12 Function of Different Areas monitor/plan movementstarget selectionsaccade decisionsaccade command(where to go)inhibits SCVHsignals to muscles(forces)
13 Posterior Parietal Cortex Intra-Parietal Sulcus: areaof multi-sensory convergencereachinggraspingPPC contains areas LIP, VIP, MST, and areas 7a and 7b. Sensory motor interfaceMany 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 AreaTarget selection for saccades: cells fire before saccade to attended object
14 Frontal eye fields Voluntary control of saccades. Selection from multiple targetsRelates 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 eyefields 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.
15 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 targetAlso implicated in smooth pursuit
16 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 FEFPlan saccade - FEF, SEFCoordinate with hands/headExecute saccade/ control time of execution: basal ganglia (substantia nigra pars reticulata, caudate)Calculate velocity/position signal oculomotor nucleiCerebellum?
17 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.
18 Brain Circuitry for Pursuit Smooth pursuit& SupplementaryVelocity signalEarly motion analysisSmooth pursuitfrontal 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.