Transcript class1P&A07
Why do we move our eyes? - Image stabilization - Information acquisition Visual Acuity matches photoreceptor density 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 Why eye movements are hard to measure. A small eye rotation translates into a big change in visual angle Visual Angle x 18mm a d tan(a/2) = x/d a = 2 tan 1 x/d 1 diopter = 1/focal length in meters 0.3mm = 1 deg visual angle 55 diopters = 1/.018 Types of Eye Movement Information Gathering Voluntary (attention) Stabilizing 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) Fixation: period when eye is relatively stationary between saccades. Demonstration of “miniature” eye movements Drift Micro-saccades Micro-nystagmus 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 V1: striate cortex Basal ganglia Oculomotor nuclei Function of Different Areas monitor/plan movements target selection saccade decision saccade command inhibits SC (where to go) V H signals to muscles (forces) Posterior Parietal Cortex reaching Intra-Parietal Sulcus: area of multi-sensory convergence grasping 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. Supplementary eye fields -Saccades/Smooth Pursuit -Planning/ Error Checking -relates to behavioral goals 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 Brain Circuitry for Pursuit & Supplementary Smooth pursuit Velocity signal Early motion analysis