Why eye movements are hard to measure. 18mm 0.3mm = 1 deg visual angle xa tan(a/2) = x/d a = 2 tan - 1 x/d Visual Angle d 1 diopter = 1/focal length in meters 55 diopters = 1/.018 A small eye rotation translates into a big change in visual angle
Types of Eye Movement Information GatheringStabilizing Voluntary (attention)Reflexive Saccadesvestibular ocular reflex (vor) new location, high velocity (700 deg/sec), body movements ballistic(?) Smooth pursuitoptokinetic 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 It is almost impossible to hold the eyes still. Drift Micro-saccades Micro-nystagmus
“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 Oculomotor nuclei V1: striate cortex Basal ganglia 1. Neural activity related to saccade 2. Microstimulation generates saccade 3. Lesions impair saccade Dorso-lateral pre-frontal
target selection signals to muscles (forces) inhibits SC saccade decision saccade command (where to go) monitor/plan movements Function of Different Areas H V
LIP: Lateral Intra-parietal Area Target selection for saccades: cells fire before saccade to attended object Posterior Parietal Cortex reaching grasping Intra-Parietal Sulcus: area of multi-sensory convergence
Frontal eye fields Voluntary control of saccades. Selection from multiple targets 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?