why do we move our eyes? - image stabilization - information acquisition

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.

18mm

0.3mm = 1 deg visual angle

x a

tan(a/2) = x/da = 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 Gathering StabilizingVoluntary (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

Vergencechange point of fixation in depthslow, 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.

DriftMicro-saccadesMicro-nystagmus

“main sequence”: duration = c Amplitude + bMin 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 AreaTarget selection for saccades: cells fire before saccade to

attended object

Posterior Parietal Cortex

reaching

grasping

Intra-Parietal Sulcus: areaof multi-sensory convergence

Frontal eye fields

Voluntary controlof saccades.Selection from multiple targetsRelates to behavioral goals.

-Saccades/Smooth Pursuit

-Planning/ Error Checking

-relates to behavioral goals

Supplementary eye fields

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

Smooth pursuit& Supplementary

Brain Circuitry for Pursuit

Velocity signal

Early motion analysis