Eye movements and visual stability

Kandel et al Ch 29, end of Wolfe Ch 8 Kandel Ch 39 for more info.

Advanced: Werner & Chalupa Ch 63

Why do we move our eyes?

- Image stabilization

- Information acquisition

Bring objects of interest onto high acuity region in fovea.

Visual Acuity matches photoreceptor density

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

Oculomotor Muscles

Muscles innervated by oculomotor, trochlear, and abducens (cranial) nerves from the oculomotor nuclei in the brainstem. Oculo-motor neurons: 100-600Hz vs spinal motorNeurons: 50-100Hz

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 motionMostly 0-35 deg/sec but maybe up to100deg/sec

Vergencechange point of fixation in depthslow, disjunctive (eyes rotate in opposite directions)(all others are conjunctive)Note: link between accommodation and vergence

Fixation: period when eye is relatively stationary between saccades.

AccelerationDepth-dependent gain, Precision in natural vision

VelocityOcular following - Miles

Acuity – babies

Rotational or translational

otoliths

Rotational (semi-circular canals) translational (otoliths)

The vestibular labyrinth

Rotational (semi-circular canals) translational (otoliths)

Hair cell responses

Neural pathways for the angular-VOR three-neuron arc

Vestibular latencyis about 10 - 15 msec

Demonstration of VOR and its precision – sitting vs standing

Saccade latency approx 200 msec, pursuit approx 100 – smaller when there is a context thatallows prediction.

Step-ramp allows separation of pursuit (slip) and saccade (displacement)

“main sequence”: duration = c Amplitude + b (also V = a Amp+d)Min saccade duration approx 25 msec, max approx 200msec

Demonstration of “miniature” eye movements

It is almost impossible to hold the eyes still.

DriftMicro-saccadesTremor

Significance??

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

Basal ganglia

1. Neural activity related to saccade

2. Microstimulation generates saccade

3. Lesions impair saccade

Dorso-lateral pre-frontal (memory)

H

V

monitor/plan movements

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

Visual stability

FEF – visual, visuo-motor, andmovement cells

-Saccades/smooth pursuit

-Planning/ Error checking-relates to behavioral

goals

Supplementary eye fields: SEF

FEF:-Voluntary controlof saccades.-Selection from multiple targets-Relates to behavioral goals.

Monkey makes a saccade to a stimulus - some directions are rewarded.

Cells in caudate signal both saccade direction and expected reward.

Hikosaka et al, 2000

Superior colliculus

Motor neurons for the eye muscles are located in the oculomotor nucleus (III), trochlear nucleus (IV), and abducens nucleus (VI), and reach the extraocular muscles via the corresponding nerves (n. III, n. IV, n. VI).Premotor neurons for controlling eye movements are located in the paramedian pontine reticular formation(PPRF), the mesencephalic reticular formation (MRF), rostral interstitial nucleus of the medial longitudinal fasciculus (riMLF), the interstitial nucleus of Cajal (IC), the vestibular nuclei (VN), and the nucleus prepositus hypoglossi (NPH).

Motor neurons

Pre-motor neurons

Oculomotor nucleus

Trochlear

Abducens

H

V

Pulse-Step signal for a saccade

Brain areas involved in making a saccadic eye movementBehavioral 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?

Relation between saccades and attention.

Saccade is always preceded by an attentional shiftHowever, attention can be allocated covertly to the peripheral retina without a saccade.

Pursuit movements also require attention.

Smooth pursuit& Supplementary

Brain Circuitry for Pursuit

Velocity signal

Early motion analysis

Gaze shifts: eye plus head

Visual Stability

Efference copy or corollary discharge

Figure 8.18 The comparator

Experiments with partial and complete paralysis of extra-ocular muscles

Stevens et al – partial paralysis – world jumps during an em

Matin – complete paralysis – no motion

Resolution: Bayesian cue combination.

Note: Visual stability vs Visual Direction Constancy