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.
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
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 motor
Neurons: 50-100Hz
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
Mostly 0-35 deg/sec but maybe up to100deg/sec
Vergence
change point of fixation in depth
slow, 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.
Acuity – babies
Rotational or translational
Acceleration
Velocity
Depth-dependent gain, Precision in natural vision
Ocular following - Miles
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 latency
is about 10 - 15 msec
Demonstration of VOR and its precision – sitting vs standing
Step-ramp allows separation of pursuit
(slip) and saccade (displacement)
Saccade latency approx 200 msec, pursuit approx 100 – smaller when there is a context that
allows prediction.
“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
Drift
Micro-saccades
Tremor
Significance??
It is almost impossible to hold the eyes still.
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
monitor/plan movements
Dorso-lateral
pre-frontal (memory)
Basal ganglia
V
H
Oculomotor nuclei
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
Visual stability
FEF – visual, visuo-motor, and
movement cells
Supplementary eye fields: SEF
-Saccades/smooth pursuit
-Planning/ Error checking
-relates to behavioral goals
FEF:
-Voluntary control
of saccades.
-Selection from
multiple targets
-Relates to
behavioral goals.
Cells in caudate signal both saccade direction and expected reward.
Hikosaka et al, 2000
Monkey makes a saccade to a stimulus - some directions are rewarded.
Superior colliculus
Pre-motor neurons
Trochlear
Motor neurons
V
Oculomotor
nucleus
Abducens
H
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).
Pulse-Step signal for a saccade
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?
Relation between saccades and attention.
Saccade is always preceded by an attentional shift
However, attention can be allocated covertly to the
peripheral retina without a saccade.
Pursuit movements also require attention.
Brain Circuitry for Pursuit
& Supplementary
Smooth 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