Post on 16-Dec-2015
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