Eye movements and visual stability
Kandel et al Ch 29, end of Wolfe Ch 8
Kandel Ch 39 for more info.
Is a set of Reichardt detectors is sensitive to motion in one direction and only in a particular
speed? It seems like an inefficient design since a great number of neurons will be required
to encode motion in all possible directions and speed, unless each of them can actually
encode for a small range of speed, although that might lower the sensitivity to speed
change. Or the visual cortices simply have enough neurons to do so.
I also want to know if information about the motion of objects stays in MT/MST etc or is it
transferred to other areas to bind with other
properties of objects that allow recognition of moving objects. Again the binding problem.
The reason I asked the binding question again is because the integration between
properties of object in the dorsal stream (eg. motion signals in MT)
and properties in the ventral stream (eg. orientation and shape for object recognition)
must differ somehow from integration within ventral stream for 'what' information.
Referring to Campbell and Robson's experiment, the lecturer mentioned
that if the cells adapt to seeing an upward motion,
they will notice a "downward" motion after the motion stops.
Is this the same kind of adaptation that sees an opposite color
when the color disappears (like in the rotating color circle that we saw two weeks ago)?
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.
Cone Photoreceptors are densely packed in the central fovea
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
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.
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.
Retinal structure
Accomodation:tension on zonule fibres
Acuity – babies
Acceleration
Velocity
Depth-dept gain, Precision in natural vision
Ocular following - Miles
otoliths
Rotational (semi-circular canals)
translational (otoliths)
VOR
Latency of vestibular-ocular reflex=10msec
Demonstration of VOR and its precision – sitting vs standing
It is almost impossible to hold the eyes still.
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
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
Dorso-lateral
pre-frontal
V1: striate cortex
Basal ganglia
Oculomotor nuclei
Function of Different Areas
monitor/plan
movements
target selection
saccade decision
saccade command
inhibits SC
(where to go)
V
H
signals to muscles
(forces)
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.
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
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.
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.
Superior colliculus
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
Brainstem circuits for saccades. Omnipause neurons (OPN) in the nucleus raphe interpositus (RIP) tonically
inhibit excitatory burst neurons (EBN) located in the paramedian pontine reticular formation (PPRF).
When OPNs pause, the EBNs emit a burst of spikes, which activate motor neurons (MN) in the abducens
nucleus (VI) innervating the lateral rectus muscle. The burst also activates interneurons (IN) which activate motor
neurons on the oculomotor nucleus (III) on the opposite side, innervating the medial rectus. Inhibitory burst
neurons (IBN) show a pattern of activity similar to EBNs, but provide inhibitory inputs to decrease activation in
the complementary circuits and antagonist muscles. Long-lead burst neurons (LLBN) show
activity long before movement onset, and provide an excitatory input to EBNs.
RF reticular formation
VN vestibular nucle
PN , pontine nucleii
Cerebellum
OV oculomotor vermis
VPF ventral paraflocculus
FN fastigial nucleus