Transcript lecture10

Describe 2 functions of eye movements and give an example of each.
Types of Eye Movement
Information Gathering
Voluntary (attention)
Stabilizing
Reflexive
Saccades
vestibular ocular reflex (vor)
new location, high velocity, ballistic
body movements
Smooth pursuit
optokinetic nystagmus (okn)
object moves, velocity, slow
whole field image motion
Vergence
change point of fixation in depth
slow, disjunctive (eyes rotate in opposite directions)
(all others are conjunctive)
Fixation: period when eye is relatively stationary between saccades.
Describe foveal over-representation in the visual cortex.
Give a reason for this over-representation. Draw a diagram to
illustrate your answer.
Describe foveal over-representation in the visual cortex.
Give a reason for this over-representation. Draw a diagram to
illustrate your answer.
Cortical
Magnification
Primary Visual Cortex: V1
Larger cortical area for neural projections
from the fovea than periphery
Give a reason for this over-representation.
- High density of cone photoreceptors in central fovea.
Describe your results in the ball-catching lab.
Draw a sketch of the brain showing the structures involved
in the generation of a saccadic eye movement. Specify the
function of these structures (to the extent that this is possible)
Function of Different Areas
monitor/plan
movements
target selection
saccade decision
inhibits SC
saccade command
signals to muscles
Sketch a muscle, including the muscle spindle and
the sensory and motor nerve fibers.
Secondary Encoding
(Less Sensitive)
MUSCLEFIBRE/SPINAL
Spindles
Golgi
Dorsal
Extrafusal
Ventral
Intrafusal
How does the speed of a movement (e.g. reaching) affect
its accuracy? Explain why.
Role of Visual Feedback
Question: why does error increase with speed?
Note: 50 cm/sec = 5cm/100msec
Draw a sketch of a feedback system and a feedforward
system. Give an example of each (for visuo-motor control).
Schematic Representation of Feedback and Feed-forward Systems
Eg: pursuit, reaching, grasping
Eye velocity=image velocity
sensory
Motor command
retinal velocity
delay
Load/fatigue/current position
Eg: saccade, throwing
wind
ballistic
Learnt motor command
guided
Describe the effect on reaching movements of large-fiber
sensory neuropathy (degeneration of the afferent fibers
from the muscles and skin) when visual feedback is
eliminated? Sketch the movements to illustrate your answer.
Consequences of loss of feedback on reaching
Large fibre sensory
neuropathy leads
to loss of proprioceptive
feedback
from muscles
Errors in direction,distance
Normal:
proprioception
only
No vision or proprioception
Vision compensates
for lack of proprioception
Describe the problems that have to be solved in order to
program a robot to flip an egg. Mention possible
solutions.
Flipping an Egg
Autonomous control: robot is pre-programmed - no human input
Problems to be solved:
1. Grasp spatula
locate handle (vision)
some mechanism to translate location into arm movement
some mechanism for controlling fingers - “passive compliance”
2. Move to pan
locate pan (vision)
translate location into arm movement
3. Lower spatula to pan
vision or
proprioception: lower until force > 0
4. Flatten
proprioception: rotate until forces on fingers are equal
5. Locate egg
vision or
proprioception: move forwards until horizontal force > 0
Flipping an Egg (ctd)
Problems to be solved:
6. Lift
need to keep spatula level: vision or proprioception (keep tension constant)
7. Flip
need to learn how much to rotate hand: rotate until forces = 0
Limitations of autonomous control:
inflexible - can’t adapt to changed circumstances
requires high precision
Why is prediction necessary?
Components of visuo-motor latency.
Photoreceptors
ganglion cells
primary visual cortex
pre-motor ctx
M1
LGN
posterior parietal ctx
muscles
Round trip from eye to brain to muscles takes a minumum
of 200 msec. Ball (our expt) only takes about 900 msec.
Prediction gets around the problem of sensory delays.
Ability to adapt to new relationships requires cerebellum
Why do we need to retain plasticity for new
visuo-motor relationships?
1. Need to adjust to changes in body size during development.
2. Need to adjust to damage/aging.
3. Need to adjust to environmental changes eg ice, loads etc.
4. Need to learn arbitrary mappings for tool use etc.
5. Need to acquire new motor skills.
6. Visuo-motor coordination is a computationally difficult problem for
the brain. Need flexibility to correct errors.
Role of Experience in
Development of Visuo-motor
coordination
Held & Hein
1
2
Both kittens get visual experience and motor experience
1. Visual experience correlated with motor
commands/proprioceptive feedback/vision of limbs
2. Gets both, but uncorrelated. Kitten 2 -abnormal visuomotor coordination.
Adaptation to different relation between
vision and movement.
George Stratton
–Wore inverting lens for 8 days
If he saw an object on the right he would reach with his right hand and discover
he should have reached with his left. He could not feed himself very well, could
not tie his shoelaces, and found himself severely disoriented. His image of his
own body became severely distorted. At times he felt his head had sunk down
between his shoulders,and when he moved his eyes and head the world slid
dizzyingly around.
As time went by Stratton achieved more effective control of his body. If he saw an
object on the right he would reach with his left hand. He could accomplish normal
tasks like eating and dressing himself. His body image became almost normal
and when he moved his eyes and head the world did not move around so much.
He began to feel as though his left hand was on the right, and his right hand on
the left. If this new location of his body was vivid, the world appeared right side
up, but sometimes he felt his body was upside down in a visually right-side-up
world.
After removing the prisms, he initially made incorrect reaching movements.
However, he soon regained normal control of his body.
Adaptation to different relation between
vision and movement.
George Stratton
– Wore inverting lens for 8 days
– Believed that we learn visual directions by
associating visual experiences with other forms of
sensory feedback (e.g. proprioceptive).
– Alternatively…
Adaptation results from learning correlation betweeen
vision and actively generated motor commands
(Held, 1965).
Role of Experience in
Development of Vision
Molyneux’s Question:
Can a person blind from birth, whose vision
is restored, tell that a circle and a square are
different shapes?
Mike May - world speed record for downhill skiing by a blind person.
Lost vision at age 3 - scarred corneas.
Optically 20/20 - functionally 20/500 (cf amblyopia)
Answer to Molyneux’s question: Mike May couldn’t tell difference between sphere and
cube. Improved, but does it logically rather than perceptually. (cf other cases)
Color: an orange thing on a basket ball court must be a ball.
Motion: can detect moving objects, distinguish different speeds.
Note: fMRI shows no activity in Infero-temporal cortex (corresponding to pattern
recognition) but there is activity in MT, MST (motion areas) and V4 (color). Other parts of
brain take over when a cortical area is inactive.
Cannot recognize faces. (eyes, movement of mouth distracting)
Can’t perceive distance very well.
Can’t recognize perspective.
No size constancy or lightness constancy/ segmentation of scene into objects, shadows
difficult.
Vision most useful for catching balls (inconsistent with Held & Hein??) and finding things
if he drops them.
MT/MST (motion)
V4 (color)
Infero-temporal cortex
Implications?
Basic object perception (recognition and segmentation) requires
experience. (Experience prior to 3 yrs not enough.)
Geometric cues about scene structure (perspective, distance) also
require experience.
Color and motion more robust - either present at birth, or acquired
before 3yrs, and preserved without continued experience.
Lab 3: Learning Visuo-motor coordination.
Virtual ball catching environment.
Ball bounces.
Hit ball with ping-pong bat.
Possible manipulations:
1. Move bat in virtual environment so it appears to be displaced from
actual hand position. Look at ability to adapt.
2. Change bounciness of ball. How does hand movement change?
3. Can Ss learn that different balls bounce in different places? Red
and blue balls with different average bounce points. Does fixation
go to different location for red and blue balls?