Lecture 15 - Objects in Motion

Question 1

aperture problem

Answer 1

when we look at the receptive fields (very small fields on the retina) each of them only gets a tiny piece of the environment (and objects are very large) = so no one receptive field can capture what’s happening at a whole object level

Question 2

overcoming the aperture problem

Answer 2

take info from lots of diff receptive fields to form that whole object

pooled convergence to get more complex receptive fields

activity in the dorsal pathway - activity in cells from V1 detecting basic motion features

MT –> MST (even more complex activity, spiral motion)

Question 3

middle temporal (MT) cortex

Answer 3

is located in the dorsal where/how stream,

for simple motion (waterfall illusion).

Question 4

understanding of complex motion is mediated by

Answer 4

MT

Question 5

how cells in MT account for the motion after effect

how is it different than color after affects?

Answer 5

– If coherent motion of stimuli is associated with cells in MT, perhaps they can become ‘fatigued’ through selective adaptation?

• Assume that the cells in MT have a spontaneous firing rate.
• After watching the waterfall stimulus, the ‘upward’ responding cells become selectively adapted and will fire less when presented with a static image.
• Cells in MT that fire spontaneously to the other directions will now have relatively more activity (especially the ‘downward’ responding cells).
• The vector sum of the activity (from convergence, from feeding forward) from these cells could cause the perception of downward movement (because you’re not getting upward motion info).

(no opponent cells like for color)

Question 6

MT neurons may converge in…..to create…

Answer 6

…..medial superior temporal
areas (MST)

….complex movements (spiral
illusion).

Question 7

Responses of a number of V1

neurons are pooled via …..

Answer 7

…..hierarchical convergence in

‘higher’ areas.

Question 8

for activity in neurons, you’re not looking for on and off you’re looking for…

Answer 8

CHAAAANGE in activity

because change is signal

Question 9

What about spiral motion after-effects?

Answer 9

theoretically the same as motion after effects

– MT cells converge on cells in the medial superior temporal – dorsal (MSTd) area.

– These cells respond to ‘optic flow’ – the relative motion of you going through your environment.

– Watching an expanding optic flow pattern can selectively adapt MSTd neurons.

basically just fatiguing cells responsive flow patterns (fatigue on outward flow and look at your hand it looks like its contracting)

Question 10

optic flow

Answer 10

if you were looking at spiral motion and all elements are going the same way it looks like the spiral was expanding: as you’re walking through the environment everything in your visual scene gets bigger (esp. in the periphery)

the optic scene flows as you travel through it: go forward things get bigger and go backward things get smaller

have cells that respond to this (MST) = highly sensitive to flow patterns

Question 11

Why don’t we perceive motion every time we move our eyes?

all the differences of things flashing across the retina

Answer 11

corollary discharge theory

Question 12

corollary discharge theory

Answer 12

one way of trying to understand how we can not see motion when we move our eyes but still see objects as we’re tracking them through space

the perception of motion depends on three different signals:

1) motor signal
2) corollary discharge signal
3) image displacement signal

Question 13

motor signal

Answer 13

the signal that your brain sends to the eye telling it to move

when the brain tells the eye to track

• signal sent to eyes to move eye muscles

Question 14

Corollary discharge signal (CDS)

Answer 14

a copy of the motor signal from the brain that is sent to another unit

• identical copy of the motor signal

Question 15

Image displacement signal (IDS)

Answer 15

objects are moving across the retina

• movement of image stimulating receptors across the retina

Question 16

Movement is perceived when an

evaluation unit, called a comparator, receives either a:

Answer 16

basically is the evaluator of two diff signals coming in:

corollary discharge signal OR image displacement signal

if it gets one of those two it detects motion

Question 17

if your eyes track an individual, you get a ____ (eyes moving),

but no ____ (image stays at the same
retinal location – the fovea).

Answer 17

CDS

IDS

Question 18

Person moves across visual field, causing image to go across retina (eye is stationary).

Answer 18

This triggers an image displacement signal (IDS), which is sent to the comparator (retina is fixed).

• The comparator did not receive an identical corollary discharge signal from motor cortex, so the movement is not
caused by eye movement.

• Therefore, the comparator signals that external motion has been detected.

Question 19

If you move your eye across a stationary scene, images will also move across the retina.

Answer 19

• the comparator will receive an IDS (images across retina) and CDS (copy of motor signals to eye).
• comparator has two signals coming in, therefore no external movement: what i’m seeing must be due to the motion that my brain is causing
• In this case, the movement on the retina can be attributed to the motion command sent to the eye.
• Therefore, the comparator signals that no external motion has been detected.

Question 20

You stare at a red image for a minute. The red image is then replaced by a blank screen and you see a greenish afterimage.

As you move your eyes around the screen, does corollary
discharge theory predict that the afterimage should appear to move?

Answer 20

yes

when you have an afterimage, you’ve been getting one color in the eye, fatiguring receptors, and the color goes away and now you get all colors but the cells that respond to cells, and the receptive

it’s always on the same place of the retina, and if you move the eye it’s getting no IDS because the image is always on the same place in the retina

you comparator get CDS (because you are moving the eye) but no IDS because the after image is always on the same place on the retina: nothing is moving across the retina = so you perceive movement and the image seems to dance around wherever the eye goes

Question 21

If you have a color afterimage on
your eye (caused by temporarily
bleached receptors), it will …

Answer 21

…..not move over the retina.

Question 22

If you then move the eye, the image (IDS) stays the same, but the ______ changes. So yes, the image should appear to move.

Answer 22

motor signal (CDS)

Question 23

_____ shouldn’t matter for
the motion. The bleached area will
appear different than the
surrounding area.

Answer 23

Screen color

Question 24

Physiological evidence for
corollary discharge theory

monkeys

Answer 24

Real-movement neurons found in monkeys (in dorsal pathway –many locations) that respond only when a stimulus moves across the retina (a) and do not respond when eyes move across stimulus (b).
• Produces IDS

Question 25

Physiological evidence for
corollary discharge theory

damage

Answer 25

Damage (lesions/strokes) to the medial superior temporal
(MST) area in humans leads to perception of movement (of
stationary environment) when eyes move.

• Apparent loss of CDS (corollary discharge
signals). = losing the signal that tells the brain that the eyes are moving, so every time they move the eyes they think the world is moving

every time they move their eyes the world seems to bounce around

Question 26

Physiological evidence for
corollary discharge theory

akinestopia

Answer 26

motion blindness = can’t process motion but can see stillness

You can also lose the ability to see motion from damage in MT.

someone sees the world as a series of still pictures

Question 27

There are different ways to experience motion

Answer 27

real, illusory, induced, and after-effect

Question 28

pooled convergence to get more complex receptive fields

Answer 28

activity in the dorsal pathway

we think that activity from cells in V1 (simple cells, complex cells) that are detecting basic motion features (little slivers of light) is converging in MT

if you converge enough of that feedforward info it will account for some large scale object effects (waterfall illusion)

if you take some of those more complex receptive fields and converge those on a higher level to MST

in MST we get even more complex activity (spiral motion)

Question 29

brain has to evaluate whether or not motion occurs and does so on the basis of…

Answer 29

whether an object moved across the retina while the eye was still

OR

the eye is tracking an object, and that object is staying on relatively the same area of the retina ( not getting image displacement signals - still on fovea or macula)

how does it resolve those two issues but doesn’t think there’s motion anytime the eye moves around when it’s not tracking an object: the comparator

Question 30

Real-movement neurons

Answer 30

found in monkeys (in dorsal pathway –many locations) that respond only when a stimulus moves across the retina (a) and do not
respond when eyes move across stimulus (b).
• Produces IDS
• [FP = Focal Point]

these cells only like it when the eyes are fixed and something moves across the retina NOT when the eyes move