Lecture 7 - Feedback

Question 1

What does feedback mean?

Answer 1

output of a system returns as input to the same system (loop)

Question 2

Positive feedback

Answer 2

feedback signal adds to the system’s input

Question 3

Formula for pos feedback

Answer 3

S = Gx + Au

Question 4

what is Δt?

Answer 4

feedback delay around the loop

Question 5

in pos feedback (biological), what is G?

Answer 5

the strength of the synapse between a collateral and cell body

Question 6

in pos feedback (biological), what is the summation junction?

Answer 6

neuron cell body

Question 7

in pos feedback (biological), what is the feedback path?

Answer 7

recurrent collateral

Question 8

in pos feedback (biological), what is A?

Answer 8

synapse between input u and the cell body (summation junction)

Question 9

how to make pos feedback an integrator? what does the equation look like?

Answer 9

G = 1, A = Δt

Assume Δt is small:

u = x’

Question 10

How to do integrator calculations on positive feedback loop?

Answer 10

old x = G(new x) + u

Question 11

what happens when G > 1 on pos feedback integrator? What kind of biological systems use this?

Answer 11

exponential increase to infinity (runaway outputs)

• systems where powerful amplification is desired
• action potential

Question 12

what happens when G < 1 on pos feedback integrator? What biological systems use this?

Answer 12

circuit is leaky integrator (exponential decrease)

• many integrators in the brain
• velocity-stroage integraror
• oculomotor integrator

Question 13

why are all known neural integrators leaky?

Answer 13

hard to make G = 1 exactly, and it’s safer to make it <1 than >1

Question 14

time constant for oculomotor integrator. What does it do?

Answer 14

20s;

in the dark if you try to hold a weird eye position, as activity leaks away, eyes drift back to straight ahead

Question 15

____ damange can increase the leak of the oculomotor integrator. What does this look like?

Answer 15

cerebelluar;

Can’t hold eccentric eye position - slow drift away from target and quickly snap back (saccade)

Question 16

nystagmus

Answer 16

slow-fast movement as you nod off to sleep

Question 17

eye movement where you slow drift - fast saccade back is called…

it is caused by…

Answer 17

gaze-paretic nystagmus;

caused by leaky oculomotor integrator (paralysis of gaze control)

Question 18

neg feedback can guide…

Answer 18

a system to a target

Question 19

parts of a neg feedback loop?

Answer 19

target → comparator → amplifier → velocity → integrator → output → feedback error (→comparator)

Question 20

what is a comparator?

Answer 20

computes the error

Question 21

define: error

Answer 21

difference between system’s output and its traget value

Question 22

define: amplifier

Answer 22

takes in the error and computes a desired rate of change (velocity command)

e.g. if target is 30 and output is 10, then error is 20 and amplifier orders a velocity in the positive direction

Question 23

define: integrator

Answer 23

computes output based on velocity command

Question 24

feedback guides the system ____ to its target

Answer 24

gradually;

go around the loop multiple times, feedback delay is small

Question 25

what happens to the velocity as output gets closer to the target value?

Answer 25

velocity decreases, causing output to get closer to target more slowly :(

Question 26

What if we boost the amplifier signal? The demostrates what advantage about feedback?

Answer 26

increases speed which output = target, but there is still drift going on; accuracy not affected; advantage: malfunctions inside the loop doesn't impair performance

Question 27

\_\_\_ control brings the output to target value as quickly as possible

Answer 27

bang-bang

Question 28

bang-bang control

Answer 28

amplifier maintains large velocity even as error decreases (non-linear). When error is very small, velocity drops to 0.

Question 29

feedback delays can cause...

Answer 29

oscillation comparator doesn't learn output has reached desired value until a bit later → overshoot

Question 30

oscillation = ____ in some neurlogical diseases

Answer 30

tremor

Question 31

quick processes can't be guided by... example? (saccade)

Answer 31

slow feedback; saccades (20 ms) can't be controlled by **vision** (50 ms delay) or **proprioceptive** feedback (monkey experiment cut proprioceptive sensor, saccades were unaffected)

Question 32

what kind of feedback does saccades use?

Answer 32

internal negative feedback

Question 33

signals carried by ocular motoneurons (commands eye muscles)

Answer 33

velocity & position signals * velocity: moves eyes * position: hold eyes still after getting into right position

Question 34

velocity commands to the eye muscles come from...

Answer 34

burst neurons

Question 35

rate of burst neurons during saccades. Not during saccades?

Answer 35

proportional to eye velocity; silent when there is no saccade

Question 36

burst neurons project to...

Answer 36

motoneurons

Question 37

fastest neuron in the brain? ____ spikes/sec?

Answer 37

Burst neurons (1000 spikes/sec)

Question 38

where does the position command projecting to eye muscles come from?

Answer 38

neural integrator

Question 39

neural integrator

Answer 39

computes eye-position commands from integrating velocity commands

Question 40

both velocity and position commands are derived from...

Answer 40

burst neurons

Question 41

what guides burst neurons (tell them how long & hard to fire)?

Answer 41

internal feedback loop

Question 42

advantage of internal feedback loop \> sensory feedback?

Answer 42

brief delay around loop (short axons)

Question 43

disadvantage of intenral feedback

Answer 43

malfunctions outside the loop aren't corrected

Question 44

burst neurons fire in a ______ mannar

Answer 44

bang-bang

Question 45

# define: saccade duration

Answer 45

time until the motor error signal reaches 0

Question 46

# define: saccade size

Answer 46

how far the eye moves in during the saccade duration

Question 47

postsaccadic drift

Answer 47

any eye movement ater the motor error signal reaches 0

Question 48

saccade duration is affected...

Answer 48

by lesions inside THE LOOP

Question 49

saccade size is affected by...

Answer 49

velocity command, motoneurons, eye muscles

Question 50

saccade drift is affected by...

Answer 50

position command, motoneurons

Question 51

optokinetic eye movements

Answer 51

track motion of large regions of the visual field

Question 52

pursuit movements

Answer 52

track small moving objects

Question 53

only ____ can pursue

Answer 53

animals with foveas (rabbits can't)

Question 54

rabbits in a rotating drum tracks movement using...

Answer 54

optokinetic nystagmus (slow, then fast snap back), whichi s generated by the optokinetic reflex (OKR)

Question 55

Pursuit vs OKR?

Answer 55

pursuit can override OKR

Question 56

is pursuit voluntary or involuntary?

Answer 56

lie on the border * voluntary: pick target * involuntary: can't force yourself to pursuit

Question 57

speed of pursuit

Answer 57

slow * latency (retina -\> eye muscles) of 130 ms * top speed 100 degrees/sec

Question 58

how to decrease delay in pursuit?

Answer 58

if target moves in predictable way

Question 59

# define: vection

Answer 59

motion illusions (IMAX, train beside yours move)

Question 60

image flow across retina

Answer 60

optical flow

Question 61

feedback for optokinesis?

Answer 61

negative feedback world velocity → retinal slip (slip velocity) → OKR adjusts eye velocity to compensate for slip

Question 62

fatigue OKR

Answer 62

waterfall effect

Question 63

the open-loop response of OKR

Answer 63

ramp of eye velocity even small constant retinal slip leads to steadlily increasing eye velocity (max 100 degrees/sec)

Question 64

what is the operator in the forward path of the OKR?

Answer 64

dynamic integrator (leaky)

Question 65

refractory period of saccade

Answer 65

200 ms (we can only make about 4 saccades/sec)