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PSL304 > Lecture 7 - Feedback > Flashcards

Lecture 7 - Feedback Flashcards

1
Q

What does feedback mean?

A

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

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2
Q

Positive feedback

A

feedback signal adds to the system’s input

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3
Q

Formula for pos feedback

A

S = Gx + Au

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4
Q

what is Δt?

A

feedback delay around the loop

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5
Q

in pos feedback (biological), what is G?

A

the strength of the synapse between a collateral and cell body

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6
Q

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

A

neuron cell body

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7
Q

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

A

recurrent collateral

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8
Q

in pos feedback (biological), what is A?

A

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

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9
Q

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

A

G = 1, A = Δt

Assume Δt is small:

u = x’

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10
Q

How to do integrator calculations on positive feedback loop?

A

old x = G(new x) + u

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11
Q

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

A

exponential increase to infinity (runaway outputs)

  • systems where powerful amplification is desired
  • action potential
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12
Q

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

A

circuit is leaky integrator (exponential decrease)

  • many integrators in the brain
  • velocity-stroage integraror
  • oculomotor integrator
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13
Q

why are all known neural integrators leaky?

A

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

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14
Q

time constant for oculomotor integrator. What does it do?

A

20s;

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

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15
Q

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

A

cerebelluar;

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

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16
Q

nystagmus

A

slow-fast movement as you nod off to sleep

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17
Q

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

it is caused by…

A

gaze-paretic nystagmus;

caused by leaky oculomotor integrator (paralysis of gaze control)

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18
Q

neg feedback can guide…

A

a system to a target

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19
Q

parts of a neg feedback loop?

A

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

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20
Q

what is a comparator?

A

computes the error

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21
Q

define: error

A

difference between system’s output and its traget value

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22
Q

define: amplifier

A

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

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23
Q

define: integrator

A

computes output based on velocity command

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24
Q

feedback guides the system ____ to its target

A

gradually;

go around the loop multiple times, feedback delay is small

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25
Q

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

A

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

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26
Q

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

A

increases speed which output = target, but there is still drift going on;

accuracy not affected;

advantage: malfunctions inside the loop doesn’t impair performance

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27
Q

___ control brings the output to target value as quickly as possible

A

bang-bang

28
Q

bang-bang control

A

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

29
Q

feedback delays can cause…

A

oscillation

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

30
Q

oscillation = ____ in some neurlogical diseases

A

tremor

31
Q

quick processes can’t be guided by…

example? (saccade)

A

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)

32
Q

what kind of feedback does saccades use?

A

internal negative feedback

33
Q

signals carried by ocular motoneurons (commands eye muscles)

A

velocity & position signals

  • velocity: moves eyes
  • position: hold eyes still after getting into right position
34
Q

velocity commands to the eye muscles come from…

A

burst neurons

35
Q

rate of burst neurons during saccades. Not during saccades?

A

proportional to eye velocity;

silent when there is no saccade

36
Q

burst neurons project to…

A

motoneurons

37
Q

fastest neuron in the brain? ____ spikes/sec?

A

Burst neurons (1000 spikes/sec)

38
Q

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

A

neural integrator

39
Q

neural integrator

A

computes eye-position commands from integrating velocity commands

40
Q

both velocity and position commands are derived from…

A

burst neurons

41
Q

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

A

internal feedback loop

42
Q

advantage of internal feedback loop > sensory feedback?

A

brief delay around loop (short axons)

43
Q

disadvantage of intenral feedback

A

malfunctions outside the loop aren’t corrected

44
Q

burst neurons fire in a ______ mannar

A

bang-bang

45
Q

define: saccade duration

A

time until the motor error signal reaches 0

46
Q

define: saccade size

A

how far the eye moves in during the saccade duration

47
Q

postsaccadic drift

A

any eye movement ater the motor error signal reaches 0

48
Q

saccade duration is affected…

A

by lesions inside THE LOOP

49
Q

saccade size is affected by…

A

velocity command, motoneurons, eye muscles

50
Q

saccade drift is affected by…

A

position command, motoneurons

51
Q

optokinetic eye movements

A

track motion of large regions of the visual field

52
Q

pursuit movements

A

track small moving objects

53
Q

only ____ can pursue

A

animals with foveas (rabbits can’t)

54
Q

rabbits in a rotating drum tracks movement using…

A

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

55
Q

Pursuit vs OKR?

A

pursuit can override OKR

56
Q

is pursuit voluntary or involuntary?

A

lie on the border

  • voluntary: pick target
  • involuntary: can’t force yourself to pursuit
57
Q

speed of pursuit

A

slow

  • latency (retina -> eye muscles) of 130 ms
  • top speed 100 degrees/sec
58
Q

how to decrease delay in pursuit?

A

if target moves in predictable way

59
Q

define: vection

A

motion illusions (IMAX, train beside yours move)

60
Q

image flow across retina

A

optical flow

61
Q

feedback for optokinesis?

A

negative feedback

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

62
Q

fatigue OKR

A

waterfall effect

63
Q

the open-loop response of OKR

A

ramp of eye velocity

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

64
Q

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

A

dynamic integrator (leaky)

65
Q

refractory period of saccade

A

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

66
Q
A

PSL304 (10 decks)

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