Lecture 7 - Feedback Flashcards
What does feedback mean?
output of a system returns as input to the same system (loop)
Positive feedback
feedback signal adds to the system’s input
Formula for pos feedback
S = Gx + Au
what is Δt?
feedback delay around the loop
in pos feedback (biological), what is G?
the strength of the synapse between a collateral and cell body
in pos feedback (biological), what is the summation junction?
neuron cell body
in pos feedback (biological), what is the feedback path?
recurrent collateral
in pos feedback (biological), what is A?
synapse between input u and the cell body (summation junction)
how to make pos feedback an integrator? what does the equation look like?
G = 1, A = Δt
Assume Δt is small:
u = x’
How to do integrator calculations on positive feedback loop?
old x = G(new x) + u
what happens when G > 1 on pos feedback integrator? What kind of biological systems use this?
exponential increase to infinity (runaway outputs)
- systems where powerful amplification is desired
- action potential
what happens when G < 1 on pos feedback integrator? What biological systems use this?
circuit is leaky integrator (exponential decrease)
- many integrators in the brain
- velocity-stroage integraror
- oculomotor integrator
why are all known neural integrators leaky?
hard to make G = 1 exactly, and it’s safer to make it <1 than >1
time constant for oculomotor integrator. What does it do?
20s;
in the dark if you try to hold a weird eye position, as activity leaks away, eyes drift back to straight ahead
____ damange can increase the leak of the oculomotor integrator. What does this look like?
cerebelluar;
Can’t hold eccentric eye position - slow drift away from target and quickly snap back (saccade)
nystagmus
slow-fast movement as you nod off to sleep
eye movement where you slow drift - fast saccade back is called…
it is caused by…
gaze-paretic nystagmus;
caused by leaky oculomotor integrator (paralysis of gaze control)
neg feedback can guide…
a system to a target
parts of a neg feedback loop?
target → comparator → amplifier → velocity → integrator → output → feedback error (→comparator)
what is a comparator?
computes the error
define: error
difference between system’s output and its traget value
define: amplifier
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
define: integrator
computes output based on velocity command
feedback guides the system ____ to its target
gradually;
go around the loop multiple times, feedback delay is small
what happens to the velocity as output gets closer to the target value?
velocity decreases, causing output to get closer to target more slowly :(
What if we boost the amplifier signal? The demostrates what advantage about feedback?
increases speed which output = target, but there is still drift going on;
accuracy not affected;
advantage: malfunctions inside the loop doesn’t impair performance
___ control brings the output to target value as quickly as possible
bang-bang
bang-bang control
amplifier maintains large velocity even as error decreases (non-linear). When error is very small, velocity drops to 0.
feedback delays can cause…
oscillation
comparator doesn’t learn output has reached desired value until a bit later → overshoot
oscillation = ____ in some neurlogical diseases
tremor
quick processes can’t be guided by…
example? (saccade)
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)
what kind of feedback does saccades use?
internal negative feedback
signals carried by ocular motoneurons (commands eye muscles)
velocity & position signals
- velocity: moves eyes
- position: hold eyes still after getting into right position
velocity commands to the eye muscles come from…
burst neurons
rate of burst neurons during saccades. Not during saccades?
proportional to eye velocity;
silent when there is no saccade
burst neurons project to…
motoneurons
fastest neuron in the brain? ____ spikes/sec?
Burst neurons (1000 spikes/sec)
where does the position command projecting to eye muscles come from?
neural integrator
neural integrator
computes eye-position commands from integrating velocity commands
both velocity and position commands are derived from…
burst neurons
what guides burst neurons (tell them how long & hard to fire)?
internal feedback loop
advantage of internal feedback loop > sensory feedback?
brief delay around loop (short axons)
disadvantage of intenral feedback
malfunctions outside the loop aren’t corrected
burst neurons fire in a ______ mannar
bang-bang
define: saccade duration
time until the motor error signal reaches 0
define: saccade size
how far the eye moves in during the saccade duration
postsaccadic drift
any eye movement ater the motor error signal reaches 0
saccade duration is affected…
by lesions inside THE LOOP
saccade size is affected by…
velocity command, motoneurons, eye muscles
saccade drift is affected by…
position command, motoneurons
optokinetic eye movements
track motion of large regions of the visual field
pursuit movements
track small moving objects
only ____ can pursue
animals with foveas (rabbits can’t)
rabbits in a rotating drum tracks movement using…
optokinetic nystagmus (slow, then fast snap back), whichi s generated by the optokinetic reflex (OKR)
Pursuit vs OKR?
pursuit can override OKR
is pursuit voluntary or involuntary?
lie on the border
- voluntary: pick target
- involuntary: can’t force yourself to pursuit
speed of pursuit
slow
- latency (retina -> eye muscles) of 130 ms
- top speed 100 degrees/sec
how to decrease delay in pursuit?
if target moves in predictable way
define: vection
motion illusions (IMAX, train beside yours move)
image flow across retina
optical flow
feedback for optokinesis?
negative feedback
world velocity → retinal slip (slip velocity) → OKR adjusts eye velocity to compensate for slip
fatigue OKR
waterfall effect
the open-loop response of OKR
ramp of eye velocity
even small constant retinal slip leads to steadlily increasing eye velocity (max 100 degrees/sec)
what is the operator in the forward path of the OKR?
dynamic integrator (leaky)
refractory period of saccade
200 ms (we can only make about 4 saccades/sec)
