Transcript Document 7675625

Cycle 6:
Oscillations and Synchrony
• What is an oscillator?
– Name two types of oscillators
• harmonic
(e.g. pendulum, spring, car on track)
• relaxation
(e.g. water drops)
– Components of oscillations:
• frequency, amplitude, phase, period
• next slide
Cycle 6:
Oscillations and Synchrony
Components of oscillators:
•
discharge/charge phase
•
duty cycle
•
(relaxation phases)
– 1 excitable “ready”
– 2 active (duty cycle)
– 3 refractory
Cycle 6:
Oscillations and Synchrony
• Difference between types of
oscillators:
– frequency estimation good for harmonic
not relaxation
– response to perturbation: relaxation
•phase reset
Concept: Oscillators can be considered at
the neuron or neural population level
Cycle 6:
Oscillations and Synchrony
• Resonance (movie)
• p. 143 “neurons were believed to be
silent unless excited by some outside
sensory input.”
– BUT Sir Adrian noted ‘spontaneous
activity’ in toad optic nerve.
• How could resonance at varying
frequencies be accomplished? p. 144
Cycle 6:
Oscillations and Synchrony
How could resonance at varying frequencies be
accomplished? p. 144
• Voltage – and Ion-gated channels with
different opening (gating) kinetics:
– Ia , I h
• Filtering:
– LPF: passive leak and capacitance p.145
– HPF: voltage-gating p.146
• K+ channels especially effective in ‘shifting
bridge’ think “Kapo”
Cycle 6:
Oscillations and Synchrony
The low-information problem and what is a
neuron’s default state p. 149:
• default state issue: oscillations are a quirky
mode seen in isolated neurons, not relevant for
information processing (e.g. anesthesia). The
non-oscillatory mode is
– 2 examples provided, p. 149
•low info issue:if a cell only fires at a given
phase of oscillation, it’s information is reduced.
Cycle 6:
Oscillations and Synchrony
Define Synchrony:
- coupling in time (what window?)
- Window depends on the ‘observer’,
e.g. for a neuron, the time it takes for its postsynaptic potential to decay to baseline, making next
input independent rather than summate. The 1/e decay
(down to 37%) is called the ‘time constant’, and it’s the
metric used to define temporal decays.
for an oscillating population, the duration of the
readiness state determines the window: ½ cycle for
harmonic oscillators, and the relevant fraction +/- for
relaxation oscillations
Cycle 6:
Oscillations and Synchrony
• Stochastic resonance
– a weak signal is transmitted better in the
presence of noise…like getting ‘jumped’ on a
trampoline, to see over a fence that was too high for
you when jumping alone. Even if that energy input
(your ‘jumper’) may fall randomly in your jump cycle
(sometimes reducing your height), when it
eventually it falls in the right window, you achieve
what you couldn’t without the energy input (seeing
over the fence).
Cycle 6:
Oscillations and Synchrony
• Stochastic resonance
• weak signal can be oscillation, if it’s subthreshold
• ‘noise’ input can also be oscillation, again, if it’s
subthreshold p. 158:
Cycle 6:
Oscillations and Synchrony
• Features of cell assemblies:
–groups of neurons whose coincident activity exceeds what
would be expected from sensory inputs.
–Reverberation, or continued activity within the population that
continues in the absence of inputs
–Flexible membership: a neuron can be a part of many
assemblies.
Cycle 6:
Oscillations and Synchrony
• Features of cell assemblies:
–groups of neurons whose coincident activity exceeds what would
be expected from sensory inputs.
–Reverberation, or continued activity within the population that
continues in the absence of inputs
–Flexible membership: a neuron can be a part of many assemblies.
–Time windows can define, and segregate assemblies, including
oscillatory ‘windows’ p.164
“The uniquely changing assemblies in each oscillatory
cycle can target anatomically unique sets of neurons.
Through assembly organization, time is translated to
neuronal network space.”
Cycle 6:
Oscillations and Synchrony
• Synchrony is cheap
– the integration time window of
neurons means that multiple
synchronous inputs effect
greater change than the same
inputs presented
asynchronously. Or, you can get
the same level of output with
fewer inputs, when the inputs
are provided in synchrony.
– Even Huygen’s clocks on the
wall synchronized, if they were
in the same wall.