Mechanisms for phase shifting in
cortical networks and their role in
communication through coherence
Paul H.Tiesinga and Terrence J. Sejnowski
Mechanisms for phase shifting in cortical networks and their role in communication through coherence
Hubel and Wiesel, 1968
*Carnivora not primate
Desimone and Duncan, 1995
Average SFC in the gamma range as a function
of stimulus contrast and attention.
Communication through neural coherence
Chalk et al. 2010
CTC is alternative to CT-FRM, size of the excitatory
postsynaptic potential decreases with increasing input
rate such that under some conditions, there is no
postsynaptic net effect of increased input rate
Fries, 2005
Mechanisms for phase shifting in cortical networks and their role in communication through coherence
Setup
Rule
If I volley preceded E volley, the E
volley was not effective in driving
the postsynaptic neuron,
(But) when the E volley preceded
the I volley, there was an output
spike.
Eff. of sensory gating depends
on the synchronization
Hence, the FR depended on the
relative phase of these two inputs,
which can be used for stimulus
selection.
Rate with which E inputs arrive
at the neuron for relative phases
A: 2 local circuits, both projecting to a 3rd circuit, each comprised of E and I cells
with at least a projection from the local I cells to the E cells.
Phase event:
Mechanisms for phase shifting in cortical networks and their role in communication through coherence
Membrane potential
of a model neuron
Neuron phase locks
(1-1.3), FR - constant
Standard deviation was equal
to the half width of the gray shading
Phase shifts occur in response
to changes in driving current.
Amount of information in the spike phase depends on its mean value,
Red cloud – just before the onset of the step;
not spherical – skipped spikes
Green cloud – Phase lock step reached; (MI maximum)
Current increased, MSP decreased
Blue cloud - amount of information in the spike phase
about the E input was reduced, whereas the IM with
the I input was increased
fixed number (5) of bins was used for each variable
to minimize the bias in entropy estimation
Black cloud - neuron is off the phase-locking step
In summary, the spike phase shifts due to external
activation(depolarizing current)during phase locking
and the cycle-to-cycle variation in spike phase can
be used to encode information about the input!
Mechanisms for phase shifting in cortical networks and their role in communication through coherence
100 I (GABA)
400 E (AMPA)
3 phases can be changed:
1. Local E and I internal phase in SC
2-3. Global phase of S/R Circuit
Short E pulses can shift
the GPh of a LC
PING (pyramidal-interneuron gamma)
mechanism
Spike time histogram
The sum of the latency and E cell
recovery period determined the
oscillation frequency
The latency normalized by the
oscillation period is the relative phase
between inhibition and excitation
Increasing the depolarizing current injected into the I cells
reduced the latency (and thus relative phase, Ca), without
significantly affecting the oscillation frequency (Ba)
The relevant phase difference is that between non-local excitation and local inhibition, which
can be changed by altering the global phase of either the sending or receiving local circuit.
Size of phase shift depends of time and strength of the pulse!
The most effective modulation
(largest shift) was obtained
when the pulse arrived about
10 ms after the I cell volley.
Mechanisms for phase shifting in cortical networks and their role in communication through coherence
21 columns x (84 E + 21 I)
P(Conn) ~ diff(pre-, postsynaptic cells)
*E conn. more orientation selective than I
P/NP:
Closest /Farthest
5 (out of 21)
preffered orientation
Peak in gamma! (Overall pop was sync)
P: negative phase – ahead of pop
During stimulus
NP: positive phase – lagging the overall pop
After stimulus
Highest FR cells fired ~60° before I cell
(or 4ms)
Lowest FR cells fired just before I cell
A mathematical mechanism for phase shifting
Phase locking occurs when neuron is driven by periodic synaptic
inputs or current injection and becomes entrained.
Const current I - transient = const (intrinsic) rate 𝑓0 𝐼 .
+ 𝑓𝑑 + A = phase-locking when
𝑓0 𝐼
𝑓𝑑
𝑝
= 𝑞.
Not all phase-locked states are equally effective in being phase shifted
2 ways to cycle through p − l states. 1) 𝒇𝒅 − 𝒇𝒊𝒙𝒆𝒅, 𝑰 − 𝒗𝒂𝒓𝒚
2) 𝑓𝑑 −𝑣𝑎𝑟𝑦, 𝐼 − 𝑓𝑖𝑥𝑒𝑑
For the leaky integrate-and-fire neuron the location of phase-locked
solutions can be calculated analytically (Coombes and Bressloff, 1999).
Mechanisms for phase shifting in cortical networks and their role in communication through coherence
Example of phase-locked solutions
The effect of noise illustrated by fixing the amplitude and
varying the noise strength
*D = var(Noise)
Width of the phase-locking steps reduced by noise
Phase-shifting is observed in vitro and in vivo
Spike density was interpreted as the spike rate in the absence of a
periodic drive
CA1 pyramidal cells in rodent
hippocampal. periodic drives are
in both theta- (5 Hz) and gamma
(40 Hz) range, but phase shifting
by increasing the level of
depolarizing current is only
possible for the theta
The range across which can be phaseshifted in vivo does depend on the
degree of phase locking.
In vivo experiments
cannot directly study the
mechanism of phase
locking because there is
no direct measurement of
the synaptic inputs that
generated the spike train,
but the LFP is often used
as a surrogate.
Because the precision of a
neuron is proportional to
the time derivative of the
membrane potential at spike
threshold (Cecchi et al.,
2000), a higher amplitude of
the sinusoidal current will
improve precision and hence
the phase-locking strength.
Mechanisms for phase shifting in cortical networks and their role in communication through coherence
In the primate visual cortex, the phase of spikes relative to oscillations in the local field potential (LFP) in the
gamma frequency range (30–80 Hz) can be shifted by stimulus features such as orientation and thus the phase may
carry information about stimulus identity.
It is possible to use phase shifting for a functional purpose, such as stimulus selection
Can be tested directly with optogenetic techniques. The necessary optogenetic
manipulation is likely to be feasible in a few years using viral transfection.
Han et al. 2009
Hp) Attention can be mediated by CTC via E volleys to I and E cells in networks
which are generating the gamma oscillations
Han et al. 2009