The spatial extent of cortical synchronization:

Modulation by internal and external factors

Adrian M Bartlett, BA Cog. Sci.Perception & Plasticity Lab

Psychology Graduate ProgramNeuroscience Graduate Diploma Program

Centre for Vision ResearchYork University

Outline

• Introduction: Distance and synchrony• References• Methods: Space constants

– Definition– Estimation

• Linear correlation of local field potentials (Pearson’s r)• Spike-triggered averaging of local field potentials (STAs)

• Results: Modulation of space constants– by Behavioural state (internal)– by Stimulus strength (external)

• Conclusions

Introduction: Distance and synchony

• Cortical synchronization is know to be strongly dependent on the distance between sites

– Constrained by small-world architecture of neocortex

– Synchronization falls off as a function of distance

– Distance of synchronization inversely proportional to oscillation frequency

References

• Destexhe A, Contreras D, Steriade M (1999) Spatiotemporal analysis of local field potentials and unit discharges in cat cerebral cortex during natural wake and sleep states. J Neurosci 19:4595-4608– ‘inside-out’– awake & sleeping cats (within-subject comparisons)– linear array of 8 electrodes in supra-sylvian gyrus (SS)

• Areas 5-7, secondary visual area, receives inputs from LGN & Area 17/18 (cat V1/2)

• 0.8-1mm depth;• 1mm distance• 0.1mm diameter

Space constants: definition

• A single scalar number used to describe rate of change as a function of space/distance

– For our purposes, how synchronization/correlation of neural activity decreases as a function of the spatial separation between recording sites

– Typically describes the slope/a parameter of a linear or exponential function fitted to the data

Space constants: definition• A single scalar number used to describe rate of change

as a function of space/distance

– Conventionally symbolized by τ (tau), which for exponentials describes the point at which the function has decayed ~63.2% to it’s asymptotic value

τ

36.8% of max

Space constants: estimation

• Spatial correlations (Pearson’s r)

– Linear correlation between voltage values for a given pair of LFP sites over time

– Averaged across electrode pairs of a given distance (Destexhe et al., 1999)

Results: behavioural state

• Awake: correlations fall off rapidly with distance (small space constant)

• SWS: large space constant

• REM: similar to awake; small space constant

Results: behavioural state

• AWAKE: Transient, local correlations

• SWS: Sustained, distant correlations

• REM: Transient, local correlations

Results: behavioural state

• Summary: Lower space constants in SWS relative to REM & waking

Results: behavioural state

• Negative correlation between high & low frequency power

• Positive correlation of low frequency power and space constant

Space constants: estimation

• And now for another method to calculate space constants…

Space constants: estimation

• spike-triggered average LFP (STA)– Average LFP voltage traces in a small window

around the time of every spike– same or different electrodes

Time of action potentials

Space constants: estimation

• wave-triggered average unit activity (WTA)– Find local minima (negative potentials) in LFP– Use peak negativities as trigger to average firing

ratesTime of peak negativities

Space constants: estimation

• spike-triggered average LFP (STA)– STA’s calculate from

spikes off the circled electrode

Space constants: estimation

• spike-triggered average LFP (STA)

– Amplitude: size of initial negative deflection

– Latency: time from spike occurance to peak amplitude

– How do amplitude and latency of STA vary as a function of distance between two electrodes?

Results: behavioural state

• R: STA• Averaged over all

electrodes

BOTTOM LINE: distal synchrony only during SWS (generally…)

Summary: behavioural state

• Dominant frequency band inversely covaries with space constant

Transition

• We saw the ‘inside-out’ approach

– How behavioural state / level of arousal affects space constants

• Now, we will take an ‘outside-in’ approach

– Do external stimuli change the degree of synchronization as a function of distance?

References

• Nauhaus I, Busse L, Carandini M, Ringach DL (2008) Stimulus contrast modulates functional connectivity in visual cortex. Nat Neurosci 12:70-76.– ‘outside-in’– Context: resolved conflicting reports of the strength of

lateral connections in V1– Anesthetized cats & monkeys– Rectangular 10x10 Utah array

• Area 18 (cat), V1 (monkey)• 0.4mm distance• 0.8-1mm depth Utah array being

implanted in a human brain

Results: external factors• STA calculated during spontaneous

activity under anesthesia

max

min

mid

FarNearSame

Results: external factors

• Independent of distance, the similarity of orientation selectivity tuning curves also predicts the degree of coupling between LFP recording sites

Results: external factors

• STA-based space constants are reduced by visual stimulation

max

min

mid

Results: external factors

• LFP correlations are overall lower and drop off faster with distance for stimulus-induced activity

Conclusions• Behavioural state modulates the spatial

extent of LFP synchrony– Large space constants & distal, sustained,

low-frequency synchrony during SWS– Small space constants & local, transient,

high frequency synchrony during REM and waking

• External stimuli de-correlate / desynchronize distal LFP synchrony– Large space constants and correlations

during spontaneous activity under anesthesia

– Reduction of space constants and correlations in a signal strength-dependent manner*

Results: exceptions

• Transient large-scale synchrony during REM & waking

Results: exceptions• Transient local

high-frequency oscillations during SWS

Results: exceptions• Transient local high-

frequency oscillations during SWS

Results: behavioural state

• L: WTA• Averaged

over 4 most distal electrodes

• R: STA• Averaged

over all electrodes

BOTTOM LINE: distal synchrony only during SWS (generally…)

Results: external factors

• Results are consistent across sites

max

min

mid

FarNearSame

Results: external factors

• Results are consistent across different orders of mammals

max

min

mid

FarNearSame

Results: external factors

• STA-based space constants are reduced by visual stimulation

max

min

mid

Results: external factors

• Space constants vary smoothly as a function of stimulus contrast.

• A weak signal leads to space constants between that seen for spontaneous and weak signals