the spatial extent of cortical synchronization: modulation by internal and external factors
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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
rates Time 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
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