The spatial extent of cortical synchronization: Modulation by internal and external factors

1. The spatial extent of cortical synchronization: Modulation by internal and external factors Adrian M Bartlett, BA Cog. Sci. Perception & Plasticity Lab Psychology Graduate Program Neuroscience Graduate Diploma Program Centre for Vision Research York University

2. 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

3. 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

4. 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-sylviangyrus (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

5. 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

6. 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 • 36.8% of max • τ

7. 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 (Destexheet al., 1999)

8. Results: behavioural state • Awake: correlations fall off rapidly with distance (small space constant) • SWS: large space constant • REM: similar to awake; small space constant

9. Results: behavioural state • AWAKE: Transient, local correlations • SWS: Sustained, distant correlations • REM: Transient, local correlations

10. Results: behavioural state • Summary: Lower space constants in SWS relative to REM & waking

11. Results: behavioural state • Negative correlation between high & low frequency power • Positive correlation of low frequency power and space constant

12. Space constants: estimation • And now for another method to calculate space constants…

13. 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

14. 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

15. Space constants: estimation • spike-triggered average LFP (STA) • STA’s calculate from spikes off the circled electrode

16. 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?

17. Results: behavioural state • R: STA • Averaged over all electrodes • BOTTOM LINE: distal synchrony only during SWS (generally…)

18. Summary: behavioural state • Dominant frequency band inversely covaries with space constant

19. 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?

20. References • Nauhaus I, Busse L, Carandini M, Ringach DL (2008) Stimulus contrast modulates functional connectivity in visual cortex. Nat Neurosci12:70-76. • ‘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

21. Results: external factors • STA calculated during spontaneous activity under anesthesia Far Near Same max mid min

22. Results: external factors • Independent of distance, the similarity of orientation selectivity tuning curves also predicts the degree of coupling between LFP recording sites

23. Results: external factors • STA-based space constants are reduced by visual stimulation max mid min

24. Results: external factors • LFP correlations are overall lower and drop off faster with distance for stimulus-induced activity

25. 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*

26. Results: exceptions • Transient large-scale synchrony during REM & waking

27. Results: exceptions • Transient local high-frequency oscillations during SWS

28. Results: exceptions • Transient local high-frequency oscillations during SWS

29. 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…)

30. Results: external factors • Results are consistent across sites max mid Far Near Same min

31. Results: external factors • Results are consistent across different orders of mammals max mid Far Near Same min

32. Results: external factors • STA-based space constants are reduced by visual stimulation max mid min

33. 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