Neural Synchrony in Attention and Consciousness Lawrence M. Ward University of British Columbia...
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Transcript of Neural Synchrony in Attention and Consciousness Lawrence M. Ward University of British Columbia...
Neural Synchrony in Attention and Consciousness
Lawrence M. WardUniversity of British Columbia
Funded by
Collaborators: Sam Doesburg, Kei Kitajo, Alexa Roggeveen, Tony Herdman, Jessica Green, John J. McDonald
Themes
Neural synchrony and its measurementNeural synchrony in attention: baton-passing in the cerebral cortexNeural synchrony in consciousness: binocular rivalry and the thalamic dynamic coreImplications for LIDA
Neural synchrony occurs when neural activity, spiking or dendritic currents,
in disparate locations rises and
falls in a fixed relationship
Gray & Singer’s cats
Ward et al’s humans
Varela et al, 2001
10 20 30 40 50
Frequency
Spectral
Power
Theta 6 Hz
10 20 30 40 50
Frequency
Spectral
Power
Gamma 40 Hz
10 20 30 40 50
Frequency
Spectral
Power
Alpha 10 Hz
Spectral power in a given frequency range reflects
local synchrony
Roles of local neural synchrony
High fidelity neural communicationPerceptual/memorial/motor…. Binding:
Increase 30-70 Hz: amplify post-synaptic effect by increasing spike co-occurrence (Fries et al, 2001)Decrease 6-15 Hz: increase post-synaptic impact by avoiding spike-frequency adaptation (Fries et al, 2001)
Fries 2005
Roles of global neural synchrony
Integration of neural activity through reciprocal (re-entrant) interactions between diverse brain regions (e.g., Varela et al, 2001)
Exchange of data (upward) and hypotheses/templates (downward); sensory/perceptual processingModulation of one region (e.g., hippocampus) by another (e.g., visual cortex) to store a memory (e.g., of a visual scene)Modulation of one region (e.g., visual cortex) by another (e.g., prefrontal cortex) to enhance processing of attended information (e.g., a sign for a sushi bar)Initiating an action in motor regions by computations from cognitive regionsConsciousness (?)
( ) ( ) ( )( )titAtfitft φζ exp~
)()( =+=amplitude phase
phase 30Hz (C3, O1, Fz)
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amplitude 30Hz (C3, O1, Fz)
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Step.2 instantaneous phase and amplitude
C3
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Fzstimulus
(sec) (sec)
(sec)
-0.2 0 0.2 0.4 0.6 0.8 1
EEG/MEG synchronization analysis: calculation of phase locking value (PLV)
Step.1 Obtain SCD of filtered signals f(t) via bandpass filtering at chosen frequencies
(sec)
10Hz
20Hz
30Hz
40Hz
stimulus
(sec)
(µV)
stimulus
C3
O1
Fz
Cz
Pz
Fz F4F3
C4
P4P3
F7 F8
T6T5
T3 T4
O1 O2
Fp1 Fp2
●
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C3
EEG (C3, O1, Fz)
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-0.2 0 0.2 0.4 0.6 0.8 1
),()(~
tfoftransformHilbertistfwhere τττπ d
t
fVPtf ∫
∞
∞−
−
−=
)(..)(
~ 1
SCD
i.e. surface Laplacian, or MEG field strength, ^
⇒ complete synchronization:1 random phase difference:0
( )
2 electrode from signal theof phase the:
1 electrode from signal theof phase the:
points time:
trialsofnumber the:
)difference phase( ),(),(),(
where
1
2
1
21
1
),(2,1
φ
φ
φφθ
θ
t
N
ntntnt
eN
tPLVN
n
nti
−=
= ∑=
Step.3 Calculation of phase locking value (PLV) for each time point
C3-O1
C3-Fz
O1-Fz(sec)
phase difference (30Hz)
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C3-O1 (30Hz)
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C3-O1 (30Hz)
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phase difference(5 trials)
(sec)
PLV (50 trials)
(sec)
High PLVLow PLV
C3-O1 (30Hz)
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(400ms) period baseline in the PLV ofdeviation standard the:
(400ms) period baseline in the PLV ofmean the:
Bsd
Bmean
PLV
PLV
Step.4 standardization of PLV
Standardized PLV
(sec)
( )Bsd
Bmean
PLV
PLVPLVtPLVz
−=)(
To reduce the effect of volume conduction of stable sources and compare between electrode pairs at different distances
C3-O1 (30Hz)
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-0.2 0 0.2 0.4 0.6 0.8 1
Step.5 statistical test using surrogate data
Standardized PLV and surrogate PLV PLV (original)
±95 percentlePLVsurrogate
Median PLVsurrogate
(sec)
significant PLV increase
(Hz)60
50
40
30
20
10
C3-O1
-0.2 0 0.2 0.4 0.6 0.8 1(sec)
1009998
3-97210
sync
desync
Note: Local and long-range PLVz
must change together for
spurious synchronization to
be indicated (Doesburg,Ward,
CC, 2007)
Alpha, gamma and attention
Local alpha power associated with active suppressionLocal gamma power associated with active processingAlpha suppression necessary for gamma binding?Roles of long-range alpha and gamma synchrony?Coordination of local and long-range synchronization?Theta-modulated gamma synchrony
e.g., Klimesch, Jensen & Colgin, Palva & Palva, Ward
Doesburg, Roggeveen, Kitajo & Ward, 2007, Cerebral Cortex
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Left Cue 11 Hz
Right Cue 11 Hz
Left Cue 39 Hz
Right Cue 39 Hz
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Left Cue 11 Hz
Right Cue 11 Hz
Left Cue 39 Hz
Right Cue 39 Hz
0 250 500 750 1000ms0 250 500 750 1000ms
P7 P8
P7 P8
Alpha/gamma local synchrony (power) indexes spatial attention orienting•Arrow cued box to attend to
•Press button only if + in attended box, not if x nor if in unattended box•Cue-target SOA 1000-1200 ms•Cue onset at 0 ms in figures
+
•At 250 ms (same as local power max gamma/min alpha) increase in global phase locking at diverse frequencies
•Lateralized in gamma band: P7 (left) for right target, P8 (right) for left target (orienting?)
•Increased synch in beta band also but not lateralized (readying?)
•Desynch at 100 ms in gamma: erasing old network?
QuickTime™ and a decompressor
are needed to see this picture.
Long-distance gamma synchrony establishes focused attention network
MEG replication
Increased lateralized synchronization in high alpha band from ~400 ms post cue onset until end of epochDecreased synchronization side ipsilateral to cueSynchronization in alpha band associated with maintenance of attentional focus at cued location
Doesburg, Ward, 2007, Proceedings BIOMAG2006
Left cue 14 Hz Right cue
What is SAM beamformer?
Synthetic Aperture Magnetometry (Vrba & Robinson 2001, Methods) Based on time/space covariance matrix of sensorsAchieves estimate of source power for each voxel in brain region from weighted linear combination of all measurements (where weights are selected to attenuate signals from all other voxels):
Optimal coefficients found by minimizing total power over time (computational tricks used in practice).
)()(ˆ ttS tmWθθ =
MEG Replication
MEG filtered at 14 HzSAM beamformer sources in parietal (SPL?) and visual cortices (n=5)PLV analysis applied to broadband source activity filtered from 6 Hz to 60 Hz14 Hz PLV shows lateralized increased synchrony similar to sensor analysis from 400-1000 ms post cue onset (right; n=2)40 Hz (gamma-band) PLV shows burst of lateralized increased synchrony at ~200-250 ms post cue onsetTheta rate gamma synch bursts at least for right parietal-occipital when attending leftDoesburg,Herdman, Ward, 2007, Cognitive Neuroscience Society
MEG replication
Beamformer sources projected to cortical surface (star=source in a sulcus so projection done by hand)Lateralized increases and decreases in synchrony between parietal and occipital sources in alpha band from 400-800 ms post cue onset (here 800 ms and 14 Hz)Occipital 14 Hz power replicates EEG data (blue bars, 800 ms)
Doesburg, Herdman, Ward, 2007, Cognitive Neuroscience Society
MEG Replication
Endogenous orienting: dorsal fronto- parietal (FEF, IPS, V1/V2)We also found FEF sources but not consistent enough for PLV analysisNot enough to activate relevant areas - must be synchronized to be functionally effective?
Corbetta and Shulman (2002) Nat Rev Neurosci; Wright & Ward, 2008, Orienting of Attention
EEG Replication: BESA beamformer sources from theta band signals
Green & McDonald, 2008, PLoS Biology
EEG analyses
BESA beamformer sources in theta band identifiedBroadband activity of dipoles at peak voxels computed based on EEG recordingsBroadband signals filtered and analytic signal, PLV etc., computed between sources
Lateralized increased synchronization in the alpha band
Doesburg, Green, Ward & McDonald, in prep.
350 ms post cue onset until target onset:
maintains attention at cued location
Left-neutral
Right-neutral
Right brain
Left brain
Baton-passing in the cerebral cortex
Auditory cues and targetsCue types: up or down glide for orient left or right (or vice versa), both for do not orient; each on 1/3 of trialsWhite noise targets (respond to all targets) for gap discrimination presented left (1/3 of trials) or right (1/3 of trials) at random regardless of cue type; probes (respond only if at cued location) presented on 1/3 of trials at randomBESA beamformer source analyses for theta-band signal
Green, Doesburg, Ward & McDonald, submitted
Valid Neutral
Invalid
RT 694 ms (25.5)
718 ms (26.6)
716 ms (26.7)
% Corr
90.9 (.016)
90.7 (.014)
90.6
(.018)
Theta-band BESA beamformer source activations
Baton-passing:
•1 Cue activates STG which in turn activates IPL and SPL
•2 IPL/SPL activate IFG
•3. IFG interprets cue
•4. IFG tells IPL/SPL where to orient
•5. IPL/SPL activate relevant STG and maintain activation until target
Green, Doesburg, Ward & McDonald, submitted
Interim Summary
Brain configured to attend to a specific location by
Confluence of burst of decreased local alpha power, increased local gamma power and increased long-range gamma synchrony at 250 ms post cueAttention maintained at specific location by increased long-range alpha-band synchrony (at least parietal-visual) that coincides with decreased local alpha powerInformation/control signals passed between brain regions by establishing and breaking synchronization in gamma band between those regions
Still not understood: mechanism of synchrony used (thalamic control?); nature of signals exchanged (control/compliance signals?); how reactivity of sensory cortex is increased by control signal; ……
Binocular rivalry: window to awareness
Stimuli
Apparent locus of fused object
Prisms
Eyes
Constant stimulation, involuntaril
y alternating experience
Corresponding retinal areas
Neural synchrony and consciousness
Cosmelli et al, (2004) Waves of consciousness: Ongoing cortical patterns during binocular rivalry. NeuroImage, 23,
128-140
Widespread 5 Hz synchrony associated with perception of the 5 Hz stimulus
Face Rings FaceFaceFace FaceRings Rings Rings
Synchrony of brain areas at 5 Hz for
different subjects; note individual
differences in both locations of active brain areas and in
amount of synchrony between them.
Binocular rivalry: window to awareness
Corresponding retinal areas
Stimuli
Apparent locus of fused object
Prisms
Eyes
Constant stimulation, involuntarily alternating experience; subjects press one of two buttons to indicate which pattern they see, neither to indicate parts of both
Long-range theta-rate bursts of gamma-band synchrony in binocular rivalry of striped patterns
Doesburg, Kitajo & Ward, 2005, NeuroReport
45Hz -450 -400 -350 -300 -250 -200 -150 -100 -50 0 50 100ms
7Hz -450 -400 -350 -300 -250 -200 -150 -100 -50 0 50 100ms
Button press at 0
Frequency (Hz)
Time (ms)
Time (ms – 0 indicates button press)
Fre
qu
en
cy
(H
z)R
L
BESA beamformer (280-220 ms pre-button-press, 36-46 Hz) applied to replication
Average PLVz over 8 active sources: R/L V1/V2, R sup occipital, R parietal, L temporal pole, L DLPF, R/L prefrontalBursts of gamma-band synchronization occur at theta rate (arrows) around button press in presence of persisting synchronization in theta band; also bursts of synchronization in beta band
Theta-modulated synchronizationLeft V1/V2 – Left DLPFC
surrogate level 0.025/0.025
Left V1/V2 – Right parietal
Right parietal – Right occipital
40-50 Hz synch
30-40 Hz synch
20-30 Hz synch40-50 Hz alt synch/desynch
40-50 Hz desynch
30-40 Hz desynch
Right brain
Left brain
Dynamic core hypothesis
Neural correlate of conscious awareness is what Tononi & Edelman called the "dynamic core"
Large-scale (brain-wide, 200-msec time scale)
Coherent (statistically synchronous) activity
Millions of neurons involved
Dynamic core hypothesis
DC simultaneously integrates activities of many brain areas (not all of them, a constantly changing subset) …And also differentiates current conscious state from many other, possible conscious states. My (radical) proposal: the thalamic dynamic core is the neural correlate of phenomenal awareness
Cortex computes, thalamus experiences Human cortex, with more neurons and more cortico-cortical fibers per thalamic fiber computes much more complex contents than do, e.g., rat, dog, or chimp cortices; pace Paul NunezCortical DC arises from synchronous activity in thalamus
Thalamic dynamic core
Dynamic core and supporting binocular rivalry data (Tononi & Edelman, Varela group); neural synchrony is keyLesions, neurosurgery, and anesthetic action point to thalamic “relay” nuclei as critical (Penfield, Alkire et al)Anatomy and function of thalamus and cortex (Mumford)We experience results (products) of computations, not the computations (processes) themselves (Lashley, Kinsbourne, Prinz, Rees, Koch, Baars)
Karen Ann Quinlan’s Brain at Autopsy (see Kinney et al 1994)
Thalamus-massive loss Cortex-little loss
Lesions: Karen Ann Quinlan
Drug/alcohol reaction; permanent vegetative state for 14 years
Penfield’s neurosurgery and stimulation mapping
Patient M.M.
treated for intractable epilepsy
M.M.’s cerebral cortex mapped via electrical stimulation by
Penfield
Neurosurgery and stimulation mapping
Penfield (The Mystery of the Mind, 1975):The mechanisms of epilepsy and electrical stimulation mapping imply that “…there are two brain mechanisms that have strategically placed gray matter in the diencephalon …, viz. (a) the mind’s mechanism (or highest brain mechanism); and (b) the computer (or automatic sensory-motor mechanism).” (p. 40).
Penfield’s “mind mechanism”
Merker (2006, BBS): argued SC is locus of
conscious analog simulation of world
General anesthesia
Alkire et al (2000) Consciousness & Cognition:
Common brain loci and mode of action of different general anesthetics imply that the critical mechanism of general anesthesia is a hyperpolarization block of the thalamic relay nuclei neurons
Alkire et al (2000) Consciousness & Cognition
Common brain areas where halothane and isoflurane anesthesia significantly depresses
activity; a. thalamus, b. midbrain reticular formation
The thalamus
Synchronizes cortical oscillations“Gateway” to cortex for major sensory systems (except smell)Evolved along with the cerebral cortex; a “seventh layer” of cortex (but with different neuron type)Each cortical area has an associated subnucleus of thalamus with massive cortico-thalamic projections and smaller thalamo-cortical projections; most thalamic subnuclei have no other inputs!
Where is the thalamus?
Thalamus
Pineal body
Gross Anatomy of some cortico-thalamic circuits
Roles of the thalamus
Relay station and gateway (attentional engagement) to cortex for sensory systemsSynchronizes neural activity in remote cortical areasActive blackboard that echoes back to cortex results of latest computations (Mumford)Site of dynamic core of neural activity that gives rise to phenomenal experience(?): thalamic dynamic core
We experience products…
Crovitz: maximum rate of consciously following strobe light = 4 to 5 Hz (250 to 200 msec per cycle) Sternberg STM scanning: no awareness of process; 40 Hz (25 ms/item) scan?LTM search & Retrieval: no awareness of memory search codes, only memoriesSpeech: not aware of composing utterences, phonemes, (co-)articulation, etc.
Conclusions
Synchronous neural oscillations occur at several scales and in particular frequency bandsLocal and global synchrony coordinate to establish an attentional network that enhances processing of attended stimuliNeural synchrony in the thalamo-cortical circuits, particularly in the thalamus, establishes a dynamic core of brain activity that supports (is?) conscious awareness
Implications for GWT/LIDA
Dynamic core is the GW GW (broadcasting) established by synchronization between various active processing modules mediated by thalamic dynamic core Transient networks of brain regions (processing coalitions) established by low frequency synchronization (carrier?), with high frequency synchronization establishing transient information exchange between processing areas in necessary temporal sequences (baton passing)Perception/action cycle governed by several temporal interactions based on processing speeds and communication requirements vis a vis available oscillation frequencies.