Monday, September 03, 2007

Models of consciousness at ASSC

By now I've made a number of references to the June 22-25 meeting of the Association for the Scientific Study of Consciousness in Las Vegas. Stuart Hameroff has just offered a report (PDF here) on the conference in the Journal of Consciousness Studies. His discussion of 'local' versus 'global' connection models of consciousness is worth excerpting here:
Dennett...reiterated one key feature of his multiple drafts model, that
activity anywhere in the brain could elicit consciousness, as long as
that particular activity was more than activity in any other brain area at
that moment. When asked precisely what type of neural activity did
the trick, Dennett passed. Put both Gazzaniga and Dennett in the ‘local’
camp (apparently with Christof Koch, who took that position in
last year’s ASSC debate). The globalists came later.

Koch (ASSC’s scientific compass) and most neuroscientists
assume axonal firings, or spikes are the bit-like currency underlying
the NCC. Gamma synchrony (coherent 30 to 90 Hz EEG, a.k.a.
‘coherent 40 Hz’) and other EEG are generated not by spikes, but by
dendritic local field potentials (LFPs), a distinction addressed in two
excellent posters from David Leopold’s NIMH group. Recording both
spikes and LFPs in monkey cortex and thalamus, they found that subjective
awareness correlates with dendritic LFPs rather than axonal

Are dendrites and gamma synchrony outside ASSC’s narrow
focus? After Singer discovered gamma correlations with consciousness
in the 1980s, Francis Crick and Christof Koch helped launch the
gamma synchrony NCC bandwagon. But they later jumped ship,
along with many others, related to an influential analysis by Shadlen
and Movshon which rejected gamma synchrony. Gamma synchrony
was rejected not because it doesn’t correlate with consciousness — it
clearly does — but because it doesn’t jive with axonal spikes, the
anointed currency of the NCC. Gamma synchrony EEG derives from
LFPs, in turn derived from post-synaptic dendritic potentials. Forced
to choose between dendritic synchrony and axonal spikes as the NCC,
Shadlen and Movshon, Crick and Koch and many others chose spikes,
and ASSC followed. Too bad.

LFPs and gamma synchrony occur both locally and globally,
compatible with both local origin theories, and global/hierarchical
views like Global Workspace (GW) and HOT. With GW guru Bernie
Baars in the house, global hierarchies surfaced in the superb session
‘Cortical Networks and Conscious Awareness’ with Alumit Ishai
(Zurich), Rafael Malach (Israel) and Giulio Tononi (Wisconsin).

Tononi, more suave than even Koch, claimed that effective connectivity
(measurable through EEG/LFPs) in thalamocortical circuits
correlates with consciousness. Tononi used transcranial magnetic
stimulation on himself, among others, to see how it disrupted normal
sleep. (What an interesting college roommate he could have been!)
Put Tononi in the globalist camp.

Alumit Ishai used fMRI to study face recognition. She found
activity in a feed-forward (posterior to frontal) hierarchical network,
including posterior visual, limbic/emotional and frontal cortex. Put
Ishai in the globalist camp (along with Koch — now in both camps —
whose tutorial covered ‘top-down’, frontal to posterior, attention

So … does consciousness require hierarchical organization as the
globalists and HOT proponents advocate? Or are the localists correct
in that consciousness can erupt in any sufficiently active brain region?
Rafael Malach from the Weizman Institute in Israel addressed this
issue. He studied continuous fMRI in subjects watching Sergio
Leone’s epic spaghetti Western The Good, the Bad and the Ugly,correlating
specific fMRI activity with precise movie scenes and frames.
While viewing the film, all above-baseline fMRI activity remained
posterior in the subjects’ visual cortical areas, with occasional snippets
of activity in sensory cortex. Malach showed that the appearance
of faces in the film (Clint Eastwood, Lee van Cleef, Eli Wallach) corresponded
with activity in viewers’ posterior visual cortex face
regions. Activity in the ‘hand’ area of sensory homunculi occurred
precisely during scenes/frames showing characters’ hands gripping a
gun, cigar or dealing cards.

Without posterior-frontal connections, Malach suggested lateral
links among basal dendrites of layer 5 pyramidal cells and cortical
interneurons worked in a distributed, rather than hierarchical,
LFP-friendly architecture to produce conscious experience. Put
Malach in his own camp — not local but not necessarily global. Call it

Could local, global and/or lateral/distributed neuronal organizations
each support different modes of consciousness in different circumstances?
When we are passively engrossed in a film, brain activity
remains posterior, e.g. in Malach’s lateral/distributed scheme. When
we become introspective or engage in command-and-control modes,
frontal cortex kicks in and more global GW/HOT networks take over.
Very localized activity could also result in consciousness (e.g. Zeki’s
famous colour consciousness in isolated V4 activity, or Damasio’s or
Panksepp’s emotional core suggestions).

So the question becomes not so much where, but precisely what
type of neural activity distinguishes consciousness from unconscious
processes. The evidence points to synchronized dendritic LFPs rather
than axonal spikes. And if Block and Kouider are correct, neural
activities supporting unconscious processes must be further divided
into (at least) two sub-types: non-conscious and pre-conscious/
access. There’s plenty of need for lower level subtlety.

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