Spindle cells for long-distance connectivity and consciousness
Another entry from the answers to Edge magazine’s question, “What do you believe is true even though you can’t prove it?”.
Stanislas Dehaene postulates that spindle cells, a type of neuron found in the anterior cingulate of humans and great apes, but not other primates, is fundamental to higher-order processing and consciousness. He postulates that these cells, which form connections through the cortex, fundamentally increase long-distance connectivity in the cortical network, which allows different brain areas to better intercommunicate.
One way that I think about this is that it allows consciousness to access information within a variety of brain areas. His way of putting this is, “we can mobilize, in a top-down manner, essentially any brain area and bring it into consciousness”.
He also believes that this increased long-distance connectivity leads to a qualitatively greater amout of spontaineous, reverberating activity in the cortical network, which corresponds to our ability to sustain a conscious narrative independent of external sensory and motor events, or, as he puts it, “the autonomy of consciousness”.
Here’s an except from Dehaene’s Edge answer:
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At a more microscopic level, it is already known that there is a new type of neuron which is found in the anterior cingulate region of humans and great apes, but not in other primates. These “spindle cells” send connections throughout the cortex, and thus contribute to a massive increase in long-distance connectivity in the human brain. Indeed, the change in relative white matter volume is perhaps what is most dramatic about the human brain.
I believe that these surface and connectivity changes, although they are in many cases quantitative, have brought about a qualitative revolution in brain function:
Breaking the brain’s modularity.
Jean-Pierre Changeux and I have proposed that the increased connectivity of the human brain gives access to a new mode of brain function, characterized by a very flexible communication between distant brain areas. We may possess roughly the same list of specialized cerebral processors as our primate ancestors. However, I speculate that what might be unique about the human brain is its capacity to access the information inside each processor, and make it available to almost any other processor through long-distance connections. I believe that we humans have a much more developed conscious workspace—a set of brain areas that can fluidly exchange signals, thus allowing us to internally manipulate information and to perform new mental syntheses. Using the workspace’s long-distance connections, we can mobilize, in a top-down manner, essentially any brain area and bring it into consciousness.
Spontaneous activity and the autonomy of consciousness.
Once the internal connectivity of a system exceeds a threshold, it begins to be dominated by self-sustained, reverberating states of activity. I believe that the human workspace system has passed this threshold, and has gained a considerable autonomy relative to the outside world. The human brain is much less at the mercy of signals from the outside world. Its activity never ceases to reverberate from area to area, thus generating a highly structured spontaneous flow of thoughts that we project on the outside world.
Of course, spontaneous brain activity is present in all species, but if I am correct we will discover that it is both more evident and more structured in the human brain, at least in higher cortical areas where “workspace” neurons with long-distance axons are denser. Furthermore, if human brain activity can be detached from outside stimulation, we will need to find new paradigms to study it, because bombarding the human brain with stimuli, as we do in most brain-imaging experiments, will not suffice. There is already some evidence for this statement: by directly comparing fMRI activations evoked by the same visual stimuli in humans and macaques, Guy Orban and his colleagues in Leuven have found that prefrontal cortex activity is five times larger in macaques than in humans. In their own words, “there may be more volitional control over visual processing in humans than in monkeys”.
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