Self Comes to Mind

experimental neuroanatomy does reveal connections to varied sensory regions, thus making the coordinating role quite plausible. Intriguingly, it has a robust projection to the important CDRegion that I mentioned earlier, the PMC. The discovery of this strong link occurred only after Crick’s death and was thus not included in the posthumously published article that he wrote with Christof Koch, in which he made his case. 1 The problem with the claustrum’s candidacy as coordinator resides in its small scale when we consider the job that needs to be performed. On the other hand, given that we should not expect any of the structures discussed earlier to perform the coordinating job single-handedly, there is no reason why the claustrum should not make a relevant contribution to the construction of the autobiographical self.
A Possible Role for the Posteromedial Cortices
     
    We need additional research to determine the specific role the PMCs play in the construction of consciousness. Later in this chapter, I review evidence from varied sources: anesthesia research, sleep research, research on neurological conditions (ranging from coma and vegetative state to Alzheimer’s disease), and functional neuroimaging studies of self-related processes. But first let’s look at the PMC evidence that appears most solid and interpretable—evidence from experimental neuroanatomy. I’ll speculate on the possible workings of the PMCs and on the reasons why they should be investigated.
    When I proposed that the PMCs would play a role in generating subjectivity, there were two strands of thinking behind the idea. One strand concerned the behavior and presumed mental status of neurological patients with focal damage to this region, which includes the damage caused by late-stage Alzheimer’s disease, as well as extremely rare strokes and brain metastases from cancer. The other strand related to a theoretical search for a brain region physiologically suitable to bring together information about both the organism and the objects and events with which the organism interacts. The PMC region was one of my candidates, given that it appeared to be located at an intersection of pathways associated with information from the visceral interior (interoceptive), from the musculoskeletal system (proprioceptive and kinesthetic), and from the outside world (exteroceptive). The factual strands are not in question, but I no longer see a need for the functional role I had envisioned. Still, the hypothesis prompted investigations that yielded important new information.
    Making headway with the hypothesis was not easy; the main problem was that the neuroanatomical information available on this region was quite limited. Some valuable studies had begun to chart the connectivity of parts of the PMC, 2 but the overall wiring diagram of the region had not been investigated. In fact, the region was known not by an umbrella term but rather by its component parts, namely, the posterior cingulate cortex, the retrosplenial cortex, and the precuneus. The PMCs, by whatever name, were definitely not yet on the radar of notable brain areas.
    In order to explore the hypothesis that the PMC was involved in consciousness, it was necessary to acquire previously unavailable knowledge about the connectional neuroanatomy of the PMCs. For this reason, our research group undertook an experimental neuroanatomical study in nonhuman primates. The experiments were conducted in Josef Parvizi’s laboratory in collaboration with Gary Van Hoesen. In essence the study consisted of making, in experimental macaque monkeys, numerous injections of biological tracers into all the territories whose neural connectivity we needed to investigate. Once injected into a given brain region, biological tracers are absorbed by individual neurons and transported along their axons all the way to their natural destinations, whatever the neurons are currently connecting to. These are the so-called anterograde tracers. Another kind of biological tracer, the retrograde kind, is taken up by axon terminals and transported in reverse, from wherever the terminals are, back to the cell bodies of the neurons, at their points of origin. The upshot of all the tracer travels is the possibility of charting, for each target region, the sites of origin of the connections the region receives, as well as the sites toward which the region sends its messages.
    The PMCs are constituted by several subregions. (In