CHAPTER 1 8Connectivity, Language, and Meaning
In the two million years between Homo habilis and us, the size of the
hominin brain has doubled, and much of this increase is related to ex-
pansion of the cerebrum. We share a convoluted cerebral cortex with
a number of other mammals, including apes, elephants, whales, dol-
phins, coyotes, dogs, and cats. And all mammals, from mice and moles
to whales to humans, have layered cerebral cortex containing similar
neural circuitry (Fig. 18.1). There is local interconnectivity within and
between cortical layers, and there is long-range connectivity between
widely separated regions of the cerebrum; for example, neurons in the
occipital lobes form connections with neurons in the frontal lobes,
and vice versa.
There is extensive connectivity going in both directions between
the cerebral cortex and many subcortical structures: thalamus, amyg-
dala, hypothalamus, substantia nigra, ventral tegmentum, locus
coeruleus, raphe nuclei, and so on. There are hundreds of trillions of
chemical and electrical synapses, and it is likely that no neuron in
the human brain is more than a very small number of synapses away
from every other neuron in the brain.
Taking our description up from the cellular microcircuitry to