188 HOW THE BRAIN MAKES POTENTIALand assimilated. So the intricate connectivity of brain networks has
increased enormously in the evolution of mammals. It has been achieved
largely by increased folding of ce re bral cortex to obtain greater surface
area. Th e theme seems to be one of expanding pro cessing capacity for
abstraction of ever- deeper spatiotemporal structure.
It is the evolution on a theme of emerging intelligent systems, but now
on a much higher plane. As with earlier systems, its very evolutionary
function is the creation of adaptable variation. Only now, at the level
of the brain, we get eff ervescences of variation that far outstrip anything
that could be generated by gene “ recipes.” And it becomes adaptable
through the internalization of environmental structure.
Th is should be borne in mind when trying to describe the origins of
individual diff erences in “brain power.” What is on the outside, or even
mea sured, is no guide to the abilities on the inside.
THE FEELING BRAINMultisensory images are created by conjunctions of sense modes, as we
have just seen. Th ey are crucial for computing impressions of the outside
world: they go far beyond the information given in sensory inputs, and
they guide reactions with predictable consequences. But there is another
sensory interface that must be included in those functions.
Since the nineteenth century, the ner vous systems of vertebrates have
been considered as dual entities. One has been described as a somatic
ner vous system, mediating sensory- motor responses to the external envi-
ronment. Th e other is a visceral ner vous system monitoring the internal
environment. We now know that the two are much more integrated and
reciprocal than the distinction suggests. Th e interactions between the two
create subjective feelings and have a crucial part to play in how the brain
interprets inputs and creates responses. Th is is oft en forgotten in de-
scribing the origins of individual diff erences in cognition and be hav ior.
It happens in even the smallest functional brains. In their small brains,
even fl ies and bees have neuronal ensembles monitoring the internal
milieu and activating neuro- secretory cells to modulate it. In vertebrates,
a small center, the hypothalamus, receives copies of signals from all sen-
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