Sensory information undergoes extensive associative elaboration and attentional modulation
as it becomes incorporated into the texture of cognition. This process occurs along
a core synaptic hierarchy which includes the primary sensory, upstream unimodal, downstream
unimodal, heteromodal, paralimbic and limbic zones of the cerebral cortex. Connections
from one zone to another are reciprocal and allow higher synaptic levels to exert
a feedback (top-down) influence upon earlier levels of processing. Each cortical area
provides a nexus for the convergence of afferents and divergence of efferents. The
resultant synaptic organization supports parallel as well as serial processing, and
allows each sensory event to initiate multiple cognitive and behavioural outcomes.
Upstream sectors of unimodal association areas encode basic features of sensation
such as colour, motion, form and pitch. More complex contents of sensory experience
such as objects, faces, word-forms, spatial locations and sound sequences become encoded
within downstream sectors of unimodal areas by groups of coarsely tuned neurons. The
highest synaptic levels of sensory-fugal processing are occupied by heteromodal, paralimbic
and limbic cortices, collectively known as transmodal areas. The unique role of these
areas is to bind multiple unimodal and other transmodal areas into distributed but
integrated multimodal representations. Transmodal areas in the midtemporal cortex,
Wernicke's area, the hippocampal-entorhinal complex and the posterior parietal cortex
provide critical gateways for transforming perception into recognition, word-forms
into meaning, scenes and events into experiences, and spatial locations into targets
for exploration. All cognitive processes arise from analogous associative transformations
of similar sets of sensory inputs. The differences in the resultant cognitive operation
are determined by the anatomical and physiological properties of the transmodal node
that acts as the critical gateway for the dominant transformation. Interconnected
sets of transmodal nodes provide anatomical and computational epicentres for large-scale
neurocognitive networks. In keeping with the principles of selectively distributed
processing, each epicentre of a large-scale network displays a relative specialization
for a specific behavioural component of its principal neurospychological domain. The
destruction of transmodal epicentres causes global impairments such as multimodal
anomia, neglect and amnesia, whereas their selective disconnection from relevant unimodal
areas elicits modality-specific impairments such as prosopagnosia, pure word blindness
and category-specific anomias. The human brain contains at least five anatomically
distinct networks. The network for spatial awareness is based on transmodal epicentres
in the posterior parietal cortex and the frontal eye fields; the language network
on epicentres in Wernicke's and Broca's areas; the explicit memory/emotion network
on epicentres in the hippocampal-entorhinal complex and the amygdala; the face-object
recognition network on epicentres in the midtemporal and temporopolar cortices; and
the working memory-executive function network on epicentres in the lateral prefrontal
cortex and perhaps the posterior parietal cortex. Individual sensory modalities give
rise to streams of processing directed to transmodal nodes belonging to each of these
networks. The fidelity of sensory channels is actively protected through approximately
four synaptic levels of sensory-fugal processing. The modality-specific cortices at
these four synaptic levels encode the most veridical representations of experience.
Attentional, motivational and emotional modulations, including those related to working
memory, novelty-seeking and mental imagery, become increasingly more pronounced within
downstream components of unimodal areas, where they help to create a highly edited
subjective version of the world. (ABSTRACT TRUNCATED)