Cerebral Cortex
The term “cognition” can be roughly translated as knowing what is going on
out there and responding appropriately. The neocortex can be considered as the
ultimate organ of cognition but no cortical area makes sense on its own. Every
student of neuropsychology knows that specialized regions of the neocortex
handle different sensory (input) or motor (output) modalities. The occipital
lobes handle vision, and the temporal lobes, sound. The sensory cortex on the
front edge of the parietal lobe receives and interprets all body sensations. The
adjacent frontal lobe motor cortex outputs body movements.
Vernon Mountcastle summarized the work done to elucidate the cellular
organization of the neocortex dominated by vertical columns of neurons usually 6
layers deep. Cortical columns are formed by the linkage of minicolumns by common
inputs and short range horizontal connections. The number of minicolumns per
column varies between 50 and 80. Long-range, intracortical projections link
columns with similar functional properties. Columns vary between 300 and 500 µm
in transverse diameter, and do not differ significantly in size between brains
that vary in size over three orders of magnitude.
Cortical expansion during evolution involved increases in surface area with
little change in thickness. There is a consistent columnar structure all over
the brain. Different inputs, outputs and variations in interconnectivity are the
basis of cortical specialization. Neurotransmitters also vary and reflect old
specializations.
The main question – what do cortical columns actually do? - remains
unanswered. In a simplified schemata, inputs come from the thalamus, other
cortical columns, cerebellum, and outputs go to the thalamus, other cortical
columns and to a complex array of effector systems in all parts of the brain.
There are dense, recursive, looping networks and more focused input and output
networks that connect the neocortex to the rest of the body and the outside
world.
We can think of local cortical processors that do their own thing, but at the
same time coordinate their activities with other local processors. In addition
there is an overall context within which the processors must work together. The
contextual field (CF) involves connections that link processors within and
between cortical regions.
The direct input of sensory information enters receptive fields (RF). RFs and
CFs interact to guide learning and information processing. Cortical computation
requires flexible evaluation of relations between input signals by specialized
processors whose activity is coordinated among regions by contextual
connections.
Phillips and Singer suggested some important distinctions. For example, the
organization of cognition into sub-systems is not based on recognizing
differences in the information processing operations that they perform, but only
differences in the information they send and receive. Some cognitive functions
require special information processing capabilities such as episodic memory and
working memory; intentional representation, creative aspects of language and
strategic planning.
They stated that: “Cognitive functions that are central to human mental life
depend on cortical activity and may not arise in a simple way from capabilities
that are common to cortex in general, however, because (i) intentional
representation and language, are not characteristic of mammals but are
restricted to a few; (ii) in contrast to skills, episodic memories cannot be
acquired in the absence of the hippocampus, and may require special
computational capabilities and (iii) the ability to dynamically create more than
one level of grouping within the same set of units, such as ((AB)(CD)), involves
special computational problems. We have argued that networks of local processors
can discover relevant information in diverse data sets… the distinction between
representation and referent is critical. The relation between representation and
referent can be iconic, symbolic, or both, and the relationship is asymmetrical.
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