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29 Aug Layers of Brain Complexity

Body of KnowledgeHierarchy: A Structural Aspect of Thought

The complexity of the cerebrum is necessitated by its function as the center of cognition. Before looking at cerebral structure, consider some of the functions of thought. Thinking involves accessing memory to find what perceived data is stored, what other data in memory it is associated with, and what is the nature of associations. For new items, thinking often involves conscious attempts to classify things into superclasses and analyze them into their constituent parts. This involves search procedures through a person’s body of knowledge and reasoning about classes of associations.

In this blog, I’m looking closely at these connections between body and mind, between cognition and communication, because I believe that this  has been lacking in some artificial intelligence or cybernetics research in the past. I have a hard time accepting views of thinking and communicating that exclude any interconnected domains of knowledge, and want to look in sufficient detail at all the factors that could contribute to our ability to understand one another.

Understanding Context Cross-Reference
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Section 1 #6

Brain Section Icon


Table of Context

The simple diagram below shows one way people deal with knowledge: we classify it into hierarchies. A simple example of hierarchical classification is the way biologists have categories for all living things. At the top are the kingdoms of plants and animals. Organisms are further categorized by phylum, genus, and species. Though little is known about how the brain makes hierarchical classifications, a description of cerebral structure can assist in analyzing how this structure may be used to accomplish tasks such as hierarchical categorization.

Cerebrum Layers

There are six layers identified in 90% of the cerebrum. The chart below briefly describes them.Cells In Cerebrum 6 Layers

# Name Depth Cell body types Fiber patterns
I Molecular 200 µm horizontal – stellate Tangential layer; (plexiform) or granule horizontal fibers
II External 200 µm pyramidal -granule Dysfibrous layer granular Martinotti shorter fibers
III External 450 µm small pyramidal-Suprastriate layer pyramidal granule – Martinotti mostly horizontal
IV Internal 200 µm pyramidal -External band of granular – granule – Martinotti Baillarger (h+v)
V Ganglionic 250 µm giant pyramidal Interstriate layer (internal granule -Martinotti horizontal with pyramidal) more vertical
VI Multiform 250 µm fusiform – pyramidal Infrastriate layer (Fusiform) granule -Martinotti vertical/horizontal

The area shown below layer VI is Sub-cortical white matter with myelinated fibers but no cell bodies . The structural morphology of cerebral cells, their organization, and their connections suggest that their structures and functions are equally diverse. The range of connectivity in cerebral cells is from dozens of synapses in small cells to tens of thousands in the largest. Pyramidal cells are the most numerous in the cortex. Small pyramidal cells can be 20-30 micrometers while giant pyramidal cells (the minority) can be from 70 to over 100 micrometers. Giant pyramidal cells (Betz cells) are characteristic of the motor area (4) in layer V.

Cerebrum1 Synapse TypesCell Shapes and Links

Links between neurons are formed at synapses. These will be discussed in greater detail in Sections 2 and 3. For now, take a look at the illustration at right. It shows the three general types of incoming synapses. Outgoing synapses can be any of the three types, but because they occur less frequently, they are all the same color on the graphic. Note that input links occur anywhere on the cell membrane, with axodendritic being the most common by far. Differences between cell morphologies and locations in the brain are usually attributed to functional specialization. Some of the cells may serve different roles, requiring limited links with certain other specialized cells. The types of links may also be different: some favor axodendritic while others prefer axosomatic or axoaxonic. Purkinje cells are known to provide the output channel, suggesting intriguing possibilities for processing and coordinating roles for the other cell types.

The cerebrum is far more complex than the cerebellum. This could be due to the complexity of functions of the cerebrum, which range from motor control to symbolic communicative processing to reasoning. These functions will be discussed more fully in later posts. For now, the foundational point for cybernetic modeling is that a simple model will not sufficiently represent this much brain complexity. A highly distributed model with large numbers of very simple processing elements is also unlikely to be able to replicate the processes handled by such a complex system.

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