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CEREBRAL CORTEX
Cell types
Pyramidal
Apical vs. basal dendrites
Projection neurons
Stellate
Smooth (inhibitory interneurons)
Spiny (excitatory)
Others
Layers
Layer are key to input/output organization and to differences between cortical areas
Name the layers and relate to cell types
Granular (layer 4; mainly stellate cells)
Supragranular (layers 2 & 3; mainly smaller pyramidal cells)
Infragranular (layers 5 & 6; mainly larger pyramidal [layer 5] and smaller
pyramidal and fusiform [layer 6])
Relation to input/output (greatly simplified)
Input from thalamic relay nuclei: to layers 4 and 3
Output
4  other layers of same area (intrinsic)
6  thalamus
5  non-thalamic subcortical (e.g., spinal cord, brainstem)
2/3  cortex (including callosal)
Interlaminar circuits (examples)
4  2/3
2/3  5
Cytoarchitecture and Cortical Areas
Layers: number varies
Neocortex: 6 layers
Allocortex: < 6 layers
Progressive reduction in complexity as approach margin of the
hemisphere
Paleocortex (olfactory cortex): 4-5 layers
Archicortex (hippocampus, edge of mantle): 3 layers
Topographic variations of laminar structure across neocortex
Thickness (or even presence) of layers
Sizes of cells
Example 1: Striate cortex
Thick layer 4 ("koniocellular" cortex)
Extra white layer splits layer 4
Clear boundary with Area 18
Example 2: Pre-central vs. post-central gyrus
Precentral
Lacks layer 4 ("agranular" cortex)
Big pyramidal cells in layer 5 (Betz cells)
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Postcentral
Thick layer 4
Biggest pyramids in are in layer 3 instead of layer 5
Cytoarchitectonic maps -- Brodmann
Good agreement on boundaries near primary areas among various
cytoarchitectonic maps and with modern maps based on other methods
Less agreement in association cortex
Modern criteria for mapping areas
Physiological
Topography
Maps of monkey cortex (Van Essen)
Difficulties in remote areas
Big receptive fields
Scrambled topographies
Receptive-field properties
Anatomical
Histochemical
Myelin architecture (e.g., middle temporal visual area or MT)
Cytochemistry (eg. blobs-to-stripes transition at V1/V2 border in
primate visual cortex)
Connectivity
Corticocortical connections
Thalamic afferent connectivity
Efferent connectivity
Summary on mapping areas
Convergent evidence is best when trying to define borders
Association areas may not have crisp boundaries
Cortico-cortical connections as related to areas
"Feedforward" (conventional, long-recognized pathway)
Primary  secondary  association  limbic/frontal
Laminar signature: terminates in layer 4 of target area
Old (simplistic) idea about “meaning” of forward
projections: Mediates transformation from pure
sensation to object recognition to integration with
other senses and emotion, memory, behavior
But parallel network more realistic
"Feedback" (a part of the networking architecture)
Laminar signature: terminates outside layer 4 of
target area (both supragranular and infragranular)
Possible function? Alteration of or integration with
sensory processing based on higher-level
perceptual, cognitive processes
Hierarchies of cortical areas based on analysis of
"feedforward" and "feedback" patterns of
connectivity
Two other classes of corticocortical connections
Intrinsic (integration within a modality over space)
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Commissural
Homotopic links
In primary cortex, regions of maps away from
the midline tend to lack commissural
connections
One role of callosum may be to form a "seam"
between two hemi-representations
Comparative (i.e., inter-species) differences in cortical organization
Location of "continental divide" between sensory and motor (central sulcus in
humans)
Frontal cortex expansion in human vs. cat/rat
Functional correlate: differences in sophistication of planning, contingent
behavior.
Expansion of "association" cortex in human
Correlate: complexity of cognition.
e.g. Language function involves auditory, motor, symbolic, memory
and visual elements
Columns
Definition
Vertically arrayed modules
Pia to white matter
Shared physiology within a column
Physiological properties change abruptly at boundary between adjacent
columns
First hint: radial fibers and cell bands
Mountcastle (father of the column concept; work done in somatosensory cortex)
Submodalities segregated in columnar zones (rapidly vs. slowly adapting
cutaneous)
Hubel and Wiesel
Ocular dominance
Orientation
Problem: some parameters vary continuously with cortical distance. How discrete is
discrete enough to indicate columnar organization?
Retinotopic location - clearly not columnar
Orientation preference - is it a continuous or discrete variable?
No anatomical marker of discreteness
Ocular dominance - even this property, except in layer 4, has a continuous, not
a discrete representation
But some evidence of anatomical modules
Ocular dominance stripes in layer 4
Blobs in striate cortex
Whisker barrels
Corticocortical connections (intrinsic and long-range)
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