• sheet of neurons and glia that make up the surface (not including meninges) of the forebrain
• relatively conserved structure from lobe to lobe
• neurons vary by density and size but not by type, general pattern of connectivity, or local projection patterns
• contains 6 layers indicative of evolutionary adaptation as opposed to the 3-4 layers found in the allocortex

• organized into 6 layers of varying cell types, densities, and connectivity patterns
• thickness of layers may differ across lobes/brain areas but there will always be 6 layers
• capable of projecting locally w/in the general region of the cell body, laterally to other cortical areas or longitudinally to subcortical targets
• also exhibit columnar organization

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• layers 1-3
• Layer 1 (outermost) contains few cell bodies and mainly consists of axons of passage
  • receives input from intralaminar nuclei of the thalamus
• Layer 2 projects to association cortices in the ipsilateral hemisphere
• Layer 3 projects to association cortices in the ipsilateral and contralateral hemispheres
• General theme of superficial: projection of and passage of axons to other cortices

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• Receives most feedforward ascending projections - sensory input from the thalamus

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• layers 5+6 - innermost layers of neocortex
• layer 5 - projects to non-thalamic structures (e.g. spinal cord)
• layer 6 - immediately external to white matter
• projects to thalamus as cortical feedback projections

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• Pyramidal cells - excitatory
• Stellate cells - excitatory
• GABAergic interneurons
  • basket cells
  • double-bouquet cells
  • chandelier cells

• One of two types of excitatory glutamatergic cells of neocortex
• Excitatory afferents typically synapse with distal (far from cell body) dendrites
• Inhibitory afferent typically project to proximal dendrites and the cell body itself
• Efferent projections go to other areas of cortex and areas outside the brain like spinal cord, thalamus, and basal ganglia
• Those leaving the forebrain typically exit through the white matter below the neocortex to form corona radiata

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• excitatory glutamatergic cells
• Found in layers 2 and 4 
• Interneurons - only send projections within the neocortex

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• Inhibitory GABAergic cells
• Only project locally to other nearby cortical neurons
• Found in all layers except Layer 1
• Axon projections can be grouped depending on cell as vertical, local, or horizontal 

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• Horizontally-projecting inhibitory GABAergic interneurons
• synapse on the soma and proximal dendrites of target neuron resulting in strong inhibition

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• vertically projecting inhibitory GABAergic interneurons
• Dense, vertical axonal arbor projects to cells w/in multiple layers in a narrow column (weaker inhibition)

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• Locally projecting GABAergic interneurons that are not always inhibitory
• synapse on axon initial segment of pyramidal neurons
• very powerful control of postsynaptic neuron

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• L4 is the input layer as it receives axons from other brain areas - primarily thalamic sensory input
• L2/L3 are the output layers as they project to other cortical areas
• Thalamocortical projections connect thalamus and L4 of a primary sensory area
• Intracortical projections connect L2/L3 of lower area to L4 of higher area
• Circuitry:
  • Thalamus → Primary sensory areas → secondary sensory areas → association cortices

• L1 and L6 are input layers while L5 and L6 are output layers
• Intracortical projections connect axons of L5/L6 of higher areas with L1 and L6 of lower neocortical areas
• Corticothalamic projections connect L5/L6 of primary sensory areas with the thalamic nucleus
•  Circuitry
  • association cortices → secondary sensory areas → primary sensory areas → thalamic nucleus

• Most information received by the cortex is relatively simple in nature and much of its output is related to motor function - lower-order processing
• More complex processing involved in recognition of people, planning, and having a sense of self involve input from many sensory association cortices sending more and more complex information up a hierarchy
• Feedforward occurs as increasingly more complex information rises up the hierarchy but feedback also occurs among these steps going down

• the dorsal or where stream of visual pathway tells you where a particular object is in view 
• information from V1 and V2 cortices are integrated with other information in association cortices of the parietal lobe
• Retina → LGN → V1 → V2 → Parietal Cortex for processing of location

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• visual or what stream of visual pathway tells you what a particular object is in view (object recognition)
• information from V1 and V2 cortices are integrated with other information in association cortices of the temporal lobe
• Retina → LGN → V1 → V2 → Temporal Lobe Cortices

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• areas involved mainly recognize and identify stimuli
• Associated disorders are known as agnosias
• Agnosias are conditions in which patients are aware of the presence of objects but do not recognize or unable to identify them
• 3 Important types
  • Associative Visual Agnosia
  • Apperceptive Visual Agnosia
  • Prosopagnosia

• caused by damage to anterior temporal lobe
• leads to an inability to recognize object or describe what it is/does
• patients are still keenly aware of its presence and capable of drawing it

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• inability to integrate parts of object to perceive it as a whole
• patient still able to identify parts of the object and can recognize whole object through touch
• unable to draw the object as they can only really perceive it in parts
• Associated with damage to occipito-temporal region

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• otherwise known as face-blindness
• inability to identify people by their faces even though they can identify parts of their face and identify people by other cues like voice
• due to damage to fusiform gyrus

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• areas in pathway mainly involved in understanding where object is in space and how it relates to body
• Disorders involve inability to associate objects w/ location and/or integrate parts of an object
• 2 important types
  • optic ataxia
  • simultanagnosia
  • contralateral neglect syndrome

• inability to use visual guidance for accurate reaching to object despite being able to see object
• able to reach accurately when given auditory cues
• associated w/ parietal-occipital association cortex damage
  • links vision with higher-level, multi-modal cortices

• inability to perceive more than one object at a time
• due to bilateral lesions of occipital-parietal junction

• also known as hemi-neglect syndrome
• tendency to ignore sensory information from and perception of side of body contralateral to a lesion
• due to lesions in right parietal cortex (cut-off of all left sensory info)

• brain is unable to regenerate through new neuron development, but cortical reorganization can result in some regain of function
• other neurons take over for those that are damaged to maintain function
• it works very well leading to almost full regain of function in some instances while it may provide an insufficient solution in others
• Occurs in response to experiences
• Good example seen in finger usage
  • fingers normally found as separate sections of SI cortex
  • monkeys taught task where two fingers used at once - led to morphing of the two finger segments
  • also can lead to growth of areas for particular control
  • associated w/ timing of activation implying that limb in question is necessary to experience reorganization
• due to changes in synaptic strength and axon collateral growth
• limb loss can lead to cortical reorganization in which part of SI involved w/ hand will be remapped to function w/ parts remaining that are close to the hand