5. cerebral cortex and higher cortical functions

cerebral cortex

seat of intelligence

basic sensorimotor, visual and auditory processing

cognition

cognition

information-processing functions carried out by the brain

• attention

• memory

• executive functions: planning, problem solving, self-monitoring, self-awareness, and metacognition

• comprehension of language and formation of speech

• calculation abilities

• visual perception

• praxis

structure of cerebral cortex

entire surface of brain

five lobes

• parietal lobe

• frontal lobe

• limbic lobe (sagittal section)

• temporal lobe

• occipital lobe

projection fibers

ascending and descending

fibers project form cortical areas to subcortical structures or from subcortical areas to the cortex

• thalamocortical fibers: main source of input to the cortex originating from non-cortical structures

callosal/commissural fibers

these fibers connect areas of the cortex in one hemisphere with areas of cortex in the opposite hemisphere

association fibers

These fibers connect areas of cortex within the same hemisphere (short fibers connect gyri of the same lobe and long fibers connect gyri of adjacent lobes)

neocortex

six layers (I-VI)

Two major cell types

pyramidal neurons: one apical dendrite and multiple dendritic trees; project OUT of the cortex to other regions of the brain and to the spinal cord

granular neurons: smaller dendritic trees and shorter axons; REMAIN in the cortex - considered interneurons

neocortex

circuitry: intrinsic circuitry

high degree of convergence and divergence within the neural circuitry of the cerebral cortex - allowing for complex integration of information

afferent fibers

input to the cortex

thalamocortical fibers

cortico-cortical fibers

diffuse inputs

afferent fibers: thalamocortical fibers

input from thalamic nuclei

excitatory

afferent fibers: cortico-cortical fibers

excitatory or inhibitory

afferent fibers: diffuse inputs

e.g. ascending reticular activating system; raphe nuclei

excitatory

local circuit neurons

processing and integrating information (layer IV)

• excitatory and inhibitory interneurons

efferent fibers

pyramidal cells - excitatory output from the cortex

• cortico-cortical fibers (i.e. association or callosal - layers II and III)

• projection fibers (layer V)

internal circuitry

Histological Organization

cytoarchitecture: not every of cortex has the same makeup

• differs based on function

homotypical cytoarchitecture

all six layers are clearly represented in most areas of the cortex

heterotypical cytoarchitecture (dont need to know difference between granular and granular cortex)

layers vary in thickness

• granular cortex: layer IV is especially thick and layer V is thin - primary sensory cortex

• agranular cortex: layer V is especially thick and layer IV is thin - primary motor cortex

  

b

brodmann’s areas: frontal lobe (dont need to know numbers, know function)

areas 4, 6, 8-12, 32, and 44-47

• primary motor cortex: area 4

• premotor and SMA (Motor planning): area 6

• frontal eye fields: area 6 and 8

• broca’s area (speech): areas 44 and 45

  • prefrontal and orbitofrontal 8-12, 32, 45-47

parietal lobe brodmann’s areas(dont need to know numbers, know function)

primary somatosensory cortex

posterior parietal cortex: visuospatial functions

• superior parietal cortex: somatosensory association cortex, dorsal stream of vision - “where” and “how” stream of vision)

• inferior parietal cortex: angular gyrus and supramarginal gyrus

temporal and occipital lobe areas (brodman’s areas)

primary auditory cortex

primary visual cortex

visual and auditory association cortices:

• wernicke’s area

• ventral stream of vision (what and who stream of vision)

limbic lobe

lateralization

right brain: big picture, help explain context

left brain: detail orientated, help with routines but not full understanding

motor and sensory functions pertaining to one side of the body are controlled by the opposite hemispehre

information from one visual field is processed by the opposite hemisphere

Functional categories: motor

primary motor cortex, supplementary motor area, premotor cortex

functional categories: primary sensory

primary somatosensory cortex, primary visual cortex, primary auditory cortex

functional categories: association areas

majority of cortex

unimodal association: areas that are adjacent and related to primary areas

multimodal association: there are large areas of cortex which receive information from several different sensory modalities and create a broader, more complete understanding of ourselves and our environment

primary motor cortex

localized to the pre central and anterior paracentral gyri of frontal lobe

somatotopic organization: motor homunculus

sends motor output to the opposite side of the body (corticospinal and corticonuclear tracts)

lesions of this area result in upper motor neuron signs contralaterallypr

primary somatosensory cortex (PSC)

localized to the post central and posterior paracentral gyri of parietal lobe

somatotopic organization: sensory homunculus

receives sensory input from the opposite side of the body (PCMLS and ALS)

lesions of this area result in decreased awareness of sensory stimuli and/or poor localization of sensory stimuli - contralaterally high

higher cortical functions: arousal and attention

arousal/alertness is first step

• starts with the reticular activating system

• if person is not alert enough, there can be no attention

• if a person is over-aroused/hyper-alert, attention will be affected

higher cortical functions: arousal and attention

attention (Frontal and parietal association areas)

• attentional capture: automatic or reflexive orienting to a stimulus

• selective attention: goal-directed focus of attention with the ability to ignore irrelevant stimuli

• sustained attention: ability to maintain vigilance over time

• divided attention: ability to perform two or more tasks at the same time, or process two or more sources of information at the same time

• alternating attention/task-switching: rapidly switching between different skills/tasks/cognitive sets

higher cortical functions: language

frontal and temporal association areas

• language is the faculty of communication to describe things, events, and to express ideas - language is a function of multimodal sensory and motor processing areas and includes speaking, reading, and writing

• hemisphere that controls language is considered the dominant hemisphere

  • almost all right-handers and 50-70 percent of left-handers are left-cerebral dominant

  • in brain lesions which impact language, about 95% of those cases are left hemisphere involvement

  • right hemisphere involved in language in regards to prosody of speech (rhythm, tone, stress, etc.)

• aphasia: disturbance in the understanding or formulation of language not due to hearing/vision/motor impairments

  

speaking a written word

aphasia

any disturbance in language, affecting the production of speech, the comprehension of speech, ability to read, ability to write

• usually due to a lesion of the left hemisphere

• fluent aphasia: can still produce speech, but meaning is impaired (usually a wernicke’s aphasia)

  • non-fluent aphasia: difficulty communicating orally and with written words, as it is the production of speech, but comprehension is usually intact (usually Broca’s aphasia)

broca’s aphasia

expressive aphasia

lesions in the left inferior frontal gyrus, aka Broca’s area or brodmann’s area 44 and 45

frontal lobe tumors and occlusions of the frontal branches of the middle cerebral artery

• often with a contralateral hemiparesis

might still be able to get key words out, grammar is off

might have problems repeating words

might have agraphia (impairments in writing)

usually able to understand spoken and written words

aware of deficits usually- can understand they are not saying what they want to say

wernicke’s aphasia

receptive aphasia

lesions in left inferior parietal lobe, brodmann’s area 39 and 40, as well as posterior superior temporal gyrus, aka wernicke’s area, or brodmann’s area 22

occlusions of temporal and/or parietal branches of middle cerebral artery

might have agraphia (impairments in writing)

might have Alexia (inability to read)

usually present with clear speech, normal prosody, but content is unintelligible (paraphasic speech)

patients usually have impaired awareness of the deficits; cannot comprehend what they are saying; usually less frustrated

conduction aphasia

disruption of the arcuate fascicles, bundle of long association fibers that connect wernicke’s to Broca’s areas

type of fluent aphasia; spontaneous expression is intact and comprehension is functional; patients will have trouble repeating phrases or translating/interpreting what they have heard into an appropriate reply

global aphasia

damage to both Broca’s and wernicke’s areas, usually due to occlusion of the internal carotid artery; results in very gross left hemisphereic damage

loss of virtually all language

may be able to use some gestures

motor control

motor planning (motor and parietal association areas)

praxis: conception and planning of a new action in response to environmental demand

apraxia: inability to execute learned purposeful movements despite having the desire to perform them as well as the motor/sensory capabilities required to execute them

• secondary to lessons of the left hemisphere (premotor cortex, supplementary motor area, and/or parietal association cortex)

forms of apraxia

ideomotor: most common; impaired motor performance in response to a verbal cue despite intact sensory/motor/language, usually with tool use or gesturing

ideational: inability to coordinate activities with multiple, sequential steps or movements

agnosia

inability to know, name, identify, or extract meaning from visual, auditory, or tactile information - parietal and temporal association areas

• visual agnosia: inability to visually recognize objects, although sensory function is generally normal (parietal and temporal areas)

  • ventral stream of vision (who/what path)

• social-emotional agnosia: inability to correctly perceive or comprehend social-emotional information conveyed by voice, gesture, or facial expression

  • prosopagnosia: inability to recognize faces

executive functions

prefrontal cortex

organization: attention, planning, sequencing, problem solving, cognitive flexibility, abstract thinking, etc.

regulation: initiation of actions, self-control, emotional regulation, moral-reasoning, decision-making, etc.

damage to the prefrontal cortex

• deficits in attention

• planning deficits: inability to anticipate or predict future events based on current information

• disorganized

• perseveration

• lack of judgment

  • socially inappropriate behavior

mirror neuron system (MNS)

distributed network of neurons located throughout the frontal (e.g. ventral premotor cortex, inferior frontal gyrus), parietal (e.g. inferior parietal lobe), temporal (e.g. superior temporal gyrus), cingulate and insular cortices

network is active when we perform a particular action and when we observe another individual perform an action

distributed network is thought to allow us to understand the actions performed by another person and predict the consequences of their actions by using mental simulation (simulation theory)

• plays a role in motor learning

• plays a role in empathy

deficits in the functioning of this network have been implicated in autism spectrum disorders and schizophrenia

theory of mind

theory-theory

• how we develop theories about peoples minds; cognitive capability to consider the wants, needs, knowledge, and mental state of another

neural network

• medial prefrontal; precuneus (superior parietal lobule)

• temporal/parietal junction (right)

  • cingulate cortex/amygdala

default mode network

consists of discrete cortical areas (bilateral and symmetrical)

• medial prefrontal cortices

• posterior cingulate cortices

• medial and lateral parietal cortices

• medial and lateral temporal cortices

typically more active when wer aren’t engaged in attention demanding goal directed tasks

thought to allow emotional processing, self referential mental activity, and the recollection of prior experiences

salience network

anterior cingulate cortices

anterior insulation subcortical nodes involved in affect and reward processing

integrates internal and external stimuli

plays a key role in identifying the most relevant stimuli in order to rot guide attention and behavior for adaptive purposes - important for switching between default mode and central executive networksce

central executive network

dorsolateral prefrontal cortex

anterior cingulate

orbitofrontal cortex

lateral posterior parietal cortex

operates during all goal-directed activities and filters out distractions that might interfere with performance

plays a key role in cognitive processing, reasoning, task flexibility, and decision making

blood supply to the cerebral cortex

anterior cerebral: serves majority of medial surface of the hemispheres

middle cerebral: serves majority of the lateral surface of the hemispheres

posterior cerebral: serves the inferior temporal lobe and medial occipital lobe

circle of willis and border zones!

anterior cerebral artery

motor cortex for LE
primary somatosensory cortex for LE

motor planning areas in medial frontal lobe (dominant hemisphere)

prefrontal cortex

posterior cerebral artery

supplies following areas

• optic radiations and primary and visual association cortices

  • hippocampal formation and fornix

middle cerebral artery

primary motor and somatosensory cortex for face, trunk, arm/hand

deep white matter (corticobulbar and corticospinal)

broca’s area: superior cortical brancha nd wernicke’s area (inferior cortical branch) in language dominant (L)

frontal eye fields

parts of frontal and parietal lobed important for lateralized attention, visuospatial analysis and emotional expression (body language/tone of voice) (R hemisphere)

optic radiations (inf cortical branch)

MCA stroke

main stem: key deficits

• contralateral hemiplegia; contralatteral somatosensory loss; contralateral homonymous hemianopia; head and eye deviation toward side of lesion (acutely)

  • right hemisphere lesion: denial, neglect, disturbed spatial perception, emotional flatness possible

  • left hemisphere lesion: global aphasia

MCA superior cortical branch deficits

superior cortical branch

• contralateral hemiparesis impacting the face and UE; possible contralateral somatosensory loss impacting the face and UE; head and eyes deviated toward side of lesion (acutely)

  • right hemisphere lesion: neglect and disturbances ins spatial perception

    • left hemisphere lesion: Broca’s aphasia

MCA inferior cortical branch deficits

possible mild weakness of contralateral face and arm possible contralateral somatosensory loss; contralateral superior Quadrantanopia

• right hemisphere lesion: denial, neglect, disturbed spatial perception

  • left hemisphere lesion: wernicke’s aphasia

anterior cerebral artery

deficits

contralateral hemiparesis involving the leg; contralateral somatosensory loss involving the leg; frontal lobe behavioral abnormalities

ACA-MCA cortical border zone deficits

weakness of proximal limb girdles of arm, leg, bothPCA

PCA

penetrating branch supplying thalamus: may produce loss of all somatic sensation in the contralateral face and body; this initial hemianesthesia may later develop into thalamic pain syndrome

unilateral cortical branches of PCA: contralateral homonymous hemianopia

bilateral cortical branches of PCA: inability to form new semantic or episodic memories; cortical blindness

lacunar strokes (aka small vessel disease)

these strokes are consequences of blockage of blood flow to a single small deep penetrating vessel supplying the subcortical white matter region, basal ganglia, internal capsule, corona radiate, thalamus, or paramedian pons

• blood vessels responsible

  • lenticulostriate arteries of the MCA

  • thalamogeniculate arteries of the PCA

  • paramedian perforating arteries of the basilar artery

common lacunar stroke syndromes: pure motor hemiparesis

pure motor hemiparesis

• most common lacunar syndrome (33-50%) usually involved the internal capsule, corona radiate, or basis pontis; presents clinically with contralateral hemiparesis

common lacunar stroke syndromes: pure sensory

typically involves the thalamus (VPL), internal capsule or corona radiate; presents clinically with contralateral sensory loss

common lacunar stroke syndromes: sensory-motor stroke

typically involves the thalamus and posterior limb of internal capsule; presents clinically with a combination of contralateral motor and sensory lossa

common lacunar stroke syndromes: ataxic hemiparesis

typically involves the internal capsule, pons, or corona radiate; presents clinically as a combination of cerebellar and motor symptoms, including weakness and clumsiness, on the ipsilateral side of the body; typically leg > arm

common lacunar stroke syndromes: dysarthria-clumsy hand

usually involves the pons, anterior limb, or gene of internal capsule; presents clinically with dysarthria and contralateral paresis/clumsiness of the arm and hand