Higher Cortical Functions

Cerebral Cortex

• Cerebral cortex is “executive suite” of brain

• Site of conscious mind: awareness, sensory perception, voluntary motor initiation, communication, memory storage, understanding

• Thin (2-4 mm) superficial layer of gray matter

  • Composed of neuron cell bodies, denrites, glial cells, and blood vessels, but no axons

• 40% of total brain mass

• Four general considerations of cerebral cortex:

1. Contains three types of cerebral cortex:

• Motor areas: control voluntary movement

• Sensory areas: conscious awareness of sensation

• Association areas: integrate diverse information

2. Each hemisphere is concerned with contralateral (opposite) side of body

3. Lateralization (specialization) of cortical function can occur in only one hemisphere

4. Conscious behaviour involves entire cortex in one way or another

Cerebral Cortex - Motor Areas

• Located in frontal lobe, motor areas act to control voluntary movement

• Primary (somatic) motor cortex in precentral gyrus

• Premotor cortex anterior to precentral gyrus - planning

• Broca’s area anterior to inferior premotor area - motor speech

• Frontal eye field - controls voluntary eye movement

Cerebral Cortex - Primary (somatic) motor cortex

• Located in precentral gyrus of frontal lobe

• Pyramidal cells: large neurons that allow conscious control of precise, skilled, skeletal muscle movements

• Pyramidal (corticospinal) tracts: formed from long axons that project down spinal cord

• Somatotopy: all muscles of body can be mapped to area on primary motor cortex

• Motor homunculus: upside-down caricatures represent contralateral motor innervation of body regions

Cerebral Cortex- Sensory areas

• Areas of cortex concerned with conscious awareness of sensation

• Eight main areas include primary somatosensory cortex, somatosensory association cortex, visual areas, auditory areas, vestibular cortex, olfactory cortex, gustatory cortex, and visceral sensory area

Primary somatosensory cortex

• Located in postcentral gyri of parietal lobe

• Receives general sensory information from skin and proprioceptors of skeletal muscle, joints, and tendons

• Capable of spatial discrimination: identification of body region being stimulated

• Somatosensory homunculus: upside-down caricatures represent contralateral sensory input from body regions

Somatosensory association cortex

• posterior to primary somatosensory cortex

• Integrates sensory input from primary somatosensory cortex for understanding of object

• Determines size, texture and relationship of parts of objects being felt

Primary visual (striate)

cortex located on extreme posterior tip of occipital lobe

• Receives visual information from retinas

Visual association area

• Surrounds primary visual cortex

• Uses past visual experiences to interpret visual stimuli (colour, form or movement)

• Example: ability to recognize faces

• Complex processing involves entire posterior half of cerebral hemisphere

Primary Auditory Cortex

• Superior margin of temporal lobes

• Interprets information from inner ear as pitch, loudness and location

Auditory Association Area

• Located posterior to primary auditory cortex

• Stores memories of sounds and permits perception of sound stimulus

Vestibular Cortex - Cerebral Cortex

• Posterior part of insula and adjacent parietal cortex

• Responsible for conscious awareness of balance (position of head in space)

Gustatory Cortex - Cerebral Cortex

• In insula just deep to temporal lobe

• Involved in perception of taste

Olfactory Cortex - Cerebral Cortex

• Medial aspect of temporal lobes (in piriform lobes)

• Part of primitive rhinencephalon, along with olfactory bulbs and tracts

• Remainder of rhinencephalon in humans becomes part of limbic system

• Involved in conscious awareness of odour

Multimodal Association Areas - Cerebral Cortex

• Receive inputs from multiple sensory areas and send outputs to multiple areas

• Sensory receptors → primary sensory cortex → sensory association cortex → multimodal association cortex

• Gives meaning to information received, store in memory, tie to previous experience, and decide on actions

• Sensations, thoughts, emotions become conscious: makes us who we are

• Broadly divided into three parts: anterior association area, posterior association area, and limbic association area

Anterior Association Area - Cerebral Cortex

• Also called prefrontal cortex

• Involved with intellect, cognition, recall, and personality

• Contains working memory needed for abstract ideas, judgment, reasoning, persistence, and planning

• Development depends on feedback from social environment

Posterior Association Area - Cerebral Cortex

• Large region in temporal, parietal, and occipital lobes

• Plays role in recognizing patterns and faces and localizing us in space

• Involved in understanding written and spoken language (Wernicke’s area)

Limbic Association Area - Cerebral Cortex

• Part of limbic system

• Involved cingulate gyrus, parahippocampal gyrus, and hippocampus

• Provides emotional impact that makes a scene important to us and helps establish memories

Techniques to study brain function

• fMRI

• PET

• EEG

Functional Neuroimaging (fMRI) of the Cerebral Cortex

• Functional imaging (PET and MRI) of brain show specific motor and sensory functions are located in discrete cortical areas called domains

• Higher functions are spread over many areas

Brain Wave Patterns and the EEG

• Brain waves are patterns of neuronal electrical activity recorded

• Reflect electrical activity of higher mental functions

• Normal brain functions are continuous and hard to measure

Electroencephalogram (EEG) records electrical activity that accompanies brain function

• Used for diagnosing epilepsy and sleep disorders

• Localizes lesions, tumors, infarcts, infections, abscesses

• Used in research and also to determine brain death

• Electrodes placed on scalp measure electrical potential differences between various cortical areas

• EEG measures patterns of neuronal electrical activity generated by synaptic activity in cortex

  • Each person’s brain waves are unique

  • Patterns changes with age, sensory stimuli, brain disease, and chemical state of body

  • Measures wave frequency in Hertz (Hz), numbers of peaks per second (1 Hz = 1 peak per second)

• Can be grouped into four classes based on Hz:

  • Alpha, beta, theta, or delta waves

Brain Wave Patterns and the EEG

• Alpha waves: (2-13 Hz) - regular and rhythmic, low-amplitude, synchronous wave indication an “idling” brain

• Beta waves: (14-30 Hz) - rhythmic, less regular waves occurring when mentally alert

• Theta waves: (4-7 Hz) - more irregular; common in children and uncommon in awake adults

• Delta waves: (4 Hz or less) - high-amplitude waves of deep sleep and when reticular activating system is suppressed, as during anesthesia; indicates brain damage in awake adult

Language

Language implementation system involves association cortex of left hemisphere

Corresponding areas on right side are involved with nonverbal language components

Broca’s Area

• Involved in speech production

• Patients with lesions in Broca’s understand words, but cannot speak

Wernicke’s area

• Involved in understanding spoken and written words

• Patients with lesions in Wernicke’s can speak, but words are nonsensible

Learning and Memory

Memory: storage and retrieval of information

Different kinds of memory

• Declarative (fact) memory (names, faces, words, dates)

• Procedural (skills) memory (playing piano)

• Motor memory memory of motor skills (riding a bike)

• Emotional memory memory of experiences linked to an emotion (heart pounding when you hear rattlesnake)

Two stage of declarative memory storage:

• Short-term memory (STM or working memory): temporary holding of information

  • Limited to seven or eight pieces of information

• Long-term memory (LTM) has limitless capacity

Factors affecting transfer from STM to LTM

• Emotional state: best if alert, motivated, surprised, or aroused

• Rehearsal: repetition and practice

• Association: tying new information with old memories

• Automatic memory: subconscious information stored in LTM

Memory consolidation involves fitting new facts into categories already stored in cerebral cortex

Hippocampus, temporal cortical areas, thalamus, and prefrontal cortex are involved in consolidation

Clinical - Homeostatic Imbalance

• Damage to hippocampus or surrounding temporal temporal lobe structures on either side results in only slight memory loss

• Bilateral destruction causes widespread amnesia

Anterograde Amnesia

• Consolidated memories are not lost, but new inputs are not associated with old one

• Person lives in the here and now

• Memory of conversation from just 5 minutes before would not be remembered

Reterograde Amnesia

loss of memories formed in the distant past

Consciousness

Consciousness involves:

• Perception of sensation

• Voluntary initiation and control of movement

• Capabilities associated with higher mental processing (memory, logic, judgment, etc.)

Clinically defined on continuum that grades behaviour in response to stimuli:

• Alertness

• Drowsiness (lethargy)

• Stupor

• Coma

Current suppositions on consciousness

• Involves simultaneous activity of large cortical areas

• Superimposed on other types of neural activities

• Holistic and totally interconnected

Clinical - Homeostatic Imbalance

• Except during sleep, loss of consciousness signals that brain function is impaired

• Fainting or syncope: brief loss of consciousness

  • Most often due to inadequate cerebral blood flow

  • Due to low blood pressure or ischemia from hemorhage or sudden, severe emotional stress

  • Coma: unconsciousness for extended period

    • Not the same as deep sleep; oxygen consumption is lowered

  • Brain death: irreversible coma

    • Ethical and legal issues surround decicions on whether person is dead or alive

Epileptic Seizure

• Torrent of electrical discharges by groups of brain neurons

• Prevent any other messages from getting through

• Victim of epilepsy may lose consciousness, fall stiffly, and have uncontrollable jerking

• Epilepsy occurs in 1 of 100 people

  • Epilepsy is not associated with intellectual impairments

  • Genetic factors play a role, but brain injuries, stroke, infections, or tumors can also be causes

Absence Seizures (formerly petit mal)

• Mild seizures in which expression goes blank for few seconds

• Typically disappear by age 10

Tonic-clonic seizures (formerly grand mal)

• Most severe; last few minutes

• Victim loses consciousness, bones broken during intense convulsions, loss of bowel and bladder control, and sever biting of tongue are common

• Aura (sensory hallucination) may precede seizure

Control of epilepsy includes anticonvulsive drugs

Vagus nerve stimulator or deep brain stimulator implantations deliver pulses to vagus nerve or directly to brain to stabilize brain’s electrical activity

Sleep and Sleep-Wake Cycles

• Sleep:state of partial unconsciousness from which person can be aroused by stimulation

• Cortical activity is depressed, but brain stem activity doesn’t change

Types of sleep:

• Two major types of sleep (defined by EEG patterns)

• Non-rapid eye movement (non-REM) sleep

• Rapid eye movement (REM) sleep

During the first 30-45 minutes of the sleep cycle, pass through first two stages (N1 and N2) of non-REM, then move into stage 3 called slow-wave sleep

• Frequency of waves declines, but amplitude increases

• EEG, blood pressure, and heart rate decrease

About 90 minutes in the REM sleep begins abruptly

• Temporary paralysis, except for rapid eye movements

• Heart rate, respiratory rate, and blood pressure increase; oxygen consumption increases greater than when awake

• Most dreaming occurs in REM

REM

Skeletal muscles (except ocular muscles and diaphragm) are actively inhibited; most dreaming occurs

Stage N1

Relaxation begins: EEG shows low-amplitude mixed-frequency and alpha waves; arousal is easy

Stage N2

Irregular EEG with sleep spindles (short high-amplitude bursts); arousal is more difficult

Stage N3

EEG has high-amplitude theta and delta waves; vital signs decline; arousal is difficult; bed-wetting, night terrors, and sleepwalking may occur

How sleep is regulated

• Alternating cycles of sleep and wakefulness reflect natural circadiain (24-hour) rhythm

• Suprachiasmatic nucleus ( a biological clock) and preoptic nucleus (a sleep-inducing center) of hypothalamus regulate timing of sleep cycle

• Hypothalamus releases orexins that help cortex to wake up

Importance of Sleep

• During sleep, consolidate new memories and discard memories no longer accessed

• Sleep presumed to be restorative stage

• During non-REM sleep pulses of cerebrospinal fluid sweep through brain, aiding in washing out of waste products, including some associated with Alzheimer’s disease

• When deprived of sleep, spread more time in REM and slow-wave sleep in next sleep episode attempting to catch up

Narcolepsy

• Sleep disorder involving abrupt lapse into sleep from awake state

• Orexins (hypothalmic “wake-up”chemicals) probably destoryed by patient’s immune system; orexin replacement possible treatment

Insomnia

• Chronic inability to obtain amount or quality of sleep needed, possibly caused by depression, anxiety, overuse of caffeine, computer/cell phone use too close to bedtime

• May be treated by blocking orexin action

Brain Injuries and Disorders

Traumatic brain injuries include:

• Concussion: temporary alteration in function

• Contusion: permanent damage

• Subdural or subarachnoid hemorrhage: pressure from blood may force brain stem through foramen magnum, resulting in death

• Cerebral edema: swelling of brain associated with traumatic head injury

Cerebrovascular Accidents

Cerebrovascular accidents (CVAs), are also called strokes

• Ischemia: tissue deprived of blood supply, leading to death of brain tissue

• Can be caused by blockage of cerebral artery by blood clot

• Hemiplegia (paralysis on one side) or sensory and speech deficits may result

• Transient ischemic attacks (TIAs): temporary episodes of reversible cerebral ischemia

Alzheimer's Disease (AD)

• Progressive degenerative disease of brain that results in dementia (mental deterioration)

• Memory loss of recent events, shortened attention span, disorientation, eventual language loss, irritability, moodiness, and confusion

• Caused by misfolding of protein that then clump together

  • Plaques of beta-amyloid peptides form in brain

  • Neurofibrillary tangles inside neurons interfere with transport mechanisms with tangles of the tangles of the protein tau, eventually killing neurons

• As brain cells die, functions are lost, and brain shrinks

• Treatment includes drugs to inhibit breakdown of acetylcholine, block toxic effects of excess glutamate that is released when neurons are damaged, or stimulate destruction of beta-amyloid

Parkinson’s disease

• Degeneration of dopamine-releasing neurons of substantia nigra

• Basal nuclei deprived of dopamine become overactive, resulting in tremors at rest

• Cause unknown, but theories include mitochondrial abnormalities or protein degradation pathways

• Treatment includes L-dopa (dopamine presursor) to alleviate symptoms, deep stimulation with electrodes, and implanting stem cells

Huntington’s disease

• Fatal hereditary disorder caused by accumulation of protein huntingtin in brain cells

  • Leads to degeneration of basal nuclei and cerebral cortex

  • Initial symptoms include wild, “flapping” movements called chorea

• Initial symptoms include wild, “flapping” movements called chorea

• Later marked by mental deterioration

  • Usually fatal within 15 years of onset

  • Treated with drugs that block dopamine effects

Diagnostic Procedures for Assessing CNS Dysfunction

• Simple test can include knee-jerk reflex with hammer tapped against quadriceps tendon

  • Abnormal responses may indicate intracranial hemorrhage, multiple sclerosis, or hydrocephalus

• CT and MEI scanning techniques allow for quick identification of tumors, intercranial lesions, plaque, or areas of infarct (dead brain tissue)

• PET scans can localize brain lesions that generate seizures

• Radioactive tracer dyes help visualize specific areas