Week 11 Flashcards
Frontal Cortex
• Posterior → motor areas • Anterior → association area – prefrontal cortex (PFC) • Important subregions • Lateral PFC (lPFC) dorsal and ventral portions (dlPFC, vlPFC) • Frontal pole (FP) • Orbitofrontal cortex (OFC) – ventromedial • Medial frontal cortex (MFC)
Prefrontal Cortex
• Massively connected – links motor, perceptual, and limbic • Large input from thalamus – connects PFC with BG, CB, brainstem nuclei • Almost all cortical and subcortical areas influence PFC either directly or by only a synapse or 2 • Many projections to contralateral hemisphere • Many connections bidirectional
Prefrontal Cortex – Sensory Input
Dorsal - motor and executive control functions for which spatial information is important Ventral – mediate emotional responses to things in the environment – emotional significance first requires recognition
Prefrontal Cortex – Motor Control
• Emotional and cognitive processes exert control over behaviour • Serial/functional hierarchy • vmPFC/OFC (emotional processes) to dlPFC (cognitive control) to motor
Prefrontal Cortex – Emotional
• vmPFC/OFC • Closely connected with limbic structures, especially amygdala • Influence cognition and directly influence ANS • Drives, motivation, valence, and the assessment of the emotional significance of sensory stimuli • Part of a system encoding emotions and emotional responses
Prefrontal Cortex
• PFC half of FL in humans • Human PFC expansion – more about white matter than grey matter • Connections! • PFC matures late in development • Not fully developed until mid 20’s !! • Grey matter peaks earlier • White matter later
PFC Myelination
• Higher cog functions (language, intelligence, and reasoning) heavily rely on intracortical and corticocortical connectivity • Do not reach full maturity until mid 20’s • Corticocortical axons that develop most between childhood and adulthood are those that run from PFC to posterior association cortex • Those axons are part of the top– down frontal efferent pathways critically involved in cognitive control
Dysfunction - Phineas Gage
• 1848, Phineas Gage working as a foreman when an accident shot a steel rod through the front of his skull • Survived and reportedly got to his feet and walked away • “I did not believe Mr. Gage's statement at that time ... Mr. Gage persisted in saying that the bar went through his head ... He got up and vomited; the effort of vomiting pressed out about half a teacupful of the brain [through the exit hole at the top of the skull], which fell upon the floor”• Significant part of his left frontal lobe destroyed • Dramatic personality shift but memory and general intelligence seemed unimpaired after the accident • Social problems - now fitful, irreverent, and grossly profane, showing little deference for his fellows. • He was also impatient and obstinate, yet capricious and vacillating, unable to settle on any of the plans he devised for future action. • His friends said he was “no longer Gage." • Most serious mental changes were temporary • Became far more functional, and socially far better adapted • A social recovery hypothesis suggests that Gage's work as a stagecoach driver in Chile fostered this recovery by providing daily structure which allowed him to regain lost social and personal skills. • 1860, he began to have epileptic seizures and died a few months later
Dysfunction - Frontal Lobotomy
• Treatment of psychiatric disorders 1930’s to 1950’s (pre drugs) • Moniz pioneered based on reported effects of frontal lobectomy in chimps (won Nobel in 1949) • Isolate PFC rather than remove – connections!!
Frontal Lobotomy
Transorbital sectioning • Electroshock anaesthesia • Icepick tapped through top of the orbit • Insert 5cm into brain and “wiggle” Leucotome • Insert 6 times with cutting wire retracted • Extrude cutting wire and rotate • Cut out a core of tissue • Anecdotal reports – patients were ‘stimulus-bound’: • Reacted to whatever was in front of them and did not respond to imaginary situations, rules, or plans for the future. • Some gained significant weight, and / or became sexually promiscuous • Could not form / sustain goals • Distracted by circumstances
Dysfunction - Frontal Astrocytoma
• Patient W.R - the man who had “lost his ego”
• Life changed after he earned his law degree - over 10
years from graduation:
• Had not taken the bar exam 4 years after graduation, or
even looked for a job
• No motivation
• Worked as instructor in a tennis club
• Family described him as ‘drifting’
• Poor state financially, borrowing from brother
• Eventually gave up tennis: became demotivated and
nonchalant during matches, not keeping score
• Lost interest in romantic pursuits
Frontal Astrocytoma
• Suffered a seizure in his last year of college
• No identifiable cause (on PET or CT) at the time
• CT re-done:
• Extremely large astrocytoma
• Traversing along the callosal fibres, invading extensively the lateral
prefrontal cortex in the left hemisphere, and considerably in the
right
• Poor prognosis: death within ~1 year
• Response of W.R.: passive, detached, no rage, minimal
anguish, general absence of concern
Dysfunction - FTD
• Frontotemporal dementia or “Pick’s Disease”
• In 1892, Arnold Pick described a man who had presented in
life with progressive loss of speech and dementia.
• When the patient died his brain was found to be atrophied.
• This shrinkage had been caused by brain cells dying in
localized areas.
• This feature of localization is very different to Alzheimer’s
disease where the atrophy is more generalized.
Atrophy of frontal
lobes and anterior
temporal lobes
‘knife-edging’ – thinning
of the gyri from
neurodegeneration
Atrophy with
ventricular dilation
Swollen neurons with
abnormal tau protein
inclusions
Frontotemporal Dementia (FTD)
Abnormal spontaneous behaviours during examination
• Inappropriate jocularity
• Echolalia (repeating the examiner’s words), echopraxia
(repeating the examiner’s gestures)
• Disinhibited approach or utilization behaviours
• Unkempt, depressed in early stages
During the first 2 years - “classic” frontal lobe syndromes:
• orbitofrontal dysfunction: aggressive and social
inappropriateness (may steal or demonstrate obsessive or
repetitive stereotyped behaviours), apathy and disinhibition
• dorsomedial or dorsolateral dysfunction: lack of concern,
apathy, or decreased spontaneity.
Speech and language
• Abnormalities often begin early and progress fast
• Memory impairment relatively less severe than
speech/language and behavioural changes
• Verbal output that is often nonfluent, with poor naming
of objects
Movement disorders
• Akinesia, plastic rigidity, or paratonia (involuntary
resistance) on motor examination
Perseveration
Frontal Lobe Dysfunction
• FL patients often seem ok – no obvious perceptual,
motor, speech, intelligence, knowledge, LT memory
problems
• Unless damage bilateral and extensive
• Apathetic, distractible, impulsive
• Trouble making decisions, planning actions,
understanding consequences of actions, following
rules
• Trouble organising and segregating timing of events in
memory and remembering sources of memories
• Loss of goal oriented behaviour –stimulus driven –
reflexive behaviour that can’t inhibit
• Poor social control, inappropriateness, irritability,
aggression
• Deficits vary with location – different regions of PFC
subserve different processes
