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Localisation of Function: Principles and methods: Brain and Behaviour…
Localisation of Function: Principles and methods: Brain and Behaviour
two competing views of structure function relationships in the brain
localist
Brain composed of specialised areas
information processing in each area is local and specific
Discrete areas are responsible for discrete functions
Gall and Spurzheim (1810)
Believed function localised in specific areas- each function had own area - like phrenology!
each mental process has own substrate in certain regions of the brain
each organ is characterised by content domain it is specialised to pr
cerebral cortex contains these organs
BROCA (1861)- like 'brocas' area in the inferior frontal sulcus
reported productive asphasia
Patients know what they want to say but don't know how to get it out
related to legion in the brocas area
Brodmann areas, 44/45
Speech production
broca realised that unilateral lesions of the left third frontal convolution not the right that meant loss of the procedural memory needed to articulate language.
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Wernicke's area - association between left temporal cortex injury and pattern of behavioural impairment that included fluent by paraphrasic speech and difficulty in language comprehension
Anti Localist
The brain is a collection of networks
information processing is 'distributed'
All areas are equally responsible for all functions
Flourens (1822)
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Challenged Galls
reported that legions to small specific brain areas in birds did not cause specific deficits
all recovered well regardless where the legion was
specific functions simply mediated by the brain:
all areas help with functions
Aggregate Field Theory
Lashley (1929)
Mass Action
Equipotentiality
Function shared by all neurons rather than a specific one
Cerebral cortex
Learning and memory
Claimed memory was equally distributed throughout the cerebral cortex
Participation of individual neurons was probabilistic rather than deterministic
Probabalistic= subject to involving chance based on or adapted to a theory of probability - variation or chance
Deterministic= all events are determined by causes external to free will
Experimental findings:
input lesions into the neocortex of a rat - given maze learning task
Neocortex: area of the brain responsible for higher order functions
sensory perception
cognition
motor commands
spatial reasoning
language
legion size related to level of impairment
small legion: small impairment
effect was independent of lesion location
visual neglect seen after lesions to the right inferior parietal lobe but also in the Right superior temporal gyrus, right lateral premotor cortex, thalamus, basal ganglia and cerebellum ( MORT et al, 2003)
Modern day evidence
ANATOMICAL
neuronal properties are variable so the way they process information must also be variable
neuron level
localised
brain = interconnected networks of neurons - must be distributed processing across these networks
network level
distributed
BRAIN ACTIVITY STUDIES
Electrophysiology and FMRI
Activity in the individual brain is task specific
localised
activity of one brain area- influences activity in the connected areas
distributed
Lesion studies
effects of carefully applied lesions = functionally specific
localised
some lesions affect ability to perform tasks when only applied to more than one area
distributed= more than one area allows a function to happen
Evidence for both localised and D info processing
explain brain behaviour relationships in terms of both
New Terminiolgy:
Functional segregation
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how does function emerge from specialised info processing in individual brain areas?
Cooney and Gazzaniga(2003)
perceptual awareness = modular -neural correlates of awareness consisting of specialised structures in perceptual processing
intergrative multimodal model of conscious experience requires functional architecture overcomes modular segregation of function
proposed model which includes both: experience emerges from interactiuons between specialised component processes via distributed neural network: so specialised areas involved but processing takes place over the whole network!
mechanism offers explanation of neural correlates of perceptual awareness, cognitive function and neurological damage
they believe a modular organisation in brain : conscious experience made of parts which are the result of modular neural circuits made to specially process specific information.
outputs of neural processing= distributed through large neuronal workspace or circuit.
outputs from array of parallel processors compete for influence within the network- intergration of these outputs determines which output will emerge as dominant= network state where information used widely over variety of cognitive processes- determine subjective experience of the individual
population of neurons activated at any second= influences development of states of the network allowing a progression of cognitive processes with no need for separate higher order executive system.
explantion for how lesions can alter or eliminate contents of subjective experience with no deficit in brain damaged patients-
evidence strong!
FMRI, single cell recordings , neural circuits segregate into functionally specific area on a cognitive and cellular level, = robust correlation between performance on specific tasks and activation of specific neural tissue regions.
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Ie. Fusiform face area region and V5 of visual cortex activated seperatley and selectively for face area and visual stimuli respectively but when these are imagined/ mental imagery of stimulus aswell - don't need the conscious experience (kanwisher et al)
functional segregation/ specialisation is only meaningful in the context of functional intergration'- (Friston, 2002)
Functional Integration
How does function emerge from distributed information processing across networks of these areas.
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Lesion studies (-) Insufficient
permenant lesions cannot help understand how an intact brain works
lesions that don't impair function
info processing in area not needed for task
processing I needed for task in normal brain but lesioned brian needs to compensate
lesions that impair function:
Essential information processing for task= happens here
Essential processing in connected areas=disprupted
different interpretations which is correct
LESION STUDIES = POOREST T resolution
Double Dissociation
patients lesion= A but intact B = deficit in A not in B and patients with lesion in B and intact A = deficit in behaviour at B not A
methods to study the intact brain - without lesions
Brain architecture
ANATOMY
Cytoarchitecture
cellular composition of the cortex - varies
information processed here therefore cannot be uniform
mapping variation distinguishes cortical areas in forms of info processing
connections
Tracers ( primates)
injected into one end of the neuron to map the network and transported out the other end
histological analysis reveals terminal sites
(-) don't leave neurons cannot cross synaptic cleft
(-) cannot trace whole circuit due to tracer dilution as travels further- distant sites harder to id.
new tracer tech
(+) virus used as tracer as they can jump the synapse
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Brain imaging-living brain
flow of info across networks= integral to info procesisng
connectivity - studies
diffusion tensor imaging
free diffusion = isotropic
=same in all directions
barriers to diffusion cause anisotropy -diffusion preferred in some directions more than others
Diffusion of water molecules
New magnetic resonance imaging
Gross morphology
MRI
FMRI
active neurons extract oxygen from blood capillairies and change oxyhaemoglobin to deoxyhaemoglobin
oxy and deoxy= different magnetic properties
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one changes to the other this is detected and localised by MRI scanners
(+) higher spatial and temporal resolution
unlike PET - no radioisotope needed
BOLD Signals
magnitude increases
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Functional
protons in water molecules in head induced emit radiofrequencies in prescence of strong magnetic fields. MRI detectors pick up signal -
use detectors to reconstruct signal location
Brain Activity - whole brain methods
Electroencephalography (EEG)
electrical potentials produced by neurons = small - several n active together= measured by an electrode on scalp
poor spatial resolution- way signal is distorted by the tissues between brain and the electrode ie bone
high temporal resolution
averaging traces to cancel out noise - EEG responses specific to particular signals event related potentials ERP's
ERP indicate change in neural activity over time
Magnetoeencephalography (MEG
MEG measures magnetic signal changes as the result of changes in electrical potential
(+)very high Temporal resolution- better spatial resolution that EEG as magnetic signals aren't as distorted as electrical
Detectors = SQUIDS =sensitive
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evoked MEG can be detected by EEG
(-) more expensive and restrictive than EEG
Positron Emission Tomography (PET)
PEt scanners -measure changes in blood flow stimulated by changes in neural activity
increases in blood flow measured by increase in conincidences at opposite directions in the PET scanner
if an isotope containing radioactive oxygen-injected into bloodstream -absorbed into the brain
H20(15)
sub atomic events= emission of photons in opposite directions
(+) reasonable spatial resolution and can image deep structures aswell as the cortex
(-) poor temporal resolution
Functional Magnetic Resonance Imaging (fMRI)
Best spatial resolution in humans
Each different in Spatial and Temporal resolution
all have the (+) able to detect activity over several locations and large distances -in the brain at the same time
all have (-) not recording directly from neurons
Behavioural Nuerophysiology
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electrodes inserted into or onto area of the brain
single unit recordings - record activity of individual neurons
recordings taken from single neurons in the monkey premotor cortex- clicking in the video is amplified of the single neuron- more clicking occurs as the neruons become more active
record the activity of the neurons directly using neurophysiology
Multi-unit recordings
recording summed activity from small pop of neurons
(+) important in animal studies for determining exactly where signal coming from -
Very high temporal resolution
Brain stimulation
Transcranial magnetic stimulation
TMS
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electrical current in TMS coil = local magnetic field
changing magnetic field generates local electrical currents in nearby brain tissue
temporarily disrupts normal activity in tissue beneath the coil
Delivers disruption for small time localised area of cortex
can target specific processes that occur in particular time points in experiments
more effective in the neocortex than subcortical
Deep Brain Stimulation
DBS
in the basal ganglia
collection of subcortical neuron clusters -form connections in the
parkinsons disease
debilitating disorder of the motor system and motor control
degeneration of the pathways within the BG connects the striatum and the substantia nigra
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studies through chemical lesions
to create the same pathology in moneys to test treatments for the disease
lesions in the sub thalamic nucleus alleviate the PD symptoms
Stimulation has many advantages over lesions:
reducing PD symptoms medication less effective
control returns in patient with Parkinsons when DBS is turned on
Allows reduced dosage of medication
symptoms caused by:
imbalance of excitatory and inhibition in the circuitry
DBS in structures can restore balance
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purpose
stimulate specific areas in BG to alleviate symptoms
Method:
Electrodes inserted into subcortical areas and connected to stimulators powered by batteries
mostly in the subthalamic nucleus of the Basal Ganglia but other
Rationale
several hypotheses but mechanisms not clear
stimulation can excite or inhibit neuronal activity
Translational science - understanding of anatomy and physiology allowed to work out areas to stimulate
stimulation in the STN and the GPi (globus pallidus) or striatum
(+) Brain activity can be recorded in locations connected to but distant from stimulation site -during stimulation
can be used to treat Parkinsons, Depression, Chronic pain, Tourettes and OCD
can these methods really study origins of uniquely human ability?
is this acceptable to consider science separate form ethical context?
can some questions of the brain only be assessed in non-humans
Can only really study at a brain surfaces and maps level in humans- perhaps optical dyes on the column level- ethically! -limited spatial resolution
MEG & ERP, FMRI PET, Lesion CAN BE APPLIED IN HUMANS
2nd row- TMS Optical dyes just below
3rd row: multi unit recording deoxyglucose , single unit recording
only in animal models -
controversy: task related signals occur in the brain
independent of neural activity
Sirotin & Das (2009)
independently measured metabolic variables - cerebral blood volume and oxygenation in primary visual cortex of awake monkeys
found two components to metabolic signal
neural responses of V1
anticipated the onset of the visual stimulus unrelated to any measurable neural activity
predictive timing - increases of cerebral blood volume in anticipation of trial onsets even in darkness
preparatory mechanism in the brain that brings additional aerterial blood to the cortex in anticipation of expected tasks
few studies - compared neuronal and haemodynamic signals in animals to test for assumed correspondance
Marshall and Fink, 2003
functional localisation studied
intact brains- temperature changes in specific areas when different tasks were performed
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gave way to PET studies
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FMRI too
multichannel cerebral thermometry (Broca, 1879)
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do the localisations observed in these new techniques find the same or similar as functional neuroimaging of cognition in healthy people?
propose that functional specilaisation is not as fixed within regions of the brain as supposed ?
GAll believed that localisation of functions like memory could be revealed through the craniology of the skull, ie those with good at memory tasks had protruding foreheads and large bulging eyes- said visual memory was located in the prefrontal cortex because of this.-
confirmed when a patient who received injury to the left frontal lobe gave him right sided paralysis and severe memory loss for words
could argue that injury on one hemisphere upset balance on both sides (finger,2000)
Passingham, Stephan & Kotter (2002)
functions of a cortical area determined by extrinsic connections and intrinsic properties
each cortico-cortical connections= connectional fingerprint- these underlie observed cell firing in different areas during different tasks
looked at through fMRI
proportion of cells which fire differs dependant on which task
