Fundamentals of Neuroscience Neuroimaging in Cognitive Neuroscience

transcript

Fundamentals of Neuroscience

Neuroimaging in Cognitive Neuroscience

James DanckertPAS 4040 jdancker@watarts.ca

Functional Neuroimaging

• Electrical activity– Event-related potentials (ERP), visual evoked potentials

(VEP) all derivative from EEG

• Stimulation– Trans-cranial magnetic stimulation – single vs. rapid

pulse TMS

• Metabolism– Positron Emission Tomography (PET) and Blood

Oxygenated Level Dependent (BOLD) functional MRI (fMRI)

EEG• Large populations of

neurons firing produce electrical potentials that can be measured at the scalp

• Signals are passively conducted through the skull and scalp and can be amplified and measured

• Difference between reference (ground) and recording electrodes are measured to give the electrical potential – electroencephalogram (EEG)

ERPs and VEPs• EEG tends to record global brain activity• ERPs (and VEPs) are a special case of EEG• average EEG trace from a large number of trials • align signal to onset of a stimulus or response – hence event-

related potential (ERP)

Pros and cons of ERPs.

• Good temporal resolution• Linked to specific physiological markers (e.g.,

N1, P3 etc. which in turn can be linked to known cognitive processes)

• Poor spatial resolution• Difficult to get at some brain regions (OFC,

temporal cortex)

Transcranial Magnetic Stimulation (TMS)

• Thompson (1910) placed head between two coils and stimulated at ~ 42 Hz• saw flashing lights – magnetophosphenes• was probably stimulating the retina and not the visual cortex

Cowey and Walsh, 2001

• TMS applies a magnetic pulse to a certain brain region to temporarily modulate the function of that region

• the induced current in the tissue is in the opposite direction to that of the coil

• the intensity of the signal drops off towards the centre and outside of the coil

circular coil induced current

• the flow of the current must cross the axon to cause stimulation or interruption of function (N3 will not be stimulated)

maximum depolarization

maximum hyperpolarization

little or no change

Spatial extent of TMS• spatial extent of induced electric field

– drops ~ 75% within 10 mm– affects 600 mm2 of neural tissue

Rapid vs. Single Pulse TMS

• for single pulse TMS duration of stimulation = 1 msec, but affects motor cortex for up to 100 msec

• for rapid or repetitive pulse TMS stimuli are delivered in trains with frequencies from 1 to 25 Hz (1 – 25 times per second)

• duration of after-effects for rapid pulse TMS anywhere from msec to several seconds

• excitatory or inhibitory reversible effects depending on site and parameters of stimulation (e.g. frequency of pulses) -> facilitates or slows down cognitive process/behavior

• when inhibitory, referred to as ‘virtual lesion technique’

• can give precise timing information (msec level) due to transient nature of effects

• rTMS is beginning to be used as a treatment for depression (focus is on DLPFC)

Transcranial magnetic stimulation (TMS / rTMS)

• Poor spatial localisation – how focal is the stimulation?

• Can’t stimulate certain areas (e.g., temporal lobe) and can only stimulate cortical surface

• Good temporal resolution• Can presumably disrupt individual processes

within a task.• Distance effects – changed interactions due

to stimulation• Can induce seizures (particularly rTMS)

Frameless stereotaxy and fMRI

• areas can be identified functionally and then used to position the coil in a TMS study using the frameless stereotaxy method

• Paus is attempting to directly combine fMRI and TMS – with TMS pulses delivered in between fMRI runs

Metabolic Imaging

• Two main techniques – positron emission tomography (PET) and functional MRI (fMRI)

• Activity in cells requires energy (oxygen and glucose)

• Increased neural activity will lead to changes in cerebral blood volume (CBV), cerebral blood flow (CBF) and the rate of metabolism of glucose and oxygen (CRMGl and CRMO)

• These changes in blood flow and metabolism can be measured using PET and fMRI

Positron Emission Tomography (PET)

• Measures local changes in cerebral blood flow (CBF) or volume and can also be used to trace certain neurotransmitters (but can only do one of these at a time)

• Radioactive isotopes are used as tracers

• The isotopes rapidly decay emitting positrons

• When the positrons collide with electrons two photons (or gamma rays) are emitted

• The two photons travel in opposite directions allowing the location of the collision to be determined

Positron Emission Tomography (PET)

PET and subtraction• Run two conditions

– stimulation (e.g., look at visual images) vs. control (e.g., look at blank screen)

• Measure the difference in activation between the two images (i.e., subtract control from stimulation)

• This provides a picture of regional cerebral blood flow relative to visual stimulation.

Motion vs. colour.• Subject views

coloured screen (left) vs. moving random black and white dots (right)

• Both task activate early visual areas (V1 and V2)

• Subtracting the two images reveals different brain areas for colour (V4) vs. motion (V5) processing

PET vs. fMRI• PET allows you to track multiple metabolic

processes so long as the emitted photon can be detected – allows imaging of some neurotransmitters

• PET is invasive – radioactive isotopes can only be administered (at experimental levels) every 4 – 5 years

• fMRI has much greater spatial resolution (~ mms)

• fMRI has greater temporal resolution – can detect activation to stimuli appearing for less than a second (PET is limited by the half life of the isotope used)

Magnet safety

• very strong magnetic fields – even large and heavy objects can ‘fly’ into the magnet bore

Cerebral blood supply.• Arterioles

– Y=95% at rest.– Y=100% during activation.– 25 m diameter.– <15% blood volume of cortical

tissue.• Venules

– Y=60% at rest.– Y=90% during activation.– 25-50 m diameter.– 40% blood volume of cortical

tissue.• Red blood cell

– 6 m wide and 1-2 m thick.– Delivers O2 in form of

oxyhemoglobin.

• Capillaries– Y=80% at rest.– Y=90% during activation.– 8 m diameter.– 40% blood volume of cortical

tissue.

– Primary site of O2 exchange with tissue.

Artery Vein

Arterioles Veneoles

Capillaries

1 - 2 cm

Neurons

Transit Time = 2-3 s

Cerebral blood supply.

• Deoxyhaemoglobin is paramagnetic• When neural activity increases more oxygenated

blood than is needed is delivered to the site• This leads to an imbalance in oxyhaemoglobin

and deoxyhaemoglobin – more oxy than deoxy• fMRI is able to measure this difference due to the

different magnetic properties of oxy and deoxyhaemoglobin

fMRI and BOLD

• blood oxygenated level dependent (BOLD) signal is actually a complex combination of:– rate of glucose

and oxygen metabolism

– CBV– CBF

• same subtraction logic used in PET is used in fMRI

fMRI – block design• fMRI (like PET) began examining brain activity using block designs

colours

motion

restrestrest

colours

fMRI – event-related design

• allows randomization of stimuli (not possible in PET)

fMRI – event-related design• BOLD response has a predictable form

• In rapid event-related designs the signal to a given trial type is deconvolved using models of the BOLD response

Linearity of BOLD responseDale & Buckner, 1997

Linearity:“Do things really add up?”

red = 2 - 1

green = 3 - 2

Sync each trial response to start of trial

Not quite linear but good enough !

Fixed vs. Random Intervals If trials are jittered, ITI power

Source: Burock et al., 1998

fMRI spatial resolution• images can be co-registered to the subject’s own brain

(not an average brain as in PET)

PET fMRI

fMRI and topologies• Using fMRI to “map” different brain functions

Penfield’s maps

Servos et al., 1998red = wrist; orange = shoulder

EXPANDING RINGS

Retintopy

• 8 Hz flicker (checks reverse contrast 8X/sec)• good stimulus for driving visual areas• subjects must maintain fixation (on red dot)

Source: Jody Culham

ROTATING WEDGES

Source: Jody Culham

time = 0

time = 20 sec

time = 60 sec

time = 40 sec

0 20 40 60

TIME STIMULUS

EXPECTED RESPONSE PROFILE OF AREA RESPONDING TO STIMULUS To analyze

retinotopic data:

Analyze the data with a set of functions with the same profile but different phase offsets.

For any voxels that show a significant response to any of the functions, color code the activation by the phase offset that yielded maximum activation (e.g., maximum response to foveal stimulus = red, maximum response to peripheral stimulus = pink)

Retintopy: Eccentricity

calcarinesulcus

left occipitallobe

right occipitallobe

• foveal area represented at occipital pole• peripheral regions represented more anteriorly

Retinotopy

Source: Sereno et al., 1995

Other Sensory “-topies”

Audition: Tonotopy

cochlea

Sylvian fissure

temporal lobe

Saccadotopy

Source: Sereno et al., 2001

•delayed saccades

•move saccadic target systematically around the clock

http://kamares.ucsd.edu/~sereno/LIP/both-closeup+stim.mpgMarty Sereno’s web page

Finding the human homologue of monkey area X!

• recent research has used monkey neurophys to guide fMRI in humans

Dukelow et al. 2001

Problems with the search for homologues

• Absence of activation doesn’t mean the absence of function• Presence of activation doesn’t imply sole locus of function• But our brains are different!• Confirmatory hypotheses

Dukelow et al. 2001

fMRI and diagnosis

• fMRI is starting to be used in patients with epilepsy

• one goal is to use this as a tool to localise language, memory etc. prior to surgery

• another goal would be to use fMRI to study the propogation of seizures

• in stroke patients fMRI can be used to chart recovery of function

Patient SP – congenital porencephalic cystPatient SP – congenital porencephalic cyst

Left Right

im a g e s

1 2 1 4 1

SP - motor stripSP - motor strip

sequential tapping

alternating tapping

motor strip

im a g e s

1 2 1 4 1

SP – somatosensory stripSP – somatosensory strip

sequential tapping

alternating tapping

somatosensorystrip

-2.0% s

1 21 41images

eimages

sequential tapping

alternating tapping

superiorparietal

images1 21 41

name animals

name objects

non-word sounds (‘ba’)

Broca’s area?

images1 21 41

name animals

name objects

non-word sounds (‘ba’)

left occipital

fMRI and cognition

• What not to do – poorly designed tasks!

• What is the right inferior parietal lobe’s contribution to movement control?

– spatial component of movements

– compare imagined movements with only a spatial component vs. movements with a sequential component

spatial only complex sequence

Bilateral FEFBilateral FEF

1 21 41 61 81 101

im a g e s

Supplementary Motor Area (SMA)Supplementary Motor Area (SMA)

1 21 41 61 81 101

im a g e s

Bilateral superior parietalBilateral superior parietal

1 21 41 61 81 101

im a g e s

Inferior parietal cortexInferior parietal cortex

Left Right

alternation only

complex sequence only

Design Problems

Boy, you must be rich then!!!

• Why is right parietal more active for less difficult tasks then?

• So did you just test task difficulty?

• Were the tasks really different in the intended way?

• What could the right vs. left parietal difference be due to?

– Attention? – possibly!– Differences in eye movements? – maybe!

– Perhaps – both tasks were spatial in nature and both tasks had a sequential component, so…

– Maybe, what of it?

– I don’t know and I don’t care, piss off! I’m gonna start again!

Confirming modularity

• Nancy Kanwisher and the parahippocampal recliner region!

Is that all there is to it?• Alex Martin and co. suggest that the FFA responds

to other kinds of objects too• Isabelle Gauthier and co. suggest that it is

expertise with faces which drives the activation

Exploring behaviours

• Prism adaptation ameliorates neglect – how?

• First, explore the direct effects of prism adaptation in the healthy brain.

• Clower et al 1996 used PET to do this but reversed the direction of prismatic shift every 5 trials.

Prism Adaptation – Rossetti and colleagues

• prisms shift world further to the right (into the patient’s ‘good’ field)

• patient’s movements compensate for the prismatic shift – in the opposite direction

• after effects lead to better processing of previously neglected stimuli

Setup.Setup.

head coil

arm brace

scanner bed

fixationtargets

rightward shifting prism

flexible arm(mounted to scanner

bed on left side)

Subject moved prism in front of right eye (left eye was patched) prior to prisms run and moved the prism to the side at the end of the run.

Protocol I.Protocol I.

11.5 sec

0.5 sec

5 runs with prisms (50 trials)

5 runs without prisms (50)

Protocol II.Protocol II.

fixation target

post-target fixation

2 sec volumes – so 2 sec for critical stimulus (the target) and 12 secfor post stimulus return to baseline (a la Bandettini).

Protocol III.Protocol III.

4T scanner at Robarts

17 pseudo-axial slices

5 mm thick

TR 2 sec

2-shot EPI sequence

Co-registered to 128 sliceanatomical

Adapting in the magnet.Adapting in the magnet.

PREPOST

-50 -40 -30 -20 -10 0 10 20 30 40 50

group meangroup mean

mean shift = 27.73 mm

Finding ROIs.Finding ROIs.

right sup parleft sup par

med frontal (SMA)

Modeling the peak activation across Modeling the peak activation across trials.trials.

Linear

Logarithmic

Left and right superior parietal Left and right superior parietal cortices.cortices.

1 2 3 4 5 6 7 8 9 10

r2=0.13

1 2 3 4 5 6 7 8 9 10

r2=0.15

Left Superior Parietal

Right Superior Parietal

Cerebellar ROIs.Cerebellar ROIs.

1 2 3 4 5 6 7 8 9 10

r2=0.14

Medial Cerebellum

Right Lateral Cerebellum

1 2 3 4 5 6 7 8 9 10

r2=0.42

SMA.SMA.

transverse sagittal

0.40.50.60.70.80.911.11.2

1 2 3 4 5 6 7 8 9 10

r2 = 0.37

Bottom line?Bottom line?

• Difficult to image the direct effects of adaptation in normals.

• Image “good adaptors” OR change protocol to look at after effects of adaptation – with all its problems…

Conclusions?• fMRI should be used for good and not evil!

I wonder if fMRI could be used to

cure cancer?

Acknowledgements

• fMRI of epilepsy patient– Stacey Danckert– Seyed Mirsitari – David Carey– Mel Goodale– Ravi Menon– Jody Culham

• fMRI of prism adaptation– Susanne Ferber – Stacey Danckert– Mel Goodale– Yves Rossetti

• fMRI of imagined movements– it was all my fault!

End of Lecture

Fundamentals of Neuroscience Neuroimaging in Cognitive Neuroscience

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