lecture 11 - neuroimaging and map plasticity in humans Flashcards
(21 cards)
Neuroimaging methods -
DIAGNOSTIC NEUROIMAGING - CT
- X-ray computed tomography
(CT) - 3D image generated by
multiple 2D scans - BUT…
- Uses radiation (x rays)
- Total exposure limits longitudinal
(follow-up) scanning
DIAGNOSTIC NEUROIMAGING - MRI
- Uses MAGNETISM to image
body structure. - Patient placed within large
STATIC magnet. - Smaller magnets change fields in
different directions at right-angles
to one another
DIAGNOSTIC NEUROIMAGING - MRI
Different ‘contrasts’
provide different kinds of
information
* These are used to build
up a fuller picture of the
area to be investigated.
* No one contrast is best
for all injuries!
‘T1’ Image
‘T2’ Image
‘Proton Density’ Image
photic stimulation
MRI image showing the activation of the visual cortex
from belliveau et al science nov 1991
Blood Oxygenation Level Dependent:
BOLD Contrast
Endogenous Contrast Mechanisms
Brain: 2% body weight,
25% body glucose use
arterioles| capillary bed | venules
flow –>. rest
arterioles | capillary bed | venues
Increased flow —> ‘active’
*Increased local flow
*Increase : ratio
*Increased MR signal
*
‘FUNCTIONAL’ MRI: FMRI
A classic experiment: show people
visual stimuli and look for ‘activation’
in the visual areas of the brain.
Flickering Checkerboard
OFF (60 s) - ON (60 s) -OFF (60 s) - ON (60 s) - OFF (60 s)
MRI/FMRI BASICS: SUMMARY
- Brains contain high concentrations of hydrogen nuclei (in water).
- Protons (hydrogen nuclei) will orient their magnetic fields when placed in
external magnetic field, and rotate (precess). - Arrangement is disrupted when radio-frequency waves are applied.
- As protons return to equilibrium, they emit signal which is detected by
MR scanner. - Oxygenated blood has different magnetic properties to
deoxygenated blood. - A special MR contrast (BOLD) holds information on the relative ratios of
deoxygenated blood: oxygenated blood. - Due to the unique nature of neuronal metabolism, BOLD contrast can be
used to track which neurons are ‘working harder’, and are thus involved
in certain kinds of experimental tasks
MAGNETOENCEPHALOGRAPHY
- The current generated by a single neuron is far too weak to be
detected by MEG or EEG; indeed, only the locally synchronous firing
of millions of neurons—approximately 40 mm2 of the cortical sheet—
can generate a measurable signal (Hämäläinen and Hari, 2002). - Magnetic fields pass transparently through the head. Conductivity
parameters not so important. - A source of activity in the brain will lead to magnetic field
components - and MEG is able to detect these signal sources
MAGNETOENCEPHALOGRPAHY - THE LAB
advs
- its silent so good for studying audio and languages
- recording a direct neuronal signal
- we have a measurable change in electric or magnetic field produced by neurones in the brain
disadv
- very sensitive and because the magnetic and electric fields from brain are so tiny it has to be in a heavily shielded room - made of aluminium and mu metal which is an impressive dampner for magnetic and electric fields
- so sensitive even if you jump up and down because of earths magnetic field it sets the sensors off
- need to make sure electromagnetic miasma from all over devices doesn’t effect the signal
WHICH PARTS OF THE BRAIN
ARE ACCESSIBLE TO MEG?
A quantitative assessment of the sensitivity of whole-head MEG to
activity in the adult human cortex.
Hillebrand A, Barnes GR.
Neuroimage. 2002 Jul;16(3 Pt 1):638-50.
MEG is supersensitive to the cortex and sensitivity drops off as you go deeper - disadv whereas fMRI is equally sensitive everywhere - things need to be large to detect them so if deep in cortex like subcortical nuclei meg doesnt detect
we can use fMRI or MEG to study cortical maps - to see if things are changing overtime or not
NEUROIMAGING – A TOOL
TO STUDY CORTICAL MAPS
IN HUMANS - CENTRAL SOMATOTOPICAL ORGANIZATION
cortical enlargement - where the size of the areas of the body within that early map are relative to the density of receptors not to the physical size of the skin surface
from non-human primate recordings we got the highly detailed map - in notes
a brief history of vibration
studying the cortical Map in a scanner can be difficult
if want to study skin surface use similar tech to physiological vibrators people use to contract their muscles but they won’t last long in an MRI so a different way is needed
‘Ol’ Faithful’ - this uses the fact that if you put something in a magnetic field and you feed an alternating current through it you get a force produced so produces a little vibration = the motor effect - can’t do those same experiments outside the scanner as no static magnetic field. also expensive
‘Ol’ Faithful’
1996-1998
‘The Claw’ 1998-
2000 - uses air puffs - but can only go so fast because of resistance within the tube
‘VibeMaster I’
2005->
CM3 – Mark IV
2010 ->
Time (yrs)
most people today use pixel electric vibrators - has pixel electric pixel in it so when theres vibrations it turns it into a signal and attach a probe to it you can give a highly timed and synchronised tactile stimulation - very little metal in them so good for MRI
can you show static organisation of a digit map with S1 in humans?
yes but can be very variable issue as to study a change you have to work out a static baseline
Does Activity in S1 Reflect Objective Stimulus
Delivery – OR Subjective Perception?
what is S1 driving in terms of our subjective experience?
the cutaneous rabbit illusion affects human primary sensory cortex somatotopically - Blankenburg et al., 2006
- two cutaneous vibrators are placed on the skin so you stimulate a train in one by tapping the arm and then the other and if the time between the end of the first train and beginning the second is short enough a significant percentage of us will feel as though there is a tapping that goes between them - between stimulation one area and stimulation two
images in notes
the illusion is feeling being touched even though we are not
so if you Map each of these locations within S1in this case using fMRI - we can see if the activation in S1 is driving that illusion
vendical rabbit condition - you are stimulated in all 3 locations
illusory rabbit - only P1 and P3 are tapped and if short enough gap in-between you feel P2 being tapped
control - no illusion buts its matching fr the amount of stimulation thats actually applied
they found when you felt the illusion there was a activation in the cortical map even though there was no afferent signal driving it
activation in these maps seems to drive our own subjective perceptions as well as being a reflection of the external world. this suggests that when you touch the fingers of the non-human primates who have lost them that its likely they experienced the phantom finger because its not a silent zone anymore.
can’t research this in non-human primates particularly in these types of mapping experiments
FUNCTIONAL
SPECIALISATIONS?
a lot of early visual experiments started off with simple stimuli and then moved on to try and look at gratings of different kinds of widths
A Tale of Tactile Imagery…
* JVP Domes (Johnson van boden and Philips) – used to assess tactile
spatial resolution on their fingertips- simple objects and spacing between domes changes - looks at tactile discrimination as a whole
- if spacing is wide its easier to tell if its placed along or across your finger than when its neater and closer together
* Analogous to grating discrimination in
vision
* Domes placed either ‘along’ or ‘across’
glabrous fingertip
* Less prone to bias than 2 point
discrimination - where you get callipers or a compass and you change the distance between it and you ask if people feel two points or one points - on your fingertips they can a be lot closer than on other regions of your body and in a way you end up with a map of receptor density if you did it all over but issue with two points is its very difficult to put both points on at the same time so your no longer just measuring spatial discrimination, how fine tuned the receptors are in space but also temporal as there is an offset between one of the points going down and the other so you dont know what kind of data your getting back so thats why the domes are better as you place them down and all points are hitting at the same time.
Van Boven, et al., 1994
image in notes of JVP domes
FUNCTIONAL SPECIALISATION?
-Sathian et al. 1997 ‘Feeling With
the Mind’s Eye’ - used the domes
PET = non-invasive neuroimaging, it is radioactive
* PET Study contrasting spatial
frequency discrimination vs.
orientation discrimination of
domes.
- he wanted to see if different areas of the brain where involved in frequency discrimination of touch (the ability to look at different frequencies of those patterns, is it closer together or further apart) and orientation discrimination (can you feel which way is orientated)
- he recorded people in the PET scanner recording their brain data and getting them to do the task
- these where early days in mapping functional specialisation of the amount of sensory cortex in humans
findings - FUNCTIONAL
SPECIALIZATION?
disappointed with findings as expected to find different activation possibly in second somatosensory cortex or in parietal cortex which would reflect the two different systems
* NO differential activity in
somatosensory areas.
* ONLY significant ‘blob’ in
V6/PO (between parietal
and occipital cortex). - at this point area barely studied in non-human primates but it seemed to be involved in the visual discrimination of gray things but bizarre to be involved in tactile discrimination of grey things as looks like a higher visual area and is miles away from other regions looked at
* Previously active in visual
discrimination of gratings.
IS THE V6/PO ACTIVATION RELATED IN A
CAUSAL FASHION TO TACTILE ORIENTATION
DISCRIMINATION?
started to move away from neuroimaging and look at a causal way to investigate the functioning of this area and used an early but widley used neurostimulation method called TMS - transcranial magnetic stimulation
- Use TMS to investigate! - set the domes up in a good way as when you give a zap with TMS need to see exact time it happens
- set up where through an electrical signal to a solenoid the grating would come down on the finger and the person would have to do the task - did that feel like the grated edges were closer together than before and which way its orientated or did you feel it —>
- Experimental Design
- Grating Orientation
Discrimination - Suprathreshold tactile
stimulation (detection) - Spatial Frequency
Discrimination - Varying TMS-Stimulus timing
delays used.
Zangaladze et al., 1999
TMS RESULTS
A and B replications (A,
N=11, B, N=6); task
ORIENTATION DISCRIM to
right D2.
* Maximum results at 180ms
after stimulus application
(similar to when EEG activity
peaks over V6 area.)
* SPATIALLY SPECIFIC
EFFECTS: only on left/midline
sites near PET focus.
on graph
air = control so TMS coil is fired into the air
measured at 10ms, 180ms and 400ms after the dome had been dropped
10ms is a slightly better control as still getting peripheral effects of the TMS but 10ms is way to short to influence that kind of processing. looks like normal
at 180ms which is roughly the amount of time according to EEG it takes for information to get there ability to do task is better when using TMS over V6 area and its only when you do it at these locations that are around that locus that they found on their imaging that that actually works
a nice demonstration of combing neuroimaging and neurostimulation
its temporally and spatially specific
beforehand this area had only been implicated in visual discrimination is also involved in the tactile processing of them
people often have eyes shut when thinking of orientation so the area may being subcontracted out by tactile system to do a particular form of neuronal computation thats very good at taking the tactile input and transforming it into visual analogue to help solve the orientation task
suggests that there are areas beyond classical somatosensory and tactile cortex that will influence processing in these areas
TMS Results 2
TMS effect is specific to only the grating orientation task – no effect of TMS near PET
blob on either tactile detection or grating spacing/spatial frequency discrimination.
* Evidence for the role of ‘non classical’ areas in somatosensory discrimination…that
involves visual imagery components…
