fmri – difficulties in mind reading

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fMRI – difficulties in mind reading. fMRI: functional (nuclear) magnetic resonance imaging Neuroimaging: get the structure of the brain Want to know how it works: connection brain parts and brain functions Aim: measure the local „thinking activity” Usage (criticism): Lie detector - PowerPoint PPT Presentation

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Page 1: fMRI  –  difficulties in  mind  reading
Page 2: fMRI  –  difficulties in  mind  reading

fMRI: functional (nuclear) magnetic resonance imaging

Neuroimaging: get the structure of the brainWant to know how it works: connection brain

parts and brain functionsAim: measure the local „thinking activity”

Usage (criticism):Lie detectorNeural- and psychological-modell checking

(think/know experiment)

Page 3: fMRI  –  difficulties in  mind  reading

How we use it

Human attempts frequent Well-planned tasks or questions Measures: order of minutes

One measure: order of 5 secondsMeasure with and without tasks or question,

further investigation based on the difference

Overlap the intensity map and brain image

Page 4: fMRI  –  difficulties in  mind  reading

The basics of fMRI

MRI: interaction between spins and magnetic field

QM based phenomenaClassical view is almost satisfactory

Find a „think-activity”-sensitive MRI measureable quantity, measure it, and then reflect to think activity

Page 5: fMRI  –  difficulties in  mind  reading

Get the signal - BOLD

Blood-oxygen-level dependence Hemoglobin: Fe2+ can absorb O2

Hemoglobin + O2 : diamagnetic molecule

Hemoglobin – O2 : paramagnetic (S=2)

Measure the oxygen-flow differencies in vein

Determine the connection between BOLD singal and toughts

Page 6: fMRI  –  difficulties in  mind  reading

Get the signal – O2 flow

The oxygen flow depends on the communicating intenstiy

Communication needs energyNeuron cells don’t have repositoryIncreased activity needs more energyBOLD signal decreasing and then

increasing

Page 7: fMRI  –  difficulties in  mind  reading
Page 8: fMRI  –  difficulties in  mind  reading

Time, accuracy and resolution Time: depends on the BOLD signal,

about less than half a minuteDo experiments with the same patient, same

time

Accuracy: easily detect maximum of BOLD

Space: resolution: in order of mm×mm×mmProblem: the motion of the patient

Page 9: fMRI  –  difficulties in  mind  reading

Increasing space resolution No new information with increasing resolution

Signal comes from multiple capillars Solution: BOLD signal minimum: more

localised Longer time or Bigger magnetic field

No (?) news: cerebral tasks are not well localised

Page 10: fMRI  –  difficulties in  mind  reading

Temporal resolution

Increasing time resolution does count (more details) usable image Two tricks to improve:

Spin echo – gradient echoEPI: echo planar imaging

x y z t const × × × »

Page 11: fMRI  –  difficulties in  mind  reading

Measurement: EOM

First: apply Short RF pulse:

( ) ( )0ddt = ´ + -M M B M M τ

( ) 01

zz z M MdMdt T-

= ´ +M B( ) { }/ ,2i iidM M T i x ydt = ´ - ÎM B

( ) ( ),, , , ,0 0 00 0 0 0zB M= Þ =B M( ) ( ) ( )( )sin ,cos ,1 0t B t t =B

Page 12: fMRI  –  difficulties in  mind  reading

Measurement: relaxation and decoherence Solutions

after„excitation”:

We can measure

Important: New rotating cylindrical coordinates with

frequency:

iM( )/ 10 1 t TzM M e-= -

/ /0 2 20 0 Li B t t T i t t TiM M e M e - -= × = ×

1 2 2 1T T T T¹ >

0L B =

Page 13: fMRI  –  difficulties in  mind  reading

Measurement: spin-echo

Page 14: fMRI  –  difficulties in  mind  reading

Measurement: spin-echo

Page 15: fMRI  –  difficulties in  mind  reading

Measurement: spin-echo

Page 16: fMRI  –  difficulties in  mind  reading

Measurement: slice-excitation

( )

( )( ) ( )

( )

*

*

,

000 1

zz

ii B G zii B i G zt

dM z tdt t eM B t e

+ ×

- ×

=

= ´ ×

=

M B( )

( )*

,

0 1 zr

i G z tdM z tdtM B t e - ××

=

= × ×

Page 17: fMRI  –  difficulties in  mind  reading

Measurement: slice-excitation and spin-echo Find for the desired excitation-

distribution:( )*1B t

( ) ( )* sin /1 1 zB t B x x x G d t= = ×

Page 18: fMRI  –  difficulties in  mind  reading

Measurement: another way, the gradient echo

Page 19: fMRI  –  difficulties in  mind  reading

Measurement: EPI

Echo Planar Imaging: increase time resolution even more!

Page 20: fMRI  –  difficulties in  mind  reading

What we get: images

Page 21: fMRI  –  difficulties in  mind  reading

An experiment: caffeine

caffeine as a contrast booster

Page 22: fMRI  –  difficulties in  mind  reading

Limits

We cannot answer any why question – only answer the question where

The brain is 3D – the image is 2D, so more experiments needed

We can measure only the neurons firing, but not the real actyvity ( block or stimulate?)

High degree of cerebral plasticity: places may vary

Page 23: fMRI  –  difficulties in  mind  reading

Mind reading The map of optic nerves to vissual cortex is

so „localised”, „continous” Training set: known videos and those fMRI

signal Unknown video: fMRI signal → video

images Ability to guess what patient think Numerical problem Further aim: movement of implants Success: camera-eye

Page 24: fMRI  –  difficulties in  mind  reading

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