Lecture 25- Introduction to the Reward System

Question 1

Which type of MRI- T1 or T2 is better at showing a tumour (acoustic neuroma)?

Answer 1

-In the T2 image the tumour contains ‘oedema’ fluid (=inflammation) which
demarcates tumour from tissue BUT does not distinguish tumour from
CSF easily - (to do this need to do ‘T2-FLAIR’)

Question 2

What can different MRI Sequences be used for?

Answer 2

To show different tissue contrasts.

Question 3

What is a T2 flair?

Answer 3

FLUID ATTENUATION INVERSION RECOVERY
- Normal T2 does not distinguish CSF from oedema of inflammation (e.g. from a tumour) easily.

-Need to do a ‘T2-
FLAIR’ sequence where CSF is DARK because it is free flowing but non free-flowing fluid like oedema is bright!

Question 4

What is Diffusion MRI?

Answer 4

-In addition to the general principles of MRI imaging: In Diffusion MRI local magnetic fields applied to the head at
different angles
-Rapid repeated scans. Rapid fast scans are not good for definition but they are good for informing how water is moving along tracts.

Question 5

What is the term for what diffusion MRI measures?

Answer 5

Anisotropy:

-Degree to which the hydrogen ions travel along white fibre bundles

-Measure of white fibre bundle cohesiveness or integrity.

Question 6

What is the difference between Isotropic (= 0), Anisotropic (= 1), and Fractional anisotropy as measured by diffusion MRI?

Answer 6

-Isotropic (= 0). Hydrogen Ions are not constrained (e.g. CSF)
-Anisotropic (= 1). Hydrogen Ions Constrained (e.g. White fibre bundles)
-Fractional anisotropy (between 0 and 1). Practical assessment of degree of white matter integrity: axon myelin, diameter and density

Question 7

What is the reward system in a general sense?

Answer 7

Reward system: a group of interconnected brain structures involved in motivation (desire, motivation, craving), associative learning (operant reinforcement and
positive conditioning) and in emotions, particularly those associated with pleasure.

Question 8

What is a known condition/ state that disrupts the reward system?

Answer 8

Addiction

Question 9

What areas are invovled in the reward system and how?

Answer 9

-Prefrontal cortex= complex, helps us to understand what we working towards, pull information from memory, planning etc.

-This has a major projection to the nucleus accumbens.

-The VTA: Ventral Tegmental Area (small area in midbrain that contains dopamine neurons) also feeds into the nucleus accumbens as well as the prefrontal cortex to tell us that we have experienced something pleasurable.

-The nucleus accumbens then projects to the ventral globus pallidus

-This then outputs to the thalamus and cortex (back to areas closely aligned with start: hence loop!)

Question 10

What system is dopamine an integral part of and what does this name mean?

Answer 10

-Mesolimbic: meso means midbrain, limbic refers to being invovled in emotions

Question 11

What is diffusion tensor imaging/ what does it indicate?

Answer 11

-Used to measure the integrity of a fibre
bundle
-Map the fractional anisotropy (FA) of a fibre bundle
- Compare FA values (0 = fully isotropic and 1 = fully anisotropic) between individuals and
groups to determine integrity of white matter tracts: are there any trends associated with certain characteristics???

Question 12

Does FA increase in adolescence?

Answer 12

Yes, but at different rates in different people. In general this trend makes sense though as the the tracts get more organised/ constrained as we grow.

Question 13

What did the diffusion tensor image study show about FA in addiction-free 14 to 15 year old boys in Oregon?

Answer 13

-These boys had parental history of adolescent binge drinking

-Looked at integrity of white matter between prefrontal cortex, ventral pallidum, VTA and nucleus accumbens (as reward centres) and duration to begin binge drinking (followed to 21 y.o. with three-monthly questionnaires ….. study type?)

-Association between pre-existing individual differences in FA and the onset of binge drinking in adolescents - NOT necessarily causal

-Specifically, Smaller FA, less organised WM, start binge drinking earlier -
greater NAcc activation - less regulated reward /risk response

Question 14

What is diffusion Tractography?

Answer 14

-3D representation of fractional anisotropy: build and identify white matter tracts based on the different directionality FAs
-Do for Long projection fibres and Short projection fibres means you build up a 3D model of fiber tracts in the brain

Question 15

What is functional T2* weighed image?

Answer 15

-In addition to the general principles of T2 MRI imaging: Adjusted scan parameters to be sensitive to
Deoxygenated hemoglobin

-Rapid repeated scans - measure “Blood-Oxygen-Level-Dependent” signal = BOLD signal

Question 16

Draw the flow chart on slide 15 that shows how the BOLD signal in a T2* image is produced…

Answer 16

Answers on slide

Question 17

What is a task functional MRI? What is an applied example of this method?

Answer 17

-Participants perform a task while in the scanner - T2* brain activity mapped (task correspond to increase in brain activity in particular area)

Applied:
-Had to respond as quickly as possible to light flash knowing that the button
press would or would not give reward, depending on the face cue
-Compare the fMRI scans in anticipation of no reward from reward
-Scans show Greater activation to reward anticipation in nucleus accumbens, midbrain dopamine areas [rewarding stimulus processing],
caudate and putamen [action related to reward]
(also parts of the frontal gyrus and cerebellum). Showing that these areas are invovled in the reward system/ pathway!

Question 18

What is the application Neurosynth used for?

Answer 18

-Mapped all the activity from lots of studies

-Show what areas are activated on average during reward/ different functions

-learning tool!

Question 19

What is a resting functional MRI scan?

Answer 19

-Participants rest in scanner without thinking about anything for 10 min – default or resting brain T2* activity mapped

-Map activity during resting, and correlate which brain regions are active at the same time = functional networks

Question 20

To sum up what are the advantages of MRI?

Answer 20

-Detailed anatomical structure
-Detect pathology with anatomical precision
-Measure white matter integrity
-Ability to assess brain function and functional connectivity at rest
and during tasks
-Doesn’t involve radiation, relatively safe and individuals can be
repeatedly scanned

-Compared to CT scanning: CT is anatomically poor contrast, cannot
measure white matter integrity, cannot give functional measure

-Compared to PET scanning: PET is anatomically poor, less available;
MRI does not need a radioactive tracer injected

Question 21

To sum up what are the disadvantages of MRI?

Answer 21

-Susceptibility to magnetic interference

-Can result in poor signal quality

-Can be a patient safety issue (e.g. metal in eyes/body)

-Requires significant technical expertise due to the physics, computation and statistical processing
(particularly diffusion and functional imaging)

-Diffusion and functional imaging are approximate, but are not a direct measure of, white matter/functional brain activity, respectively

-Compared to CT scanning: cannot visualise bone (marrow only for T1) or acute tissue bleeding; relatively expensive, longer procedure, can be frightening due to confinement

-Compared to PET: much more difficult to accurately
quantify metabolic or molecular processes