Lecture 25 - Neuroimaging and Reward Intro Flashcards
Diffusion MRI for
structural connectivity visualisation
Diffusion MRI process
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
Rapidly repeated at all angles, instead of slow to get anatomical definition you go fast to get the hydrogen atoms to move in all different directions (want to see how and where they are moving so you do not get very good definition from diffusion MRI but you will get movement of water along tracts
Diffusion MRI measures
Measure anisotropy:
Degree to which the hydrogen ions travel along white fibre bundles
Measure of white fibre bundle cohesiveness/integrity i.e. how good that white matter bundle is
Diffusion MRI - Isotropic
= 0
Hydrogen ions are not constrained (e.g. CSF)
When not constrained by anything the ions are going to move all around in the head in many directions, therefore say they are isotope
Diffusion MRI - Anisotropic
=1
Hydrogen ions constrained (e.g. White fibre bundles)
Trying to move water along a white matter bundle, the hydrogen ions are going to very much constrained so when we go in a certain direction even though magnetic field is trying to make it go all around it is going to go in one direction therefore called anisotropic movement
Isotropic =
0
Anisotropic =
1
Diffusion MRI - Fractional anisotropy
(between 0 and 1)
Practical assessment of degree of white matter integrity: axon myelin, diameter and density
Certain degree of movement allowed in certain directions because the bundles are not completely compact all the time, so get a fraction, the fraction is affected by the axon myelin, diameter and density
Fractional anisotropy =
between 0 and 1
Application of Diffusion MRI
looking at the reward system and its networks
Reward system define
Reward system: a group of interconnected brain structures involved in motivation (desire, motivation, craving), associative learning (operant reinforcement (learning something that you did associates with some kind of outcome that makes you want to do this thing more or less) and positive conditioning (associate the reward with the action being done e.g. teaching a dog to sit with a treat)) and in emotions, particularly those associated with pleasure
Reward system
look at poster
Reward system circuit
Prefrontal cortex is a complex structure in the front of the brain that helps us decide the kind of things we might want to aim towards obtaining (motivation) and perceive something from memory we have experience before and start wanting to work our actions towards that
Nucleus accumbens is part of the basal ganglia which is small but a central component to this reward system
Ventral tegmental area in the midbrain is one area where the cells that make dopamine live and dopamine is a pleasure chemical that reinforces/motivates and tells the brain that we have experienced something that is potentially pleasurable that we may want to do again that is rewarding and reinforcing and these are connected together the VTA delivers the dopamine to the prefrontal cortex and to the nucleus accumbens and other areas as well and this part of the sopamine projection is called the mesolimbic (medial surface of brain)
Prefrontal cortex actually makes its major projects ions onto the nucleus accumbens (red arrow is green on recording), anything that is being remembered/desired/planned about is informing the nucleus accumbens so it is an input pathway if you want to the reward system
Nucleus accumbens then has a projection out (not anatomically correct on this diagram) to the ventral pallidum (globes pallidus is part of the lent information nucleus and this is the ventral part of it) and from here it goes to the thalamus and from the thalamus it goes back to the cortex and back to areas that are often closely aligned to the prefrontal areas where the activity actually started
This is a loop that is responsive for motivation, positive emotions, associative learning (learning about the value of the stimulus)
This is the circuit that is known to be disrupted in addiction
Using diffusion MRI to measure the integrity of a fibre bundle - Diffusion Tensor Imaging (DTI)
Map the fractional anisotropy (FA) of a fibre bundle
- Compare FA values (0 = fully isotropic and 1 = fully anisotropic) between individuals and
groups; FA increases during adolescence but at different rates between people ( not all developing at the same rate, as you are getting older, your white matter tracts are getting more myelinated and therefore assume they are getting more organised and constrained therefore an increase in FA as the individual grows)
For reward network specifically lock at connections between nucleus accumbens and ventral tegmental area
Using diffusion MRI to measure the integrity of a fibre bundle - Diffusion Tensor Imaging (DTI) - Study looking at addiction-free 14 to 15 year old boys in Oregon with a 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 (cohort study type)
Association between pre-existing individual differences in FA and the onset of binge drinking in adolescents - NOT necessarily causal (correlation rather than causal)
The greater the FA, the longer it took them to develop a drinking habit
The less organisation of the fibre bundle the more likely they were to develop binge drinking habits earlier in their lives
Adolescence with lower FA in the medial orbital gyrus white matter display greater activation of nucleus accumbens during decision making involving risk and reward
Can map the FA onto a scan, brighter the tracts are the greater the FA I.e. more organised, impressive fibre connection between frontal areas here and the down in the areas posteriorly that are related to the reward system
Smaller FA, less organised WM, start binge drinking earlier - greater NAcc activation - less regulated reward /risk response
Less FA = less organised white matter
Frontal areas are linked in behaviour regulation therefore if you are not connecting to them and have less white matter connection then you are more likely to show risky behaviour
