Diffusion-weighted imaging – measures the direction of water movement by comparing responses to magnetic fields in different directions. Shows neural fibre bundles.
Functional MRI – measures how tissue magnetic interactions change over time. Examines the blood flow and oxygenation. Neural activity changes when subject is exposed to a stimulus.
MRI & fMRI advantages:
- High spatial resolution
- Straightforward analysis / interpretation
- Safe and non-invasive
- Easy access
Disadvantages:
- Indirect measure of neural activity
- Low signal to noise rations
- Awkward environment
- Poor temporal resolution
How does MRI work?
- Place brain in a strong static magnetic field
- Use weaker, changing magnetic field to change magnetization energy
- Remove gradient field
- Measure energy emitted and reconstruct into image
- fMRI: measure changes/distortions produced by magnetically-active substances
The signal depends on:
- PD: proton density
- T1: realignment with magnetic field
- T2: proton misalignment due to tissue interactions
- T2*: T2 and magnetic field inhomogeneities
Deoxygenated blood causes signal loss.
Why does fmri work?:
- Deoxyhemoblobin affects T2*
- Blood response follows neural activity
- Step 2 overcompensates
Important: effect 2 does not compensate accurately for effect 1
So oxyhemoglobin concentration increases due to increased blood flow.
Relation between neural activity and BOLD (black is neural activity):
[note: deze afbeelding uit het college is door de WorldSupporter redactie verwijderd wegens vermoedelijke inbreuk op het auteursrecht]
- MUA: action potentials (spikes) = neural output
LFP: synaptic activity = neural processing
fMRI is slightly better correlated with LFP. So BOLD signals reflect synaptic activity.
What limits the temporal resolution of fMRI?
- Slow blood flow changes
- NOT frequency of measurement
Perception: a translation of the physical environment into a pattern of neural activity that can be used by our brain to guide behavior. Perception is a set of tricks to extract useful (not an accurate!!) information from the environment.
The ganglion cells look at change in color: contrast.
Visual convergence:
We need to match the details of the image to the properties of the (visual) cell:
So the top cell responds well to the first image and the bottom cell responds well to the third image. So we use the differently sized receptive field cells to perceive different properties in images.
The images we see is mapped in the visual cortex:
Orientation-cells only respond to cells in a specific orientation. This is the relation between preferred orientation and track distance:
There are two visual pathways:
- Dorsal visual stream: where
- Ventral visual stream: what
V2 looks for patterns in the images it receives. V1 does not.
The middle stages of visual processing look at common patterns in the output from the first stage.
We use our imagination to use previously seen patterns and structures. So we use context from things we’ve seen before to map images.
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