Download fMRI: Biological Basis and Experiment Design

fMRI: Biological Basis and Experiment Design
Lecture 13: BOLD
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Neurons per voxel
Neural signaling
Neural/vascular link?
HRF
– linearity
1 light year = 5,913,000,000,000 miles?
BOLD relies on T2*-weighted images
Signal is low where T2* is
short.
Signal will increase where
blood flow increases.
Predicting BOLD signal in a single voxel
Neural activity = input + local computation + output
Cortico-cortical cxns
Intrinsic cxns
astrocyte
Excitatory neuron
Inhibitory neuron
Output: spikes
Thalamic input: spikes
Energy budgets
Lennie (2003) “The Cost of Cortical Computation”, Current Biology 13:493.
Attwell and Laughlin (2001) “An energy budget for signaling in the grey matter of
the brain,” J. Cer. Blood Flow & Metab. 21:1133.
• Stimulus
• Input
• Local computation
Molecules of ATP
Predicting BOLD signal, Part I: neural activity
• Output
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• Total
time (ms)
• Total Neural Activity
• Oxygen extraction
fraction
Molecules of ATP
BOLD signal, Part II: Hemodynamic response
• CBF
time (sec)
(ms)
• CBV
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• BOLD
Cannonical HIRF
Key assumptions of convolution model
• Linearity:
Homogeneity
+
Additivity
=
Superposition
• Shift-invariance: no refractory period
Modeling BOLD with convolution