BOLD Signal Response

“impulse” response

to a brief stimulus

brief initial “dip”

(HbO2 Hbr, local flow changes)

more sustained positive BOLD

response (larger scale flow changes,

excess of HbO2 created, reduction

in conc. of Hbr)

post-stimulus undershoot,

return to normal flow but

slow CBV recovery (giving

effective increase in [Hbr])

4s

8s

12s

time / secs

Signal Intensity

fMRI “dip”: Menon et al., MRM 33:453; Ernst & Hennig, MRM 32:146; Hu et al., MRM 37:877

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