( ( ( ( Dilating or ) constricting ) ( blood vessels ) ) ( usually ( ( is ( ( one cause ) ( of ( migraine headaches ) ) ) ) . ) ) )
(ROOT (S (S (VP (VBG Dilating) (CC or) (VBG constricting) (NP (NN blood) (NNS vessels)))) (ADVP (RB usually)) (VP (VBZ is) (NP (NP (CD one) (NN cause)) (PP (IN of) (NP (JJ migraine) (NNS headaches))))) (. .)))
Dilating or constricting blood vessels usually is one cause of migraine headaches.
(ROOT (S (SBAR (WHADVP (WRB When)) (S (NP (NP (NN blood) (NNS vessels)) (PP (IN in) (NP (DT the) (NN head)))) (VP (VBP dilate)))) (, ,) (NP (NP (DT a) (NN migraine)) (, ,) (NP (DT a) (JJ severe) (NN headache)) (, ,)) (VP (MD can) (VP (VB occur))) (. .)))