( Migraines ( ( are ( headaches ( that ( are ( thought ( to ( be ( dilation ( of ( ( blood vessels ) ( in ( the brain ) ) ) ) ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (NNS Migraines)) (VP (VBP are) (NP (NP (NNS headaches)) (SBAR (WHNP (WDT that)) (S (VP (VBP are) (VP (VBN thought) (S (VP (TO to) (VP (VB be) (VP (VBN dilation) (PP (IN of) (NP (NP (NN blood) (NNS vessels)) (PP (IN in) (NP (DT the) (NN brain))))))))))))))) (. .)))
Migraines are headaches that are thought to be dilation of blood vessels in the brain.
(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))) (. .)))