( Migraines ( ( are ( thought ( to ( be ( ( ( caused ( by ( extreme constriction ) ) ) and ) ( then ( dilation ( of ( ( blood vessels ) ( in ( the head ) ) ) ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (NNS Migraines)) (VP (VBP are) (VP (VBN thought) (S (VP (TO to) (VP (VB be) (VP (VP (VBN caused) (PP (IN by) (NP (JJ extreme) (NN constriction)))) (CC and) (VP (ADVP (RB then)) (VBN dilation) (PP (IN of) (NP (NP (NN blood) (NNS vessels)) (PP (IN in) (NP (DT the) (NN head)))))))))))) (. .)))
Migraines are thought to be caused by extreme constriction and then dilation of blood vessels in the head.
(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))) (. .)))