BLOOD OXYGENATION LEVEL DEPENDENT SIGNAL TIME COURSES DURING PROLONGED VISUAL-STIMULATION

Previous functional magnetic resonance imaging (MRI) studies using ext ended visual stimulation have reported disparate results, Two studies have shown that blood oxygen level dependent (BOLD) contrast decays ov er time which is cited as evidence of recoupling between oxygen utilis ation and cerebral blood flow during stimulus presentation, These find ings have serious implications for the design of functional MRI experi ments because they raise the possibility that BOLD contrast may not ac curately reflect neuronal activity, Another study reported no decay of BOLD contrast, These studies used different visual stimuli and imagin g techniques, We have performed a series of experiments, using differe nt MRI techniques (echo-planar imaging and fast low angle shot) and tw o different visual stimuli to assess which of these factors may explai n the previous results, In all of our experiments the signal time cour se from areas of significant activation remained largely elevated thro ughout the duration of stimulation and this is not affected by the ima ging method used, Our data, in accordance with that of Bandettini et a l., suggest that recoupling between blood how and oxygen extraction is not a general phenomenon in the human brain when visual stimuli are p resented for an extended time. (C) 1998 Elsevier Science Inc.