Abstract #2787

Meltzer J, Constable R, Negishi M

A mixed block / event-related fMRI study was used to explore the extent to which transient aspects of the hemodynamic response give rise to activation and deactivation in a block design contrast between two memory tasks, one of which requires the hippocampus but induces hippocampal deactivation. In the hippocampal-dependent condition, enhanced post-stimulus undershoots were observed, in some regions equal to or exceeding the magnitude of the early positive BOLD peak observed in that condition. This work provides evidence that the relative magnitude of the poststimulus undershoot can determine whether a region is considered activated or deactivated at the block-design level.