Regardless of the increasing observations of the negative BOLD, its origin and relationship to neuronal activity remains poorly understood and controversial. The NBR may originate from reduced neuronal activity [176, 180, 183, 185, 186], or vascular blood stealing [187, 188], or perhaps both [181, 182]. In addition, the NBR can be either dependent or independent upon the task or stimulus content [17, 185]. For example, in response to a particular stimulus, the NBR may be found at non-stimulated regions within the relevant sensory system [177–183]. Such a task-dependent NBR is often hypothesized to reflect neuronal suppression at regions complementary to those with increased neural activity [182]. On the other hand, decreases in BOLD (or CBF) signals are also observable at locations that change little across a wide variety of tasks [130, 176]. These task-independent NBR regions are believed to be functionally active [130, 185] and connected [189] in the resting state, collectively responsible for a default-mode brain function [130, 190]. At these regions, the NBR may be induced as a consequence of decreased default-mode activity when the resting-state brain function is interrupted by the execution of attention demanding tasks [17].
