It is worth mentioning that the above theoretical model assumes the baseline activity remains unchanged from the resting-state or control condition to the stimulus condition. However, it is possible that the baseline activity changes by itself or in response to certain stimuli. This depends on the brain location of interest as well as the stimulus type and property. For instance, there are a number of default-mode regions that are functionally active during the resting state but are likely suppressed given certain stimuli or tasks that request attentional resources [130]. It is also possible that the baseline activity at some brain regions spontaneously modulates over time, with or without any relationship to the external stimulus. As a result, if the stimulus is given when the baseline activity modulation reaches a lower (or higher) level than during the resting-state control state, the corresponding BOLD signal change reflects not only the stimulus evoked neuronal response but also the modulation of the spontaneous baseline activity.
