Despite extensive research of the Default network, a set of regions which tend to reduce their activity relative to rest in response to stimulus-driven tasks, its function is still debated. Specifically, it is still not clear to what extent the activation profile of this network is driven by processes related to external stimulation (inhibitory or anticipatory), or is driven by specific thought contents. To address this question, we examined the ability of thoughts, generated in the absence of external stimulation, to modulate default network activation. In a set of experiments, several types of long lasting stimulus-free thoughts were elicited by brief (<1s) auditory cues. Sustained (40s) brain activations, far outlasting the cue, were demonstrated during these stimulus-free conditions. Importantly, brain activity in the default network showed a striking modulation associated with stimulus-free thought content. More specifically, a preferential activation was observed in essentially the entire default network during volitional-prospection thoughts when compared to the other stimulus-free thought conditions. Furthermore, several regions of the default network showed long-lasting above rest activations during the volitional-prospection condition. Our results demonstrate that default network activation can be modulated in the absence of external stimuli, thus pointing to the importance of thought-content in default-network specialization. Furthermore, together with previous research, these results support the notion that intrinsically oriented processing is a core specialization of the default network. Finally, our stimulus-free experimental paradigm introduces a new method for studying default network functionality.
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