The Basal Forebrain Influences the Brain’s Default Mode

What's the science?

The ‘default mode network’ is a unique network of brain regions that is dysregulated in many clinical conditions, like epilepsy and depression. Typically, these regions are most active while a person is resting or reflecting internally. Brain activity in the default mode network at a particular frequency (gamma frequency) is high during rest.The basal forebrain is a brain region thought to contribute to arousal and attention - for example, it accelerates learning and boosts neural signals in response to sensory stimulation. This week in PNAS, Nair and colleagues assessed the relationship between gamma frequency power (strength) in the basal forebrain and the default mode network, to understand how these regions might influence one another. 

How did they do it?

They studied the basal forebrain and default mode network in rats using recordings from electrodes in these regions, specifically measuring the gamma power. During the recordings, the rats were placed in their home cages where they groomed themselves and were quiet but awake, or they were placed in a large new area, where they tend to engage in active, exploratory behaviour. Next, they recorded the firing (activity) of basal forebrain neurons and tested whether it was correlated with fluctuations in gamma power, in order to understand whether the gamma frequency was generated locally. Finally, they tested whether patterns of gamma frequency activity in the basal forebrain predicted activity in the default mode network or vice versa.

What did they find?

They found that gamma power was high while the mice were in their home cages, and low while the mice explored an area. This change in gamma power was not due to movement. They also found that the neuronal firing of basal forebrain neurons was related to gamma power, indicating that these neurons were responsible for generating the signal. Finally, they found that gamma power in the basal forebrain predicted gamma power in the default mode network.

Hugo Gambo, Eeg gamma, image by BrainPost, CC BY-SA 3.0

What's the impact?

This is the first study to observe a link between the the default mode network, active during rest, and the basal forebrain, traditionally understood to increase arousal. Basal forebrain neurons are also activated during rest along with the default mode network. This study suggests that the default mode network is influenced by the basal forebrain, which could be a new clinical target for disorders in which the default mode network is affected.

Nair et al., Basal forebrain contributes to default mode network regulation. PNAS. (2018). Access the original scientific publication here.