The Brain Is Always Active

Welcome to the new year for which I wish you all the best. Let me start the new year with a question that will guide my work and hence these blogs in the new year: Why is our brain’s spontaneous activity so important?

The brain shows an intrinsic activity that remains independent of external stimuli or tasks. This high level of continuous activity in the brain is described as spontaneous, intrinsic or resting state activity. The term resting state activity is rather paradox since it signifies the opposite of what the term itself says: the brain is never really at rest, and if it is at rest, it is dead, brain death, as the neurologist says. Such spontaneous activity must be distinguished from task-evoked or stimulus-induced activity that is related to stimuli or tasks external to the brain itself. Neuroscience has long favored task-evoked or stimulus-induced activity since it is easily accessible and can be directly probed by applying specific stimuli or tasks to subjects in the scanner. This is the main focus of for instance cognitive, affective, and social neuroscience that use stimuli or tasks to probe the respective stimulus-induced or task-evoked activities.

However, more recently, the brain’s spontaneous activity came into focus. What does it do though, why is it important, and how does it impact our stimulus-induced or task-evoked activity? We currently do not know the answers to these questions. I hypothesize that the brain’s spontaneous activity is a major game changer in our understanding of the brain and how it yields mental features like consciousness and psychopathological symptoms in psychiatric disorders. This was elaborated in my major two volume work “Unlocking the brain” Vol I Coding, Vol II Consciousness (Northoff 2014, Oxford University Press). Here I want to focus on the last question namely how the brain’s spontaneous activity impacts our stimulus-induced or task-evoked activity.

Zirui Huang from our group conducted an fMRI study with very short stimuli (1s max) consisting of short autobiographical (and semantic) questions and longer intervals between the stimuli to allow for recovery of neural activity to the baseline, e.g., resting state levels. Intuitively, one would now assume that the stimulus-induced activity is merely added to or supersedes the ongoing spontaneous activity – this would be additive interaction between spontaneous and stimulus-induced activity. In that case the spontaneous activity would not really exert any impact on the stimulus and the degree of activity it elicits in the brain. Spontaneous and stimulus-induced activity would basically act in parallel in a non-additive way. This was the traditional model that was assumed so far. However, the data obtained by Zirui Huang did not provide evidence for such model.

Alternatively, there would be direct interaction between both with the spontaneous activity level impacting the degree of activity the stimulus can elicit. Specifically, ZIrui Huang observed that low levels of spontaneous activity prior to the stimulus, e.g., low pre-stimulus resting state activity levels, elicited high levels of stimulus-induced activity after the onset of the stimulus. Conversely, high levels of spontaneous activity prior to the stimulus, e.g., high pre-stimulus resting state activity, lead to low levels of stimulus-induced activity after the onset of the stimulus. In short, pre-stimulus high lead to low stimulus-induced activity, while low pre-stimulus activity entailed high stimulus-induced activity. Hence, the relationship between pre-stimulus and stimulus-induced activity levels was characterized by a reciprocal or opposite relationship. This is a clearly non-additive pattern of rest-stimulus interaction since in the case of an additive pattern one would have expected high pre-stimulus levels to lead to high stimulus-induced activity levels (as well as low pre-stimulus levels leading to low stimulus-induced activity levels). Zirui Huang also showed the mechanisms underlying such non-additive rest-stimulus interaction which shall be neglected here (refer to Huang, 2015 for details).

Why is such non-additive rather than merely additive rest-stimulus interaction important? It demonstrates for the first time that the spontaneous or resting state activity of the brain has an active say or impact on what external stimuli or tasks can elicit activity in the brain. Our brain is not a merely mechanical response apparatus that responds to external stimuli or tasks. Instead, our brain is rather a dynamic organ that provides its own spontaneous activity by means of which is can impact and manipulate its own processing of external stimuli or tasks.

You may be puzzled why such purely neuronal feature of the brain, e.g., non-additive rest-stimulus interaction is so important. This has major implications for our understanding of how the brain can yield mental states like consciousness which shall be discussed in the next blog.

Huang Z, Zhang J, Longtin A, Dumont G, Duncan NW, Pokorny J, Qin P, Dai R, Ferri F, Weng X, Northoff G. (2015). Is There a Nonadditive Interaction Between Spontaneous and Evoked Activity? Phase-Dependence and Its Relation to the Temporal Structure of Scale-Free Brain Activity. Cereb Cortex. 2015 Dec 7. pii: bhv288. [Epub ahead of print]

Northoff Georg (2014a) Unlocking the brain. Vol I Coding. Oxford University Press, Oxford, New York

Northoff Georg (2014b) Unlocking the brain. Vol II Consciousness. Oxford University Press, Oxford, New York