What causes the brain to initiate spontaneous actions?

A new study sheds light on how the brain initiates spontaneous actions without any external triggers.

The research not only explains how these actions emerge but also offers insight into the mysterious ramping of neural activity before movement – a phenomenon that has puzzled scientists for years.

Origins of spontaneous action 

The study was led by Jake Gavenas during his time as a PhD student at the Brain Institute at Chapman University. He collaborated with two faculty members, Uri Maoz and Aaron Schurger, to explore the origins of spontaneous action. 

Together, the researchers simulated activity in neural networks and compared the results to real human intracortical recordings during spontaneous movement. 

The findings offer a fascinating explanation for how the brain’s neurons work together to trigger movement without any clear external signal.

Brain signals and conscious actions

One of the study’s key discoveries is that rapid fluctuations in the activity of individual neurons can combine within a network, resulting in much slower fluctuations when viewed at the population level. 

This helps explain a phenomenon that many people have experienced but never fully understood – such as when you’re standing on a diving platform, contemplating a jump. Nothing outside of you is telling you to jump, but at some point, you make the decision internally, and then you move. 

The brain, specifically the motor cortex, is coordinating muscle contractions to make this happen, but the question is: where do the initial signals to jump come from, and how do they relate to your conscious decision to act?

Spontaneous voluntary actions

Since the 1960s, neuroscientists have observed that electrical activity in the brain begins to increase one to two seconds before a spontaneous voluntary action.

Many researchers believed this slow ramping up of activity represented the brain preparing to move, following a preconscious decision. But what remained unclear was how this ramping process starts, as it seemed to emerge from nowhere.

This slow-ramping brain activity has sparked years of debate among neuroscientists and philosophers about free will and conscious control.

The big question has been whether this ramping signals that the brain is preparing to move even before we consciously decide to act, raising concerns that our actions might be predetermined at an unconscious level. Understanding where this ramping activity comes from has been a central problem in neuroscience.

Neural activity before spontaneous actions 

Researchers like Maoz and Schurger have challenged the view that the brain makes a preconscious decision to move. 

In 2012, Schurger proposed an alternative explanation, suggesting that the slow ramping in neural activity is part of a broader process. In this process, slow background fluctuations in the motor cortex accumulate until they hit a certain threshold, triggering movement.

In this framework, the key moment isn’t when the slow ramping begins, but rather when the fluctuations cross a threshold to initiate action. This would explain why scientists see ramping activity before movement – because, looking back from the point of action, slow fluctuations would have been leading up to the threshold crossing all along. 

This explanation, while compelling, left one crucial question unanswered: where do these slow background fluctuations in neural activity, often called 1/f noise, come from, considering that individual neurons fluctuate quite rapidly?

Slow-ramping signals 

The study is the first to provide an explanation for how slow background fluctuations arise from networks of neurons, none of which, individually, operate on such slow timescales. 

The research shows that when neurons interact in a network, their fast, individual fluctuations can combine into slower, population-level fluctuations. These slow fluctuations can then lead to a threshold-crossing event, triggering movement and giving rise to the ramping activity that appears before spontaneous actions.

“We see similar slow-ramping signals before other kinds of spontaneous behaviors, like coming up with creative ideas or freely remembering things that have happened to you. A similar process might therefore underlie those phenomena, but only time and further research will tell,” Gavanas said.

How the brain triggers movement

In short, this study provides a possible explanation for the slow, spontaneous fluctuations in neural activity, which are widespread in neural systems and critical to understanding how the brain triggers movement without external cues.

Moreover, the study highlights an important point about how researchers interpret their findings. According to Maoz, it reveals the bias we have as researchers to think that our results uncover a causal mechanism, when it may really be just a correlation.

This observation reminds us that in neuroscience, understanding the difference between correlation and causation is crucial for making sense of how the brain functions.

The research is published published in the journal Nature Communications. 

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