A recent study investigates how the brain triggers spontaneous actions. This research, which shows that spontaneous actions can occur without any external stimuli, also sheds light on the gradual increase in neural activity that often happens before movement begins. This phenomenon is well-known but not fully understood.
A recent study, published in Nature Communications this week, was led by Jake Gavenas, PhD, during his time as a PhD student at the Brain Institute at Chapman University. He was assisted by two faculty members, Uri Maoz and Aaron Schurger. The research focuses on how the brain initiates spontaneous actions. In addition to highlighting how these actions can arise independently of environmental cues, the findings also have implications for understanding the gradual buildup of neural activity before a movement starts—a phenomenon that is frequently observed yet not comprehensively explained.
In this study, Gavenas and his colleagues sought to answer this question. They modeled spontaneous activity using simple neural networks and compared these simulations to intracortical recordings from humans as they moved spontaneously. The results revealed an interesting insight: a network of rapidly fluctuating neurons can collectively create very slow fluctuations at the population level.
To illustrate, consider standing on a high dive, trying to muster the courage to jump. No external cues prompt you to leap; that choice is internally driven. Eventually, you feel you have decided to jump, and then you do. During this time, your motor cortex in the brain sends out electrical signals that coordinate muscle contractions throughout your body, allowing you to run and jump. But where do these signals originate in the brain, and how do they connect to your conscious decision to move?
Since the 1960s, neuroscientists have noted that brain activity begins to increase about 1-2 seconds before a spontaneous voluntary action occurs. Many believed that this pre-movement ramping indicated the brain was gearing up to move following a subconscious decision to act. However, despite extensive research, the source of this gradual increase in activity seemed rather mysterious. This led to debates among neuroscientists and philosophers regarding the implications for free will and conscious control: if the brain prepares to move 2 seconds (or more, in some studies) prior to your conscious decision, could it mean that your actions are largely predetermined without your awareness? Thus, figuring out the neural origin of this ramping activity remains a significant issue in neuroscience.
Some neuroscientists, like Maoz and Schurger, have challenged the idea of preconscious information being present seconds before an action. Particularly, Schurger proposed in 2012 that the gradual increase in brain activity is part of a broader mechanism where slow fluctuations in the motor cortex must reach a certain threshold to trigger movement. If these slow fluctuations are responsible for determining when this threshold is crossed, then examining movement onset will naturally reveal slow ramping beforehand, even if this ramping is not due to an early subconscious decision. Here, the critical moment is not when the slow-ramping begins but when the threshold is crossed. However, this perspective raises an important question: where do these slow background fluctuations, often referred to as 1/f noise, originate, given that individual neuron activity is typically more erratic?
Gavenas’s research offers the first explanation for the emergence of these slow background fluctuations from neural networks, where no single neuron operates on such a long time scale. These slow fluctuations may therefore play a role in the threshold-crossing event believed to initiate movement, which links to the slow ramping observed prior to spontaneous actions and perhaps beyond. Gavenas noted, “We observe similar slow-ramping signals before other spontaneous behaviors, like generating creative ideas or recalling past experiences. A similar mechanism may underpin these experiences, but further research will clarify this.”
In conclusion, this study is groundbreaking because it provides a possible explanation for the source of slow, spontaneous fluctuations in population-level neural activity, a common occurrence in neural systems. Furthermore, as Maoz points out, “It highlights the tendency among researchers to interpret our findings as revealing a causal mechanism, whereas they might actually just indicate a correlation.”
