A study found that both the cortex processes sensory and motor signals, challenging past ideas and suggesting that these signals are connected in impacting decisions. Credit: SciTechDaily.com
Innovative research demonstrates how the brain combines sensory data and movement signals, affecting how we respond to what we hear.
When you hear a phone ring or a dog bark, you have to figure out if it's coming from your own space or elsewhere. Similarly, determining if the footsteps you hear belong to your child or an intruder requires quick decision-making. Researchers at the Champalimaud Foundation have shed some light on the processes that may occur in our brains during these moments, bringing us closer to understanding how the brain translates perceptions into actions.
Understanding Brain Functions While Making Decisions
Every day, we make numerous decisions based on sounds without much consideration. But what exactly happens in the brain at such times? A new study from the Renart Lab, published today (May 10) in Current Biology, delves into this topic. Their discoveries enhance our comprehension of how sensory information and behavioral choices are intertwined within the cortex – the brain’s outer layer that shapes our conscious perception of the world.
The cortex is split into sections that handle different tasks: sensory areas process information from our surroundings, while motor areas oversee our actions. Interestingly, signals pertaining to future actions, traditionally expected in motor areas, also appear in sensory areas. Why do movement-related signals exist in areas designated for sensory processing? When and where do these signals emerge? Examining these inquiries could clarify the origin and role of these perplexing signals, and how they influence – or do not influence – decisions.
Innovative Research Approaches
The researchers addressed these questions by creating a task for mice. Postdoc Raphael Steinfeld, the lead author of the study, shares: “To understand what signals related to future actions may be doing in sensory areas, we carefully considered the task that mice would need to complete. Prior studies often relied on “Go-NoGo” tasks, in which animals indicate their choice by either making an action or staying still, based on the stimulus's identity. However, this setup blends signals connected to specific movements with those related to general movement. To isolate signals for specific actions, we trained mice to choose between two actions. They had to determine if a sound was higher or lower than a set threshold and communicate their decision by licking one of two spouts, left or right.”
Nevertheless, this was not enough. “Mice quickly learn this task, often responding as soon as they hear the sound,” Steinfeld explains. “To separate brain activity linked to the sound from that linked to the response, we introduced a crucial half-second delay. During this period, the mice had to refrain from making their decision. Crucially, this delay allowed us to temporally separate brain activity associated with the stimulus from that associated with the choice, and track the unfolding of movement-related neural signals over time from the initial sensory input.”
The researchers wanted to create an experiment tough enough for the mice to make mistakes, so they could understand how the brain processes information and choices. If the mice were always right, it would be hard to distinguish between their reactions to the stimulus and their actual decisions. By allowing for mistakes, the researchers could separate how the brain encodes sound from how it makes decisions. For example, when the mice heard the same sound but made different choices, the researchers could see if the activity of certain neurons changed. If it did, it would mean that the neuron contained information about the decision-making process.
Deepening Understanding of Neural Connections
After six months of intense training, the researchers were able to start recording the brain activity of mice while they completed the task. They focused on the auditory cortex, which processes what we hear and is known to be important for the task. Alfonso Renart, the principal investigator, explained that the cortex of mice and humans is made up of six layers, each with specific functions and connections to other brain areas. The researchers simultaneously recorded activity across the layers of the auditory cortex for the first time in a task like theirs, where sensory and motor signals could be clearly separated.
The researchers discovered that signals linked to the detection of sound appeared quickly but faded fast, while signals related to the mouse's decision emerged later and were more focused in the deeper layers of the cortex. These findings suggested that the brain processes sensory information and decision-making separately in terms of time and location within the auditory cortex.
Even though there was a time difference between how the brain reacted to the stimulus and how it prepared for a decision, further analysis showed an interesting link: neurons that responded to a specific sound frequency also tended to be more active for the actions associated with those sounds. This indicated that the activity of these neurons adapted through experience rather than being fixed.
Origin and Role of Choice Signals
The researchers found that the early sensory signals in the auditory cortex did not predict the mice's eventual choice, and the choice signals appeared much later. This suggested that the sensory signals in the auditory cortex did not directly cause the mice's actions, and that the choice signals likely originated in higher brain regions involved in planning and executing movements, which then sent feedback to the auditory cortex.
However, if these signals for movement don’t control actions, what could their role be? Perhaps they mainly help to bring together and pass on information. For example, these signals might adjust the brain’s understanding to match a developing decision, making what we perceive more stable. Alternatively, they could prepare the brain for the expected sensory results of actions, such as the sound made by movement, ensuring that our sensory experiences match our movements.
Future Research and Implications
However, these ideas still need to be confirmed. Renart ponders, “One might ask, if the sensory signals from the auditory cortex don’t directly influence choices, and the signals for choices we see there aren’t actually created by it, then what is the purpose of the auditory cortex exactly?” “We could suggest that the auditory cortex is more focused on creating a conscious experience of sound than on changing sensory input into movement, but that’s a discussion for another time.”
Nevertheless, we can’t dismiss the possibility of a causal role, especially since the deeper layers of the auditory cortex send information to the posterior striatum, which is part of the brain’s command center for habits and movements. In the future, studies will try to identify the exact source of these movement signals and whether they really do cause behavior. For now, we can add another piece to the puzzle of how brains turn perception into action, and the inner processes that come into play when you hear footsteps at night.
Reference: “Differential representation of sensory information and behavioral choice across layers of the mouse auditory cortex” by Raphael Steinfeld, André Tacão-Monteiro and Alfonso Renart, 10 May 2024, Current Biology.
DOI: 10.1016/j.cub.2024.04.040
