Wavelet Transform Coherence Under Controlled Temporal Correspondence and Social Co-Presence: Implications for fNIRS Hyperscanning

Human beings are wired to connect. When we share a moment with another person, whether it is a conversation, a shared laugh, or a coordinated movement, our bodies often fall into step. We might lean forward at the same time, match our breathing, or move our hands in rhythm. Scientists call this interpersonal synchrony, a phenomenon where our behaviors and even our internal biological rhythms align. In recent years, researchers have taken this observation to a new level by looking inside the brain. Using a portable technology that measures blood flow in the brain, they can now record the neural activity of two people at the same time while they interact. This approach, known as hyperscanning, has revealed that when people connect, their brain waves often seem to synchronize too. But a critical question remains: does this brain-to-brain matching happen because the two people are truly influencing each other, or is it simply because they are both reacting to the same task or environment?

To answer this, a team of researchers at the University of Trento in Italy designed a careful experiment to test how we measure this brain connection. They focused on a specific mathematical tool used to compare brain signals, called wavelet transform coherence. This tool is like a high-precision ruler that checks if two brain signals are moving in time with each other across different speeds. The researchers wanted to know if this ruler could reliably detect a difference between signals that were meant to match and signals that were meant to be different. More importantly, they wanted to see if the presence of another person changed how the brain worked, and whether that change affected the measurement itself. By isolating the timing of a simple task from the social experience of doing it with a partner, they uncovered that the way we measure brain synchrony is deeply tied to the social context in which it happens.

The study involved forty-four adults who were paired up into twenty-two teams. Each person wore a cap equipped with sensors that measured blood flow in the motor areas of the brain, the regions responsible for controlling movement. They also wore headphones to listen to a series of beeps. The task was straightforward: whenever they heard a tone, they tapped their finger on a key for ten seconds, and when the tone stopped, they remained still. The researchers created a specific pattern of tones and silence that repeated in blocks. Some blocks had the same timing structure, while others had different structures. The goal was to see if the brain signals from matching blocks looked more similar to each other than signals from non-matching blocks.

The experiment took place in two distinct settings. First, each person performed the tapping task alone in a quiet room. Later, the pairs performed the same task together, sitting side by side, able to see each other's hands moving, but instructed not to talk or deliberately coordinate their actions. This setup allowed the researchers to compare the brain signals of the same person doing the task alone versus with a partner, and to compare the signals between two different people doing the task together versus separately.

The results revealed a clear picture of how the brain responds to this simple motor task. When people tapped their fingers, the part of the brain on the opposite side of the body that controls movement lit up, which is exactly what scientists expect to see. This happened whether they were alone or with a partner. However, a significant difference emerged when they were together. During the joint session, the part of the brain on the same side as the tapping hand also became active. This suggests that simply having another person present, watching the same task, changes how the brain implements the movement, recruiting additional areas that remain quiet when we act alone.

The most surprising findings came from the analysis of the brain signal matching. When the researchers looked at the data from people performing the task alone, the measurement tool failed to distinguish between the matching and non-matching blocks. Even though the timing of the taps was strictly controlled and different blocks had different structures, the brain signals did not show a clear pattern of similarity that the tool could detect. The signals were too variable to tell the difference.

The story changed completely when the participants performed the task together. In the joint session, the measurement tool successfully identified the matching blocks. The brain signals from periods with the same timing structure showed a much stronger connection than those with different structures. This effect was strongest in the motor regions of the brain. Crucially, this improvement in detection happened not just between the two people, but also within the same person. When a single individual did the task with a partner present, their own brain signals became more consistent and easier to match with other similar blocks.

This finding challenges the idea that brain synchrony is a simple, context-independent measure of connection. The researchers found that the social presence of a partner altered the very nature of the individual's brain signals. The partner's presence seemed to make the brain's response to the task more consistent and organized. Consequently, the tool used to measure synchrony became much more sensitive. It was not that the two people were suddenly "syncing up" in a magical way that didn't exist before; rather, the social context changed the individual signals themselves, making the underlying patterns of timing more visible to the measurement.

The study also looked at how well the tool worked when comparing two different people. When the partners were tested separately, the tool found very little evidence that their brain signals matched based on the task structure. However, when they performed the task together, the tool detected a clear increase in matching signals, particularly in the motor areas. This confirms that while shared tasks can create some similarity in brain activity, the act of being together in the same space amplifies this effect significantly.

These results suggest that when we see two brains moving in sync, we cannot assume it is solely a result of a deep interpersonal bond or a direct neural link. The social environment itself shapes the signals. The presence of another person can change how an individual's brain processes a task, making the signals more consistent and easier to detect as synchronized. This means that measures of brain synchrony are not just a window into the space between two people, but also a reflection of how each person's brain is functioning within that specific social moment.

The researchers conclude that tools used to measure brain connection must be interpreted with care. They are not neutral rulers that give the same reading regardless of the situation. Instead, they are sensitive to the functional and social context in which the brain is working. The study demonstrates that social co-presence can alter the temporal dynamics of individual neural signals, effectively changing the raw material that synchrony metrics are built upon. This insight is vital for understanding human interaction, reminding us that the brain does not operate in a vacuum. When we connect with others, our brains do not just link up; they change. The very act of being together reshapes the neural landscape, creating a more coherent and synchronized signal that reflects the complexity of shared human experience.