The Sudoku Effect: Study Examines How Shared Cognitive Tasks Shape Inter-Brain Synchronization
What happens when people tackle the same mental challenge, and what happens when one person secretly works against the group? A new study used electroencephalography (EEG) to simultaneously record brain activity from multiple people during cooperative games, individual puzzles, and rounds involving a hidden adversary. Researchers found lower inter-brain synchrony when a covert saboteur was present, while people solving different puzzles alone showed levels of synchrony comparable to those observed during teamwork.
Note: This article is intended for general information and educational purposes. It summarizes scientific research in accessible language for a broad audience and is not an official scientific press release.
When people work together toward a common goal, their actions often become coordinated. They may anticipate one another’s decisions, adapt to shared challenges, and gradually develop a sense of group cohesion. These everyday observations have led neuroscientists to ask a deeper question: do people’s brains also become synchronized when they share the same experience or work toward the same objective?
Published on March 31, 2026, in Frontiers in Neuroergonomics, a study explored this question using a technique known as EEG hyperscanning, which allows researchers to record brain activity from multiple individuals simultaneously. The research was conducted by Alex Kennedy, Nathan Shields, Sean Farrell, and Alejandro Lopez Valdes from Trinity College Dublin, with affiliations spanning the School of Engineering, the Trinity Centre for Biomedical Engineering, the Trinity College Institute of Neuroscience, and the Global Brain Health Institute in Dublin, Ireland.
The researchers investigated how inter-brain synchrony changed when small groups participated in cooperative games, completed individual cognitive puzzles, and encountered a hidden adversary who was instructed to work against the group’s objective. Their findings support the idea that shared cognitive demands may contribute to inter-brain synchrony, even when people are not actively cooperating.
What the Researchers Investigated
Previous hyperscanning studies have reported inter-brain synchrony during a variety of social interactions. However, most of this research has focused on pairs of participants rather than larger groups. The authors aimed to investigate inter-brain synchrony in groups of three individuals, known as triads. More specifically, they wanted to examine two competing explanations for why synchronization occurs across brains during shared activities.
One explanation, known as the cooperative interaction hypothesis, proposes that synchronization emerges primarily from social interaction and cooperation.
The second explanation, known as the comparable task hypothesis, proposes that synchronization may result from people engaging in similar cognitive processes, regardless of whether they are actively cooperating.
To explore these possibilities, the researchers compared brain activity during cooperative and non-cooperative tasks while also examining the effect of deliberate disruption within a group.
How the Study Was Conducted
The study involved 36 healthy adult participants recruited from Trinity College Dublin. All participants were over 18 years of age and reported no neurological disorders.
Researchers conducted two separate experiments in a naturalistic setting designed to resemble an ordinary group activity rather than a highly controlled laboratory environment. Participants sat around a table while wearing wireless EEG devices equipped with 24 recording channels that continuously measured electrical activity from the brain.
Experiment One
The first experiment involved two groups of three participants who completed a series of experimental blocks consisting of individual puzzle tasks and cooperative card games.
Participants alternated between:
- A cooperative card game called The Mind, in which players attempted to place numbered cards in the correct order without speaking or using physical signals.
- Individual cognitive puzzles, including Sudoku, crosswords, and word-search activities.
- Short resting periods before each task.
Because the cooperative game ended immediately when the group succeeded or failed, task durations varied naturally from round to round.
Experiment Two
The second experiment involved 10 separate groups of three participants.
The same cooperative game and puzzle tasks were used, but all task sessions were standardized to the same duration. If a card game ended early, new cards were immediately redistributed so that task engagement continued for the full four-minute session.
This experiment also introduced an additional condition.
In some cooperative rounds, one participant secretly received instructions to work against the group’s objective by intentionally placing an incorrect card at a moment of their choosing. The remaining participants were unaware that deliberate sabotage had been introduced.
Measuring Inter-Brain Synchrony
After collecting the EEG recordings, the researchers removed non-neural artifacts such as eye blinks and muscle movements. They then calculated inter-subject correlation (ISC), a measure used to quantify the similarity of brain activity across participants.
What Makes This Study Different
According to the authors, most previous hyperscanning studies have focused on interactions between two people. This research expanded the investigation to three-person groups performing both cooperative and individual tasks.
The study also addressed an important methodological issue. Earlier work suggested that analytical choices, such as the length of the observation window, could influence measures of synchronization. The researchers therefore carefully examined how experimental duration affected their results.
Another distinguishing feature was the inclusion of a hidden adversary condition. Rather than comparing cooperation and competition directly, the study explored what happened when one participant quietly worked against the group’s goal while appearing to cooperate.
This design allowed the researchers to examine how subtle disruptions within a group might influence inter-brain synchrony.
Key Findings from the Study
Why the Results Were Unexpected
Many discussions of inter-brain synchrony focus on cooperation, communication, and shared social experiences. Based on that perspective, it would be reasonable to expect people actively working together to show substantially stronger synchronization than people solving different tasks on their own.
However, the results of this study painted a more complex picture.
1. The Sudoku Effect: Solo Puzzles Matched Teamwork
One of the most notable findings was that participants displayed similar levels of inter-brain synchrony during cooperative gameplay and individual puzzle solving.
In Experiment One, inter-subject correlation values were 0.286 during cooperative gameplay and 0.267 during individual puzzles. In Experiment Two, the corresponding values were 0.186 and 0.177. Statistical analyses indicated that these differences were not significant.
According to the authors, these findings support the comparable task hypothesis. Although participants completed different puzzles independently, all of the tasks belonged to the broader category of cognitively demanding brain teasers. The researchers suggest that similar cognitive demands may contribute to similar patterns of neural activity across individuals.
In other words, participants did not need to be actively cooperating to exhibit levels of inter-brain synchrony comparable to those observed during teamwork.
2. Active Tasks Produced More Synchronization Than Rest
The study also found higher levels of inter-brain synchrony during active task performance than during resting periods. In both experiments, synchronization levels during cooperative gameplay and puzzle solving exceeded those observed when participants simply sat quietly before beginning the tasks.
The authors interpret this finding as evidence that task engagement contributes to the emergence of shared neural activity patterns.
This distinction proved important. While teamwork did not produce significantly greater synchrony than puzzle solving, both forms of active cognitive engagement were associated with higher synchrony than passive rest.
3. The Hidden Adversary Reduced Inter-Brain Synchrony
Perhaps the most intriguing result emerged from the adversary condition. When all members of the group pursued the same objective, average inter-brain synchrony reached 0.189. When a hidden adversary was present, synchrony declined to 0.166. This difference was statistically significant.
Importantly, participants generally did not report noticing intentional sabotage during post-experiment debriefings. Most appeared to interpret incorrect plays as ordinary mistakes rather than deliberate attempts to undermine the group’s performance. In other words, the disruption was not obvious to the participants themselves, yet lower inter-brain synchrony was still observed during those rounds.
The study did not determine exactly why this reduction occurred. However, the findings suggest that subtle disruptions in group dynamics may be reflected in measures of inter-brain synchrony.
4. Winning and Losing Did Not Significantly Change Synchrony
The researchers also compared successful and unsuccessful rounds of the cooperative game. Average synchrony was slightly higher during successful rounds than unsuccessful ones, but the difference was not statistically significant.
According to the authors, this suggests that synchronization may be more closely related to the process of engaging in a shared activity than to the final outcome of that activity.
The finding highlights an interesting distinction: whether the group succeeded or failed appeared less important than whether group members remained aligned in pursuing the same objective.
5. Experimental Duration Influenced Synchronization Measures
Another important finding involved methodology rather than behavior. The researchers observed a significant relationship between the duration of analysis windows and measured synchronization levels. Shorter analysis windows tended to produce higher inter-brain synchrony values.
The authors emphasize that future hyperscanning studies should carefully standardize analysis durations to avoid introducing artificial differences into synchronization measures.
Authors’ Conclusions
The authors conclude that their findings provide stronger support for the comparable task hypothesis than for the cooperative interaction hypothesis.
While previous interpretations of similar data emphasized the role of cooperation, the current analyses suggest that comparable cognitive demands may account for a substantial portion of observed inter-brain synchrony.
The researchers note that participants solving different puzzles independently displayed synchronization levels similar to those observed during cooperative gameplay. According to the authors, this finding suggests that shared categories of cognitive processing may contribute to neural alignment across individuals.
The study also demonstrated that introducing a hidden adversary was associated with reduced inter-brain synchrony. The authors note that additional research will be required to better understand the mechanisms underlying this effect.
The researchers acknowledge several limitations. The study did not collect detailed information about participants’ personality traits, sociability, or preferences for games and puzzles. In addition, the individual tasks all belonged to the general category of cognitive brain teasers, making it difficult to isolate the effects of specific task characteristics.
The authors recommend that future studies incorporate additional measures such as eye tracking and explore a broader range of cooperative and non-cooperative activities.
Understanding the Broader Context
Research on inter-brain synchrony remains a rapidly developing area of neuroscience. Hyperscanning technologies now allow scientists to examine neural activity across multiple people simultaneously, creating new opportunities to investigate communication, cooperation, attention, and group behavior.
This study contributes to that growing body of research by examining how synchronization emerges in small groups and how it changes under different task conditions.
The findings also highlight the importance of distinguishing between social interaction and shared cognitive demands when interpreting measures of inter-brain synchrony.
Conclusion
Can people’s brains become synchronized simply because they are facing similar mental challenges? According to this study, the answer may be more nuanced than previously thought.
The researchers found comparable levels of inter-brain synchrony during teamwork and individual cognitive puzzles, higher synchronization during active tasks than during rest, and lower synchronization when a covert adversary was introduced into the group.
Together, these findings add to a growing body of research suggesting that inter-brain synchrony may be influenced by both social interaction and shared cognitive demands. As researchers continue exploring how shared tasks and group dynamics influence neural activity, studies like this may help clarify what drives synchronization across multiple brains during real-world interactions.
The information in this article is provided for informational purposes only and is not medical advice. For medical advice, please consult your doctor.
References
Kennedy, A., Shields, N., Farrell, S., & Lopez Valdes, A. (2026). Analysis of inter-brain synchrony in group-based electroencephalography to assess task-dependent interactions. Frontiers in Neuroergonomics, 7, 1774423. DOI: https://doi.org/10.3389/fnrgo.2026.1774423
