A recent study by researchers from Osaka University has identified specific brain activity patterns associated with daydreaming and mind-wandering. The researchers found that these self-generated thoughts are linked to sharp-wave ripples in the hippocampus, a brain region crucial for memory formation and retrieval. The findings have been published in Nature Communications.
The ability to think about things that are not present is a hallmark of human cognition, allowing us to plan for the future, reminisce about the past, and imagine hypothetical scenarios. However, the exact brain mechanisms behind these self-generated thoughts remain unclear.
Understanding these processes could have significant implications, particularly for individuals with conditions such as autism, attention deficit disorder, and other cognitive disorders where self-generated thoughts may differ from the norm. Additionally, this knowledge could contribute to the development of brain-computer interfaces that enable communication for individuals who are paralyzed and cannot speak or move.
“We are developing a brain-computer interface that decodes a person’s thoughts from brain activities and enables them to communicate without physical movement. This technology can be applied to aid communication in severely paralyzed patients such as amyotrophic lateral sclerosis (ALS),” explained study author Takufumi Yanagisawa, a professor at the Institute for Advanced Co-Creation Studies at Osaka University.
“For this purpose, it is necessary to clarify what kind of brain activity occurs when a person thinks. Therefore, in this study, we investigated people’s spontaneous thoughts over a period of 10 days and examined the brain activity during these thoughts. In particular, we focused on activity in the hippocampus, which is thought to be important when people imagine something.”
To explore the connection between brain activity and self-generated thoughts, the researchers utilized data from patients with drug-resistant epilepsy. These patients were already undergoing a procedure to locate the origin of their seizures, which involved implanting electrodes in the hippocampus to monitor brain activity continuously.
Over a span of ten days, the patients’ brain activity was continuously recorded while they went about their daily activities within a hospital setting. In addition to the brain recordings, the patients were asked to complete an hourly questionnaire on a tablet device about their thoughts and emotions. This questionnaire consisted of 17 questions designed to assess their mood and the nature of their thoughts, asking them to rate their experiences on a scale from 1 to 7.
The researchers also recorded physiological data using a wristband device worn by the patients. This device measured heart rate, physical activity, and other physiological signals, providing additional context for understanding the brain activity. By combining these detailed recordings with the patients’ self-reported thoughts and feelings, the researchers aimed to identify any patterns linking brain activity to specific types of thoughts.
The study researchers that sharp-wave ripples in the hippocampus were more frequent during periods when patients reported vivid, imaginative, and less task-related thoughts — essentially, when their minds wandered. These brain waves are characterized by their specific pattern: a sharp wave followed by a rapid burst of synchronized neuronal activity, which can be recorded as ripples. They typically occur at frequencies between 100 and 200 Hz and last for about 50 to 200 milliseconds.
These ripples were predominantly observed at night but also appeared during wakeful periods of rest or low activity. This pattern suggests a strong link between hippocampal activity and the generation of spontaneous thoughts.
“The thought state that induces spontaneous thought correspond to a specific brain activity, hippocampal sharp-wave ripple, which is the same activity involved in memory fixation during sleep,” Yanagisawa told PsyPost. “The thoughts that spontaneously come to mind may also have a memory-related function. It has also been suggested that it is possible to estimate from brain activity whether a person is in a state of mind wandering.”
Despite the study being conducted on patients with epilepsy, the researchers took care to filter out data directly related to epileptic activity. They found that the observed patterns were consistent with findings from studies on healthy individuals and animal models, reinforcing the broader applicability of their results.
The researchers noted that the sharp-wave ripples occurred in a consistent manner across the patients and that these ripples were associated with thought patterns that were less connected to immediate tasks or future planning. Instead, these thoughts were more likely to be vivid, visual, and sometimes intrusive or undesired.
“We found that there is a common relationship between hippocampal activity and thought in different people,” Yanagisawa said. “Although the detailed content of thoughts are different among people, we found that it is possible to estimate whether people are mind-wandering or thinking about things they don’t want to think about, regardless of the person.”
While the study provides valuable insights, there are some caveats to consider. For instance, the correlation between sharp-wave ripples and thought content does not establish a causal relationship. It is unclear whether these brain waves directly induce specific types of thoughts or simply coincide with them. Further research involving controlled manipulation of these ripples could help clarify their role in thought generation.
“In the future, more data will be collected and technology will be developed that can decode the content of thoughts in detail,” Yanagisawa explained. “This will enable us to understand how people’s natural thoughts arise and the technology to decode them, which will then be applied clinically as a communication technology for patients with severe paralysis.”
“In this study, patients who had electrodes implanted intracranially for the purpose of epilepsy treatment cooperated with us, allowing us to record their thoughts while directly measuring the hippocampal EEG. With current technology, accurate measurement of hippocampal EEG requires techniques such as this one that require surgery.
“However, it may be possible to estimate hippocampal EEG using technology that does not require surgery, such as scalp EEG,” Yanagisawa said. “In the future, anyone will be able to know the state of their hippocampus in real time, which may enable them to look at their thought state objectively.”
The study, “Hippocampal sharp-wave ripples correlate with periods of naturally occurring self-generated thoughts in humans,” was authored by Takamitsu Iwata, Takufumi Yanagisawa, Yuji Ikegaya, Jonathan Smallwood, Ryohei Fukuma, Satoru Oshino, Naoki Tani, Hui Ming Khoo, and Haruhiko Kishima.