How sharp ripples shape our imaginative minds

Researchers identified sharp ripples in the hippocampus as a key brain activity linked to self-generated thoughts like daydreaming. By studying epilepsy patients, the team found that these brain patterns are linked to vivid and imaginative thoughts, suggesting a broader impact on conditions such as autism and ADHD. Credit: SciTechDaily.com

Osaka University researchers found that mind wandering, especially when associated with less pleasant and more vivid and imaginative self-generated thoughts, is associated with a specific pattern of brain activity related to memory and sleep.

Part of what makes us human is our ability to think about people, places, or events that aren't presently present, but we still don't know exactly how our brains do this. Now, researchers at Osaka University have identified a specific type of brain activity linked to these types of thoughts, such as when we daydream or let our minds wander.

Sharp ripples and memory

When we think about things that aren't actually happening, like when we dream, the brain is essentially making information rather than receiving and processing it. For this reason, researchers classify it as a “self-generated” brain state.

In a recent study published today (May 22) in the scientific journal Natural communications, Japanese researchers have identified that these self-generated states are associated with a specific pattern of brain activity known as “sharp wave ripples.” These ripples begin in the hippocampus, a brain region essential for creating and retrieving memories.

An epileptic patient with intracranial electrodes implanted in the hippocampus answering questions about the content of the patient's thoughts while measuring electroencephalography. Credit: 🄫Medical Fig.

Methodology for studying brain activity

To study the relationship between these sharp ripples and different types of thoughts, the research team used information collected when patients with drug-resistant epilepsy are about to undergo surgery (to remove the starting point of the epileptic activity in the brain). Intracranial electrodes are implanted in the hippocampus of these patients and brain activity is monitored continuously, so that surgeons can identify the epileptic region and be sure not to remove a part of the brain that would have unintended consequences.

“We asked patients undergoing this electroencephalographic brain monitoring for 10 days to complete an hourly questionnaire regarding their thoughts and emotions,” explains the study's lead author, Takamitsu Iwata. “We mainly wanted to see if we could identify any connections between the recorded brain activity and how patients were feeling and thinking at the time.”

Questionnaire used to probe the content of thoughts and feelings. Credit: Adapted from Iwata et al., Nature Communications, Sharp hippocampal undulations correlate with periods of naturally self-generated thoughts in humans; DOI: 10.1038/s41467-024-48367-1; under CC BY 4.0 license

Results on brain activity and thought patterns

In general, sharp hippocampal ripples were generated in patients at night (presumably during sleep). Additionally, the research team noticed a link between increased high-wave activity and more vivid or imaginative and less desirable or task-related thoughts, that is, when their minds wandered .

“Although our study was conducted entirely on people with epilepsy, we did our best to remove epilepsy-related data so that the results were applicable to healthy populations,” says Takufumi Yanagisawa, lead author of the study. “The similarities between many of our results and those of previous studies, using other species or methods, indicate that our approach worked well.

The larger the letters in the word cloud, the greater the effect on the high wave ripples. Credit: Adapted from Iwata et al., Nature Communications, sharp hippocampal undulations correlate with periods of naturally self-generated thoughts in humans; DOI: 10.1038/s41467-024-48367-1; under CC BY 4.0 license

Implications for understanding brain functions

There is growing evidence that self-generated brain states, including mind wandering and intrusive thoughts, have complex links to intelligence, autism, attention deficit disorder, and happiness /well-being. A better understanding of the brain regions and the activity behind these states could therefore help people suffering from various pathologies.

Reference: “Sharp hippocampal undulations correlate with naturally self-generated periods of thought in humans” by Takamitsu Iwata, Takufumi Yanagisawa, Yuji Ikegaya, Jonathan Smallwood, Ryohei Fukuma, Satoru Oshino, Naoki Tani, Hui Ming Khoo and Haruhiko Kishima, May 22, 2024, Natural communications.
DOI: 10.1038/s41467-024-48367-1