The hippocampus is a structure found on both sides of the brain in the temporal lobes, near the ears. It plays an important role in memory and thinking about the past and future. This led a team of researchers, led by Cornelia McCormick at the Wellcome Centre for Human Neuroimaging, to wonder if people with damage to both hippocampi are still capable of mind-wandering – after all, when we mind wander or day-dream, a lot of the time it is about things we’ve done or plan to do. And if these patients can mind wander, will the content of their mind-wandering thoughts be different from healthy controls?
For their new paper in the Journal of Neuroscience, the researchers shadowed 6 male patients with bilateral hippocampus damage for two days during daylight hours, occasionally prompting them to report what they were thinking about, and compared their descriptions with those obtained from 12 age-matched healthy controls over the same period. The patients with hippocampus damage mind-wandered just as much as the controls, but the form and content of their mind-wandering was very different.
The researchers followed the patients (who had small but highly specific lesions to the hippocampus, caused by a form of encephalitis) and controls while they were attending a psych lab for two days of brain scans and other tests. Twenty times during these eight-hour days, the researchers prompted the participants with the question: “What were you thinking about just before I asked you?”. They timed these prompts deliberately during quiet periods when mind-wandering might be especially likely, and immediately transcribed the participants’ answers.
Coding the answers, the researchers found extremely high rates of mind-wandering – defined as having thoughts disengaged from the outside world (or perceptually “decoupled”) – among both patients and controls, of around 80 to 90 per cent. The high rates of mind-wandering are not such a surprise, at least among the controls, because of the way the researchers had timed their prompts. For comparison, past research suggests that we mind-wander about 30 to 50 per cent of our waking lives.
Crucially, the patients’ mind-wandering thoughts were very different from the healthy controls’. Whereas the controls’ mind-wandering was mostly episodic (about events past and present) and consisted of visual scenes, the patients’ mind-wandering was largely semantic (about facts) and verbal. “Small, selective lesions of their hippocampus dramatically affected the nature of their mind-wandering,” the researchers said.
This contrast in mind-wandering between patients and controls is in the context that on neuropsychological testing the hippocampus patients had impaired episodic memory, but performed normally in all other ways, including on tests of working memory, so it is highly unlikely they were simply unable to recall their mind-wandering thoughts. They also had no trouble recalling the nature of two other experiments they took part in, which covered similar time-scales to the mind-wandering research.
We know from research with healthy people that mind-wandering depends on activity in a network of brain regions known as the “default mode network”, which includes the hippocampus. Also, people who report particularly rich and detailed mental time travel during mind-wandering tend to have stronger connectivity between their hippocampi and other key regions of the default mode network. However, this new study is the first to suggest strongly that an intact hippocampus is necessary for “normal” mind-wandering involving mental time travel and vivid visual scenes. The results also complement other research showing that hippocampus patients struggle to imagine the past and future when prompted to do so, revealing that this impairment extends to the content of their spontaneous thought.
“By showing [that the hippocampus] plays a causal role in a phenomenon as ubiquitous as mind-wandering, this exposes the impact of the hippocampus beyond its traditionally perceived role in episodic memory, placing it at the centre or our everyday mental experiences,” the researchers concluded.