Daydreaming, thinking about the past, planning for the future, or just letting your mind wander off from the current moment occupies our thoughts for a large part of every day. However, a new study has revealed patients suffering from a specific kind of early-onset dementia may have completely lost the ability to do this and seem perpetually "stuck in the moment".

The research set out to compare how different structural changes in the brain can affect a person's ability to engage in more abstract, introspective thought processes. Three groups of subjects were gathered for the study. Alongside 37 cognitively healthy controls, the research included 24 Alzheimer's patients and 37 subjects with frontotemporal dementia.

Frontotemporal dementia is a type of early-onset dementia that usually begins to manifest in a person's late 40s. It is defined by a progressive loss of spindle neurons in the frontal and/or temporal lobes.

"Individuals with frontotemporal dementia become very rigid in their thinking," explains Associate Professor Muireann Irish from the University of Sydney. "They are unable to visualize alternatives, to think of solutions to problems, or to deviate from their everyday routines. In previous work, we have shown that their ability to remember the past and to imagine the future is severely compromised. Simply put, these individuals are stuck in the moment."

To empirically evaluate each subject's mind-wandering capacity, they were shown static geometric shapes on a computer screen. The idea is that a healthy person's mind will inevitably move on to think about broader things due to the minimal stimulus. So after each specific presentation of a stimulus the subjects were asked to recount to researchers what thoughts occurred to them while watching the screen.

"We found all of the healthy older adults engaged extensively in mind wandering, allowing their thoughts to drift away from the immediate stimulus to more interesting scenarios and ideas," explains Irish. "But the participants with frontotemporal dementia were completely tethered to the stimulus in front of them. When asked what they were thinking about, they either reported 'Nothing' or that they were thinking only about the stimulus itself."

In a surprising finding however, the researchers discovered the Alzheimer's subjects reported the same volume of mind wandering as the healthy control group. This intriguing discovery suggests the very specific neuronal damage that occurs in frontotemporal dementia seems to correlate with the brain networks associated with introspective thought. And it also suggests that those patients suffering from Alzheimer's disease still have the ability to let their mind drift in ways that are different from other kinds of dementia.

The researchers believe this novel discovery offers a compelling insight into how certain neurodegenerative alterations can manifest in specific behavioral changes. "Moreover," adds Irish, "it allows us a unique glimpse of what it would be like to lose a fundamentally human capacity."

The new research was published in the journal PNAS.