Could déjà vu be explained by grid cells?
—Robyn Ganeles, San Francisco

Neuroscientist Edvard I. Moser of the Norwegian University of Science and Technology responds:

This is a great question, because grid cells, which are involved in processing spatial information about our surroundings, are located in a brain region that is part of a larger memory system thought to be responsible for the feeling of familiarity. After considering their function in detail, however, I think it seems more likely that a different system of neurons, place cells, plays a stronger role in providing us with the sense that a new locale is familiar—a feeling called “déjà visité.”

In any environment, the brain must keep track of the distinct locations within the surrounding area (say, at the kitchen table versus in front of the refrigerator). It also must note how these different locales relate to one another (the table is three feet to the right of the fridge, for instance). Place cells are involved in the former type of processing; each place cell corresponds to a specific location in an environment and fires when you pass through that spot.

In contrast, grid cells work in a network to produce a kind of internal coordinate system, noting information about distance and direction. These neurons do not correspond to a specific location but become active across several regularly spaced points in any setting. The geometric arrangement of these cells, relative to one another and to the external setting, ultimately helps us form a mental map of a certain environment.

Grid cells are located in the entorhinal cortex, a brain region that processes information before sending it to the hippocampus, the area where place cells are located. Because we know that place cells have a unique firing pattern for nearly every experience, it is likely that the hippocampus, and not primarily the entorhinal cortex, decides whether a location is novel or being revisited. When a strange place is experienced as familiar, it may be because the activated ensemble of place cells at that location happens to be similar to a pattern of activity that was elicited by a previous locale.

Do the typical sleep schedules of elderly people have a physiological benefit, or do they simply reflect generational trends?
—Shannon Atkinson, Raleigh, N.C.

Michael V. Vitiello, professor of psychiatry and behavioral sciences at the University of Washington, replies:

It is unlikely that the “typical sleep schedules of elderly people” either reflect generational trends or convey any physiological benefit. There is no evidence to support the idea that the typical sleep schedules of older adults in developed countries—marked by earlier sleep and rise times, less total sleep and more nighttime wakefulness—are cohort effects (that is, result from having grown up or lived during a specific time period). The few studies that have looked at how sleep patterns change as people age show slow and progressive changes in sleep patterns across the human life span. If there were specific cohort or generational differences, this pattern of regular, progressive change would be much less predictable.

Given that the sleep patterns of older adults appear long after these individuals are capable of reproduction, they probably do not confer any physiological benefit. It is much more likely that they reflect biological and social changes that occur as people age. Biological changes include alterations in the body’s underlying circadian rhythm, which helps to regulate the timing and depth of sleep, and age-related reductions in the homeostatic sleep drive, the metabolic process that causes the inclination to sleep after a period of wakefulness.

Other factors likely to influence the quality and timing of sleep in older adults include the increased prevalence of illnesses, such as osteoarthritis, that can directly disrupt sleep, and the presence of primary sleep disorders, such as obstructive sleep apnea or restless legs syndrome. Various behaviors and environmental factors can also disrupt sleep. Many of these variables, such as irregular sleep sched­ules, staying in bed too long, and bedding or a bedroom that is not conducive to sleep, can be adjusted for improved slumber.