What did you do last Friday night?

Searching for an answer, you might first recall where you woke up on Saturday: your girlfriend's place, so you must have had a date the night before. Then you think of your favorite restaurant near her place, and recall that it was closed, so you opted to cook at home. Next you remember that you watched The Shining, because you had just finished reading the book, and that you argued over whether the novel was better.

Our brains are constantly searching mental time and space in this way: looking for the hooks that will retrieve events from the past. Scientists have spent more than a hundred years studying this phenomenon, but the mechanics of what psychologist Endel Tulving called "mental time travel" — the ability to relive the past in the mind — remains largely mysterious. If we can understand how our minds retrieve events, then we might find ways to keep those hooks glinting clearly in the murk of memory, especially as we age.

What seems to happen to older people, particularly those with dementia or Alzheimer's disease, is that the part of the brain responsible for searching memory degrades, making it harder to isolate moments in history. If you ask your grandmother, "Remember that time we went to the beach?" she may well recall other details from the summer in question, but struggle to come up with the specific memory of a beach on her own. This is because, according to one hypothesis, she lacks the ability to conjure the "gist" of a memory — to orient her attention to the area of the mind where the memory she is looking for should live.

One of the first brain regions to degrade in Alzheimer's and related forms of dementia is the front tip of the structure used to make memories: the seahorse-shaped part beneath the temples, called the hippocampus. This area is the focus of a recent paper, published in Proceedings of the National Academy of Science, on how time and space are mapped in our brains.

The scientists from Ohio State University gave subjects smartphones, worn on a string around their necks for a month. The phones were programmed to snap photos throughout the day, tagged with the time and location. Later, the subjects — nine women between ages 19 and 26, from the university area — viewed 120 of the photos, and tried to retrieve a vivid memory of what was happening in each. By studying brain scans of the subjects viewing the pictures — some of which were remembered, others forgotten — the researchers honed in on a region representing the where and when of memories. Also: how these memory coordinates are represented in the brain.

"Place cells" were found in rodents decades ago, a discovery for which Norwegian and British scientists shared last year's Nobel Prize in Medicine. These neurons in the hippocampus fire for particular locations in space, as an animal walks around. During sleep, they "replay" sequences from waking activity, firing in the same order. This suggests the experience is being rehearsed, and converted into memory. The same thing happens during quiet waking, researchers found later, and this "cortical reactivation" of experience correlates with learning. A recent study on people with epilepsy found that place cells tuned to locations in a video game space reactivated when the same scenes were remembered later.

But the time and space in all these experiments were limited to the narrow and artificial context of a lab. "We won't understand memory for our life until we understand the structure of our lives," explains Per Sederberg, the head of the psychology lab at Ohio State University that published the new study.

The study by Sederberg's lab is among the first to study memory in a broader context. It is also the first to use a machine-learning tool to track not only where the brain is active, but how patterns of brain activity tag memories in time and space. By comparing neural patterns tied to memories of different locations and times, the researchers found that one brain region, the front tip of the left hippocampus, represents memories in terms of where and when they took place. The closer together two events were in time and space, the more similar were in the patterns of brain activity representing the events.

The authors' conclusion is that this region of the brain represents the "gist" of experiences. Since it encodes similar memories of events that took place in close proximity, this area is able to integrate experiences that are alike, in order to extract the essence of what makes them related. This may be why the front of the hippocampus is necessary to target memory searches, and is damaged in memory disorders.

"What if I ask you, 'What did you do for your 19th birthday?'" says Sederberg, who has studied human memory for 20 years. The implicit questions we use to isolate such an event, he points out, start with the gist of what we want, and gradually narrow: Where was I when I was 19? College, second year. Who were my friends then? What were our favorite restaurants? Ah yes, the Indian restaurant... Sederberg suggests that gist is likely supported by the hippocampus' front portion, where memories, his study shows, are represented at scales as large as 30 kilometers, across a time period as broad as a month.

"This experiment shows what you can do with human fMRI [functional magnetic resonance imaging] that you can't with animal studies," says Jeffrey Zacks, a professor of psychology at Washington University in St. Louis and an expert on event memory, who was not involved in the study. "In this case, stretching out the temporal and spatial scale."

On the other hand, he cautions, the focus of the study on space and time may overstate the extent to which memory is organized in these objective dimensions.

"Psychological space is not the same as physical space," he points out. "Our mental maps are distorted, chunked into landmarks and events."

When you remember reading The Shining last week, then recall watching the movie, the leap you make is through meaning, not time or space. Although experience flows past our senses in a continuous stream, people recall the past in discrete episodes. As Zacks' lab and others have shown, this organization of time into units is influenced by age, knowledge, and experience. So time and space can't be the only coordinates our brain uses to map the past.

Some researchers have even questioned whether space is strictly what "place cells" encode, since locations correlate with other factors, such as emotion. The part of a maze where a mouse expects to find cheese, cocaine, or an orgasm button is likely to be exciting, for example, while one where he has been shocked will produce fear. The humans in the new experiment have different degrees of familiarity and emotion associated with the places they passed during the month of their participation: a workplace might trigger stress; a family home comfort; a romantic partner's dorm room excitement. As a researcher once said to Zacks, "Time is not just a line that you chop into events. Lives aren't flat."

Nevertheless, the new discovery is a huge step forward in the mapping of memory, even if the picture is not yet complete.

"They do an excellent job of motivating the experiment to address a specific hypothesis," Zacks says. The result is impressive, he says: "a unified, cross-species account of the medial temporal lobe's function in memory."

Memory scientists have ideas where to cast their hooks next. Sederberg says his lab needs to replicate its findings on a bigger sample, and with men. Next, he wants to collect life logs and brain scans from older people and Alzheimer's patients. If, say, a person with Alzheimer's is shown to represent life memories differently than a healthy person, then brain scans might help diagnose the condition or track improvement. People with traumatic brain injuries that affect memory can be better treated if the holes in their memories are identified more precisely. Phobias and traumatic memories could also be studied this way.

The best prospect for memory disorders, though, may be not brain scans, but the life-logging tool itself: mapping the structure of daily life. Memory often improves when people are shown how to distinguish events, on video.

"What if you showed people their actual lives, instead of a movie?" Sederberg wonders. "If you showed people segmented periods of the day, before they went to bed, would their memories improve?"