It began about a decade ago at Syracuse University, with a set of equations scrawled on a blackboard. Marc Howard, a cognitive neuroscientist now at Boston University, and Karthik Shankar, who was then one of his postdoctoral students, wanted to figure out a mathematical model of time processing: a neurologically computable function for representing the past, like a mental canvas onto which the brain could paint memories and perceptions. "Think about how the retina acts as a display that provides all kinds of visual information," Howard said. "That's what time is, for memory. And we want our theory to explain how that display works."

Episodic memory, the conscious recollection of past events, is typically experienced from a first-person (egocentric) perspective. The hippocampus plays an essential role in episodic memory and spatial cognition. Although the allocentric nature of hippocampal spatial coding is well understood, little is known about whether the hippocampus receives egocentric information about external items. We recorded in rats the activity of single neurons from the lateral entorhinal cortex (LEC) and medial entorhinal cortex (MEC), the two major inputs to the hippocampus. Many LEC neurons showed tuning for egocentric bearing of external items, whereas MEC cells tended to represent allocentric bearing.

Researchers have discovered how our brain keeps track of time. They say a special network of brain cells expresses our sense of time within experiences and memories. It essentially provides timestamps for events, and keeps track of the order of them - rather like a filing system. The illustration shows the episodic time from the experience of a 4-hour-long ski trip up and down a steep mountain, including events that alter the skier's perception of time. The idea is that experienced time is event-dependent and may be perceived as faster or slower than clock time.The newly discovered neural record of experienced time is in the lateral entorhinal cortex (LEC) in green.