How Your Brain Maps Time and Space

Have you ever wondered how your brain keeps track of the order of events in your life, allowing you to reminisce about the past and plan for the future? Recent studies conducted by neuroscientists have delved into the intricate workings of the human brain, uncovering the role of specialised neurons in encoding the dimensions of time and space, shedding light on the mechanisms that underpin our ability to reconstruct memories.

Unveiling the “Time Cells”

The human brain’s ability to keep track of the order of events in a sequence is thought to be facilitated by specialised neurons known as time cells. These neurons, located in the hippocampus, play a crucial role in linking distinct events of an experience with temporal fidelity, allowing for the proper recall of the sequence in which they occurred. Previous evidence for these sequence-tracking time cells was found in rats, but understanding how episodic memory is encoded in the human brain remained a mystery.

Neuroscientist Leila Reddy and her team at the Brain and Cognition Research Center (CerCo) in France conducted experiments with 15 epilepsy patients, monitoring electrical activity using implanted microelectrodes in the hippocampus.  The experiments involved presenting participants with a sequence of images and recording specific neurons firing in response during different phases of the task. According to the researchers, the neurons involved are evidence of time cells. During experiments, the researchers observed that time cells were not only active when participants were engaged in specific tasks, but also during periods of inactivity. Even in the absence of external stimuli, these “multi-dimensional” time cells fired at successive moments, encoding an evolving temporal signal. This suggests that the human brain’s time cells are capable of encoding information related to time while also responding to various sensory stimuli.

These findings suggest that the complex behaviour of these time neurons may contribute to recording the “what, where, and when” of experiences, weaving together coherent memories from a multitude of inputs and playing a role in the proper recall of sequences of events.

The Brain’s GPS: Time and Place Cells in Harmony

Building on the foundation laid by Reddy’s research, two studies led by Dr. Itzhak Fried from the UCLA David Geffen School of Medicine provided further insights into the simultaneous representation of time and place in the human brain. Using recordings of single neurons in patients with depth electrodes implanted for epilepsy treatment, Fried and his team identified “place cells” acting as the brain’s GPS system.

In the first study, where Dr. Daniel Schonhaut and Dr. Zahra Aghajan were co-authors, researchers first identified “time cells” that activated during waiting periods between patients’ searching and retrieval phases in a virtual gold mine navigation game. These “at-rest” time cells sequentially activated, suggesting a role in counting seconds during idle intervals. During game navigation, distinct “place cells” emerged as participants moved to specific locations, and a new set of “time cells” appeared at specific points in navigation. Notably, when patients alternated between searching and retrieving gold, place cells remained unchanged, firing consistently at the same locations, while time cells changed.

This study provides the first evidence of the coexistence of time and place cells in the human brain, suggesting that, at the neuron level, temporal and spatial contexts are dissociable. The researchers propose that these neuronal classes constitute a biological basis for a cognitive map of spatiotemporal context, serving as the scaffolding onto which memories are written.

Decoding Time: A Cinematic Journey in the Brain

In a second study, researchers explored how the brain tracks time during extended periods, such as watching a movie without interruptions. Fourteen neurosurgical patients watched an hour-long movie while their single-neuron activity was recorded from multiple brain regions. Surprisingly, some neurons displayed striking periodic activity across various time scales, ranging from tens of seconds to several minutes. This unique set of neurons allowed the decoding of time within the video, indicating their role in encoding temporal information.

When participants took a memory test after watching the movie, many time cells shifted their dominant timing to shorter scales, suggesting a potential role in temporal compression for memory retrieval. The researchers noted that the temporal periodicity of these cells they termed “temporally periodic cells” (TPCs) remained consistent even when the video was presented at different speeds. The ability to extract precise temporal information from TPCs highlights their role in encoding time and potentially contributing to memory retrieval.

In summary, the human brain’s remarkable ability to encode and retrieve memories appears to be orchestrated by a symphony of specialized neurons. Understanding these neural mechanisms not only deepens our knowledge of the brain’s inner workings but also opens avenues for exploring memory-related disorders and potential interventions.