How does the brain process experiences over time?

Experts at UCLA Health have begun to uncover one of the fundamental mysteries in neuroscience – how the human brain encodes and interprets the flow of time and experiences. 

Brain cell activity reflects experiences 

The experts directly recorded the activity of individual neurons in humans and revealed that specific brain cells fire in a way that reflects the order and structure of a person’s experiences. 

Notably, the brain retains these firing patterns after the experience has ended and can rapidly replay them during periods of rest. 

The brain also utilizes these learned patterns to prepare for future stimuli, offering the first empirical evidence of how brain cells integrate “what” and “when” information to retain representations of experiences over time.

Cognition in the human brain

Dr. Itzhak Fried, the study’s senior author, noted that these findings could be pivotal in developing neuro-prosthetic devices designed to enhance memory and other cognitive functions. 

The study’s results may also provide insights into artificial intelligence’s understanding of cognition in the human brain.

“Recognizing patterns from experiences over time is crucial for the human brain to form memory, predict potential future outcomes and guide behaviors,” said Fried, director of epilepsy surgery at UCLA Health.

“But how this process is carried out in the brain at the cellular level has remained unknown – until now.”

Cognitive map in the brain

Previous research, including work by Fried, used brain recordings and neuroimaging to explore how the brain processes spatial navigation.

The studies showed that two regions of the brain – the hippocampus and the entorhinal cortex – play key roles. These areas, both critical for memory, work together to create a “cognitive map.” 

Hippocampal neurons function as “place cells,” marking specific locations like an ‘X’ on a map, while entorhinal neurons act as “grid cells,” measuring spatial distances. Initially discovered in rodents, these cells were later found to also exist in humans.

Further studies have demonstrated that similar neural processes are involved in representing non-spatial experiences such as time, sound frequency, and object characteristics. 

One of Fried’s seminal findings was the discovery of “concept cells” in the human hippocampus and entorhinal cortex, which respond to specific individuals, places, or distinct objects and are essential for memory.

Studying how the brain processes experiences 

To study how the brain processes experiences over time, the UCLA team recruited 17 participants with intractable epilepsy, all of whom had depth electrodes implanted in their brains for clinical treatment.

The researchers recorded the neural activity of these participants as they performed tasks involving pattern recognition and image sequencing. 

The experiment began with an initial screening phase where about 120 images of people, animals, objects, and landmarks were shown repeatedly over a 40-minute period. 

Participants were instructed to complete various tasks, such as determining whether an image showed a person. The images included famous actors, musicians, and places, based partly on the participants’ preferences.

Modifications in neural activity

The main experiment involved three phases, during which six images were displayed in random or structured order across a pyramid-shaped graph. 

Participants were tasked with behavioral responses unrelated to image positioning, such as identifying whether an image showed a male or female or if the image was mirrored from previous phases.

In their analysis, Fried and his team found that hippocampal-entorhinal neurons began to modify their activity in alignment with the image sequence on the pyramid graph. 

Remarkably, these patterns formed without any direct instruction to the participants. The neuronal patterns also reflected the likelihood of upcoming stimuli and remained encoded even after the tasks were completed.

Representing space and time

“This study shows us for the first time how the brain uses analogous mechanisms to represent what are seemingly very different types of information: space and time,” said Fried. 

“We have demonstrated at the neuronal level how these representations of object trajectories in time are incorporated by the human hippocampal-entorhinal system.”

These findings provide an unprecedented glimpse into how the brain processes the flow of time, potentially opening doors to new research in memory and cognition.

The study is published in the journal Nature.

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