A groundbreaking research effort has started to clarify one of the key enigmas in neuroscience: how the human brain processes and understands the continuum of time along with experiences.
A groundbreaking research effort led by UCLA Health has started to clarify one of the key enigmas in neuroscience: how the human brain processes and understands the continuum of time along with experiences.
The research, which appears in the journal Nature, has captured the activity of individual neurons in humans. It discovered that certain brain cells fired in patterns that corresponded closely to the order and structure of a person’s experiences. Remarkably, the brain retains these distinct firing patterns even after the experience has ended and can swiftly replay them during periods of rest. It can also use these established patterns to prepare for future stimuli that follow the initial experience. These results deliver the first concrete evidence on how certain brain cells amalgamate information regarding “what” and “when,” enabling the brain to form and sustain representations of experiences throughout time.
Dr. Itzhak Fried, the senior author of the study, mentioned that these findings could contribute to the advancement of neuro-prosthetic devices aimed at improving memory and other cognitive functions, as well as provide insights into artificial intelligence’s comprehension of human cognition.
“Identifying patterns from experiences over time is essential for the human brain to construct memory, forecast potential future outcomes, and inform behavior,” noted Fried, who serves as the director of epilepsy surgery at UCLA Health and is a professor of neurosurgery, psychiatry, and biobehavioral sciences at the David Geffen School of Medicine at UCLA. “However, the way this process occurs in the brain at the cellular level has remained a mystery until now.”
Prior investigations, including work by Dr. Fried, employed brain recordings and neuroimaging techniques to comprehend how the brain manages spatial navigation. This research indicated that two regions of the brain—the hippocampus and the entorhinal cortex—play crucial roles. These regions collaborate to create what is known as a “cognitive map.” Neurons in the hippocampus serve as “place cells,” indicating when an animal is located in a specific area, similar to marking an ‘X’ on a map, while neurons in the entorhinal cortex act as “grid cells,” providing a measure of spatial distance. Initially identified in rodents, these cells were later discovered in humans by Fried’s team.
Additional research has shown that similar neural mechanisms also represent non-spatial experiences, including time, sound frequencies, and the characteristics of objects. One significant discovery by Fried and his colleagues involved “concept cells” found in the human hippocampus and entorhinal cortex, which respond to specific people, places, or unique objects, and are vital for memory formation.
To investigate how the brain processes events over time, the UCLA study recruited 17 participants with severe epilepsy who had been fitted with depth electrodes in their brains for treatment purposes.
Researchers recorded the brain activity of participants as they engaged in a complex set of tasks that included behavioral challenges, pattern recognition, and image sequencing.
Initially, participants went through a screening phase where they viewed about 120 images of individuals, animals, objects, and landmarks on a computer over a span of approximately 40 minutes. They were instructed to complete various tasks, such as discerning whether an image depicted a person. The images included famous faces and well-known locations and were chosen partly based on each participant’s preferences.
Subsequently, a three-phase experimental procedure followed, where participants performed behavioral tasks in reaction to images shown in random placements on a pyramid-shaped graph. Six images were selected for each participant.
In the first phase, images appeared in a pseudo-random arrangement. The second phase was based on the image’s position on the pyramid graph, while the final phase repeated the first. During each image viewing, participants engaged in unrelated behavioral tasks, such as determining the gender of individuals in the images or whether an image was a mirror of the previous one.
Through their analysis, Fried and his research team noticed that the neurons in the hippocampus and entorhinal cortex began to adjust their activity to align with the sequence of images displayed on the pyramid graphs. These patterns emerged naturally, without explicit instructions to the participants, as stated by Fried. The neuronal firing patterns also indicated the likelihood of upcoming stimuli and retained their organization even after the task was finished.
The study’s lead author was Pawel Tacikowski, along with co-authors Guldamla Kalendar and Davide Ciliberti.
“This research reveals for the first time how the brain employs similar mechanisms to depict what seem to be vastly different types of information: space and time,” Fried concluded. “We have shown at the neuronal level how these representations of object movements over time are integrated within the human hippocampal-entorhinal system.”
