Have you ever wondered how the human brain meticulously keeps track of the order of events in a sequence? Recent scientific exploration hints at the presence of ‘time cells’—neurons residing in the hippocampus that may act as the glue binding our memories in the correct sequence, facilitating accurate recollection.
Previous research, notably in rats, identified specific neuron assemblies believed to support event recollection and action sequence planning, providing initial evidence for these sequence-tracking time cells.
While much was known about the role of time cells in rats, the encoding of episodic memory in the human brain remained a mystery for an extended period. Seeking answers, a team of researchers, led by neuroscientist Leila Reddy from the Brain and Cognition Research Center (CerCo) in France, delved into the electrical activity of the brains of 15 epilepsy patients. Microelectrodes implanted in the hippocampus enabled a closer look without invasive procedures beyond those already part of the patients’ epilepsy treatment.
The researchers, as outlined in their 2021 study, explored whether human hippocampal neurons represented temporal information during the learning of a sequence of items, crucial for creating episodic memories.
Conducting experiments alongside medical tests, the participants were tasked with memorizing a predetermined sequence of images. Electrodes recorded the firing of specific neurons in the hippocampus during image display, gaps between images, and pauses where participants predicted the next image in the sequence.
According to the researchers, the observed neurons demonstrated characteristics of time cells, their activity modulated by temporal context within well-defined time windows. Interestingly, some neurons were active not only during memorization but also in the absence of visual stimuli, implying an ongoing encoding of the flow of time even in seemingly uneventful moments.
“Time cells were observed to fire at successive moments in these blank periods,” as explained in the researchers’ paper, shedding light on the evolving temporal signal during these gap periods without external event-driven influences.
The researchers propose that time cells in the human brain exhibit a “multi-dimensional” nature, capable of encoding temporal information while responding to various sensory stimuli. This multi-dimensional behavior may play a pivotal role in stitching together the ‘what,’ ‘where,’ and ‘when’ of experiences, contributing to the formation of coherent memories.
Addressing the phenomenon of subjective ‘mental time travel,’ the researchers emphasized its centrality to episodic memory, wherein the ability to vividly recall specific events in a particular place and temporal order is paramount. Their findings, reported in The Journal of Neuroscience, provide compelling evidence that human hippocampal neurons indeed represent the flow of time in our experiences.
