Hippocampal Cells Track Moments in Time

Post by Peter Imoesi

What's the science?

The ability of humans to recall the ‘what, where, and when’ for a series of past events is referred to as episodic memory. The ability to link these distinct events or experiences together has been associated with a set of cells known as hippocampal time cells in rodents. These cells have been found to play a crucial role in the sequential organization of memory. This week in the Journal of Neuroscience, Reddy and colleagues implanted microelectrodes into the hippocampus of epileptic patients to measure the activity of hippocampal time cells and their association with episodic memory.    

How did they do it?

All participants had been diagnosed with epilepsy that was non-responsive to drug treatment. Microelectrodes were implanted into the hippocampal region of the patients. Computed tomography (CT) co-registered with a magnetic resonance imaging (MRI) was used to confirm the exact location of the microelectrodes. Participants performed two sets of a sequence learning experiment in front of a computer. In the first experiment, patients were presented with a predefined set of images in sequential order. Each image was presented for 1.5 seconds, with a 0.5-second inter-stimulus interval (delay) before the next. Subjects were probed on their learning of the image, and a “trial” of interest was considered to be two consecutive probe events. The second experiment was identical except for a 10-second gap periodically inserted between some images, during which participants saw a blank screen.

What did they find?

The authors were able to record from 429 hippocampal neurons in the first experiment and 96 hippocampal neurons in the second experiment. In the first experiment, the authors established time cells within the hippocampus were controlled by temporal context. Specifically, they found that the activity of 111 cells was related to time, and the activity of another 50 cells was related to the identity of the images. The activity of a few other neurons was found to be influenced by a combination of time, image identity, and other factors. In the second experiment with the gap periods, the authors found a total of 26 hippocampal neurons were controlled by time. In addition, a few cells were responsive during the 10 second gap period. This suggests there are some hippocampal neurons that specifically respond to a changing temporal context.

What's the impact?

This study demonstrated the role of time cells within the hippocampal region of the brain. These time cells have the capacity to store sensory information in sequential learning in the presence or absence of a stimulus. The characterization of time cell function in humans will play a pivotal role in understanding the mechanisms underlying episodic memory.

Reddy et al. Human hippocampal neurons track moments in a sequence of events. Journal of Neuroscience (2021). Access the original scientific publication here.