“I am hopelessly in love with a memory.
An echo from another time, another place.”
Humans are obsessed with memory.
There are references to memory and how we remember things throughout history—in film, music, art, science, and philosophy. Humans have a wistful affection for the past and we often spend time replaying vignettes from our lives.
Philosophers and scientists began searching for a place in the brain that stores memory, which was referred to as the “engram.” In the 1920s, Karl Lashley performed memory experiments on animals in search of this elusive engram.1 But it wasn’t until the 1950s that researchers were given the first real glimpse into the the neuroscience of memory.2
A Brief History of Memory Research
Henry Molaison (known as H.M. until his death in 2008) underwent a resection of his temporal lobe in order to lessen the burden of his epilepsy. Shortly following this surgery, he became unable to create and store new memories.2 Thousands of studies later, neuroscientists have accumulated mounting evidence pointing to the hippocampus as the seat of episodic memory in the brain.3
The hippocampus, which is Greek for seahorse, is a bilateral curve-shaped structure located deep in the brain. Almost all of H.M.’s hippocampus was removed during this surgery, which resulted in his prominent short-term memory loss.2
H.M.’s hippocampal memory loss has been replicated in animal studies in which the hippocampus is damaged or removed and in human studies using fMRI, where hippocampal activity was shown to be associated with memory events.3
Since then, neuroscientists have used more powerful techniques to study memory. Different regions of the brain can be analyzed by measuring gene expression, and electrical activity that is emitted by neurons can be studied to understand memory processing.4
A Moment in Time
Scientists have proposed that there are three components that make up a single memory: “what,” “where,” and “when.” By integrating these three parts, we can successfully recall the time, place, and event that make up a single memory.
Memory researchers have learned that different regions of the hippocampus, such as the dorsal (top part) and ventral (bottom part) or subregions (CA1 and CA3), contribute differently to memory for “what,” “where,” and “when.” 5,6
Researchers have also demonstrated that the hippocampus works with many other brain structures in order to encode and store memories.6
The Hippocampal Circuit and Memory for Time
Although memory for “what” and “where” have been well described in the literature, less was known about memory for time until recently.6
Neuroscientists knew that the hippocampus is involved in memory for the order of events and memory for sequences but memory for “when” remained less clear.
Researchers at Boston University recently identified cells in the hippocampus that they call “time cells,” which encode specific moments in time as an animal performs a behavioral task.7
They later found that these “time cells” can also be found in the entorhinal cortex. This region was previously believed to only be involved with spatial processing.8 But their data suggests that specially tuned cells in the hippocampus record the temporal context of a memory, which is then integrated with “what” and “where” information, allowing for recall of a single memory event.
The Latest Research on Human Memory
Another recent study (published in September 2015) at the Ohio State University looked at how the human hippocampus stores memories. Although the hippocampus has previously been shown to represent space and time, they wanted to know how the hippocampus represents time over longer scales necessary to support human episodic memory events.
The researchers used fMRI to record brain activity while participants relived their own experiences cued by photographs. They found that the anterior hippocampus “records memories formed over a month and occurring over distances of up to 30km,” and that “a similar pattern of spatiotemporal organization also exists for organizing distinct memories that are distant in space and time.”
We are still just scratching the surface of this incredibly complex process. Memory is a beautiful gift and one we often take for granted until it starts to slip away.
1. Lashley KS. In search of the engram. Symp Soc Exp Biol. 1950;4:454.
2. Scoville WB, Milner B. Loss of recent memory after bilateral hippocampal lesions. J Neurol Neurosurg Psychiatry.1957;20:11-21.
3. Burgess N, et al. The human hippocampus and spatial and episodic memory. Neuron. 2002;35:625-641.
4. Kandel ER. The molecular biology of memory: cAMP, PKA, CRE, CREB-1, CREB-2, and CPEB. Mol Brain. 2012;5:14.
5. Tsien JZ, et al. The essential role of hippocampal CA1 NMDA receptor-dependent synaptic plasticity in spatial memory. Cell. 1996;87(7):1327-1338.
6. Eichenbaum H, et al. The medial temporal lobe and recognition memory. Annu Rev Neurosci. 2007;30:123-152.
7. Eichenbaum H. Memory on time. Trends Cogn Sci. 2013;17(2):81-88.
8. Kraus BJ et al. During running in place, grid cells integrate elapsed time and distance run. Neuron. 2015;88;3(4):578-589.
9. Nielson DM, Smith TA, Sreekumara V, Dennisa S, Sederberg PB. Human hippocampus represents space and time during retrieval of real-world memories. PNAS. 2015;112(35):11078-11083.