Seahorse and friends: a stroll down memory lane.

I think it’s fitting to begin my first official ‘factual’ post with a fun filled, informal yet informative, trip through my own area of research.

I study a region of the brain known as the hippocampus. This area takes its name from the Latin for seahorse (from ancient Greek: hippos, “horse” and kampos, “sea monster”). The picture below shows a human hippocampus removed from its surrounding brain structures and nicely illustrates where this name comes from:

                                (Source Professor Laszlo Seress, University of Pecs.)

Our hippocampi are located bilaterally (one on either side of the brain) nestled on the underside of the temporal lobe. The image below shows the hippocampus in blue (looking decidedly less like a seahorse) sitting within the temporal lobe, shown in red.

If you were to take the hippocampus pictured above and cut it in half you would find that the inside is not homogeneous, in fact it consists of a number of interlocking cellular semicircles arranged in a circuit (looking rather similar to a swiss roll).

Cells within these curves receive information from a number of brain regions, including visual, olfactory (smell) and auditory (hearing) systems. The cellular circuits found within the hippocampus are believed to be vital for combining such information together to form memories. Indeed, individuals who lack a functioning hippocampus often suffer from a specific inability to form new memories (anterograde amnesia), as illustrated in the films below.

At this point I have the desire to be insanely pedantic, and it’s my blog so I will be! In both these movies the characters are said to have lost their short term memory however, this is not technically correct. Although there is significant debate over the precise functions of the hippocampus, the memory loss experienced by these patients is more likely to represent an inability to transfer new experiences into a long term store rather than representing damage to a short term specific memory system. Or to be significantly less pedantic we can always just say that their swiss roll seahorse is on the frits!

Interestingly damage to the hippocampus does not interfere with all forms of memory. Indeed, individuals with hippocampal damage retain the capacity to learn new skills and often express a sense of familiarity towards recently observed objects, although without any conscious awareness of these memories. For example the, now famous, patient HM (Henry Gustav Molaison) who suffered severe damage to both his left and right hippocampi, retained the capacity to learn new skills such as mirror writing. However, whenever HM was asked to conduct a mirror writing task he argued that he was unable to do so and was always shocked upon discovering his proficiency.
Try it yourself: Print out the star pattern below and hold a small mirror next to this, now (looking only in the mirror) attempt to trace around the star remaining within the two outer lines – the task is much harder than you might think!

Therefore it is believed that the hippocampus is only part of a much larger memory system. Indeed, there are a number of structures situated around the hippocampus within the medial temporal lobe which may all be crucial for different types of memory. I.e. emotional/ fear memory has been linked to the amygdala (a fun link for auditory learners out there ^_^) also object recognition may require the perirhinal cortex. However, the hippocampus is still ultimately recognised as being central to the process of linking many separate aspects of an experience together to form a full flavored memory.

My specific line of research explores how individual cells within the hippocampus form long lasting memories. This basically involves me being a ‘fly on the wall’ listening in on conversations between these cells and understanding how different external factors can influence their communication. This is important for understanding not only how and why we form certain memories but also what happens when things go wrong, for example during debilitating diseases such as Alzheimer’s.

Post by: Sarah Fox