Researchers in the UK are challenging the long-established idea that our brains use different mechanisms for making long and short term memories: they suggest that while some mechanisms are separate, other mechanisms are shared.
Led by Professor Emrah Duzel of the Institute for Cognitive Neuroscience at University College London (UCL), the researchers have written about their findings in a paper to be published online this week in the Proceedings of the National Academy of Sciences.
Amnesia, a condition that is commonly caused by injury to the hippocampi located deep inside the temporal lobes, severely affects a person’s ability to make long-lasting memories.
However, as long as they are not distracted, patients with amnesia can recall short pieces of information like phone numbers quite well for short periods of time. This is what led to the belief that the hippocampus is involved in making long-term but not short-term memories, a notion that Duzel and colleagues’ work now throws into doubt.
For the study, Duzel and colleagues studied patients with “temporal lobe epilepsy with bilateral hippocampal sclerosis” which severely impairs the hippocampi.
They asked them to memorize photographs of normal every day scenes such as a living room with table and chairs. The researchers then tested the patients’ memory of the images while recording their brain activity with magnetoencephalography (MEG). They did this twice: after a short period of 5 seconds and then after 60 minutes.
When they tested the patients’ standard memory, for instance could they distinguish between images they had seen before and new images, they could do this easily at 5 seconds, but with difficulty at 60 minutes. This was expected and fit in with the established idea that short term memory would be good while long term memory would be impaired.
However, when they then asked the patients at 5 seconds to recall details of the scenes they had memorized, such as was the table on the left or the right of the chairs, they couldn’t.
The researchers said that detailed short term memory requires the coordinated activity of a network of visual and temporal brain areas, whereas standard short-term memory drew on a very different network. It was clear that the the coordination between visual and temporal brain areas was disrupted in the patients with hippocampal sclerosis.
“As we anticipated, the patients could not distinguish the studied images from new images after 60 minutes – but performed normally at five seconds,” said Duzel.
“However, a striking deficit emerged even at five seconds when we asked them to recall the detailed arrangement of objects within the scenes,” he added.
Duzel said these findings show there are:
“Two distinct short-term memory networks in the brain: one that functions independently of the hippocampus and remains intact in patients with long-term memory deficits and one that is dependent on the hippocampus and is impaired alongside long-term memory.”
The first author of the paper, Nathan Cashdollar from the Institute of Neurology at UCL said:
“Recent behavioural observations had already begun challenging the classical distinction between long-term and short-term memory which has persisted for nearly half a century.”
“However, this is the first functional and anatomical evidence showing which mechanisms are shared between short-term and long-term memory and which are independent,” he added, explaining that the study also highlights that patients with “impaired long-term memory have a short-term memory burden to carry in their daily life as well”.
— Proceedings of the National Academy of Sciences
Source: University College London.
Written by: Catharine Paddock, PhD