While it is widely accepted and scientifically proven that human beings have a selective memory – we forget distracting memories to focus on what matters – little is known about the capacity to forget in other mammalian species. However, new University of Cambridge research published in Nature Communications reveals that rats, much like humans, possess the ability to actively forget distracting memories.
In “A retrieval-specific mechanism of adaptive forgetting in the mammalian brain,” researchers Pedro Bekinschtein, Noelia V. Weisstaub, Francisco Gallo, Maria Renner, and Michael C. Anderson point out that the human brain is estimated to include approximately 86 billion neurons, and 150 trillion synaptic connections. In order to properly process and store memories, and avoid information overload, the human brain has to actively forget distracting memories.
“People are used to thinking of forgetting as something passive. Our research reveals that people are more engaged than they realize in actively shaping what they remember of their lives. The idea that the very act of remembering can cause forgetting is surprising and could tell us more about people’s capacity for selective amnesia,” Anderson said.
Although so-called selective amnesia improves the efficiency of human memory, it can sometimes lead to problems. For instance, when law enforcement questions a witness, the individual is often unable to recall whatever information their brain deemed distracting, so law enforcement officers use the technique of repeated questioning to help the witness recall details important to the case.
As it turns out, the ability to selectively memorize information is not unique to our species. Rats, too, actively forget distracting memories using a similar brain mechanism, which suggests that other mammals do this as well, the Cambridge University researchers claimed.
In order to prove their hypothesis, the researchers group-housed 129 male adult Wistar rats between two and three months of age. To demonstrate that rats have selective amnesia, the researchers – given rats’ sense of curiosity, and the tendency to explore new environment – devised a simple task.
They allowed rats to explore two objects (i.e. cup, toy, ball) – object A, and object B – in an open arena. The animals explored object A for five minutes, then were removed from the arena, and subsequently placed back in it with object B.
In order to prove that rats displayed selective amnesia, the researchers performed so-called “retrieval practice” on the aforementioned object A. During this stage of the experiment, the researchers repeatedly placed the rat in the arena with the object they wanted the animal to remember (A), together with an unfamiliar third object.
Given rats’ sense of curiosity, they preferred to explore new objects. This, according to the researchers, implies that they had remembered one of the two objects (A), considering it old news, and instead focused on the unfamiliar object.
Thirty minutes later, in order to explore how retrieving object A affected rats’ subsequent memory for B, the researchers placed the rodent into an arena with object B and an entirely new object. The rats explored the new object, and the object B equally. Meaning, they remembered their experience with object A and trained themselves to forget object B, displaying selective amnesia.
In control conditions, meaning relaxing time instead of retrieval practice, rats remembered object B as well.
“Rats appear to have the same active forgetting ability as humans do – they forget memories selectively when those memories cause distraction,” Anderson explained.
But what controls the active forgetting mechanism?
According to the Cambridge University researchers, an area towards the front of the rat’s brain called the medial prefrontal cortex. The researchers tested this by temporarily “switching off” the medial prefrontal cortex using muscimol. When dosed with muscimol, the rat would lose the ability to selectively memorize information, even when undergoing the retrieval practice.
“[Rats] use a similar prefrontal control mechanism as we do. This discovery suggests that this ability to actively forget less useful memories may have evolved far back on the ‘Tree of Life’, perhaps as far back as our common ancestor with rodents some 100 million years ago,” said Anderson.