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10 June 2015

Happy summer memories? It could all come down to smell

As it gets warmer, the molecules in our environment vaporise and fill the air with scents. These make their way into the outermost extreme of the brain – the nose – and stimulate neurons into firing.

By Michael Brooks

Why do we have many happy memories of summers past but far fewer of winter joy? It may all come down to the laws of physics. As it gets warmer, the molecules in our environment vaporise and fill the air with scents. These make their way into the outermost extreme of the brain – the nose – and stimulate neurons into firing.

The result? We remember – and those memories are most likely to have been formed in childhood. That is because smells are often the only triggers for memories formed in the first decade of life, before we associate events with specific verbal or visual labels. They are also likely to be happy memories, formed at a time before there was very much to worry about.

Winter can’t compete because its cold prevents natural scent stimuli from forming. Even summer’s bad smells – rotting, uncollected garbage, for instance – tend to provoke a more primal reaction. We are disgusted, and disengaged, before memory kicks in. When it comes to emotional engagement, summer wins every time.

Memory has long fascinated scientists, and we are starting to make significant progress in understanding its mechanisms. Thanks to new tools, we can now be fairly confident that, for one thing, memory formation and recall operate by two very different processes.

An article in the journal Science in April showed how researchers had tested and manipulated the memories of mice using a technique called optogenetics. This adds a light-activated protein on to a neuron and allows a pulse of light, in theory, to do the same job as an evocative smell, switching on a group of neurons and stimulating recall.

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The basic idea is that we encode memories when a group of neurons takes on a particular physical structure. We have always associated this with observations that the connections between them – the synapses – are strengthened. But it seems we may have been mistaken. The synaptic strengthening appears to be merely a door that opens up the memory to easy recall.

Memory does seem to involve a change to the physical and chemical properties of a group of neurons. This altered network is called the memory engram. We know this because scientists have been able to create a memory of an event in a mouse, then stimulate the specific engram using light or an emotional trigger. In each case, they could induce a response in the mouse that indicated its recall of that specific memory.

But things got really interesting when the researchers prevented synaptic strengthening using a chemical called anisomycin, which stops neurons from making new proteins. When mice were given anisomycin after a new experience that formed a memory, natural stimulation – a trigger equivalent to a smell – did not bring recall. If, however, the researchers used light to activate the engram directly, the recall occurred.

So, memory is sometimes beyond the reach of natural triggers. Memory seems to reside in the architecture of the engram – or a group of engrams – but in the absence of the synaptic strengthening process, we simply cannot access it.

The researchers claim their work has implications for amnesiac patients and those with dementia. It is too early to speculate whether this really is a route to improving failed or failing memory, but given that most dementia patients also suffer problems with their sense of smell, there may be reason for hope.

There’s no hope for winter, though. Its memories are still there, but – perhaps thankfully – there is no natural trigger to stimulate recall. If you want to remember the cold and the misery, optogenetics might be your only solution.