The brain is a complex tangle of neurons that allow us to process lots of different stimuli at once. But according to new research from Duke University, that multi-tasking ability doesn't necessarily come down to lots of neurons doing their own job. The team has found evidence that individual neurons can process multiple signals simultaneously, by rapidly alternating between them.
According to the team, previous studies usually only looked at how single neurons react to a single stimulus, like one note or an image, but this is a very limited test that doesn't take into account the complexities of the real world.
These studies have also found that individual sensory neurons are sensitive to a range of sound frequencies, which raises questions: If two of those sounds were played at once, how does the neuron process both? Wouldn't one sound sort of hog the neuron and keep the other sound undetectable?
"The question we asked is, how do neurons preserve information about two different stimuli in the world at one time?" says Jennifer Groh, an author of the new study. "If I am a neuron and I'm able to respond to both an image of a pillow and the couch it is resting on, how does the brain infer that both the pillow and the couch are present?"
The fact that we can listen to multiple sounds at once implies that our neurons can multitask. To investigate how, the Duke team first implanted electrodes in the brains of monkeys – specifically in the auditory pathway – where they could measure the firing rates of neurons in response to two different sounds.
These monkeys had been trained to look in the direction of sounds they heard, so the team sat them down in a dark room, played sounds and watched their brain waves. The team found that an individual neuron would respond to each of the two sounds with different firing rates.
But of course the interesting thing was when both sounds were played together – the neurons appeared to rapidly switch between the two firing rates associated with each sound. In some cases, it was as quick as half a second, while other times it took longer.
"We found that there are periods of time when a given neuron responds to one stimulus, and other periods of time where it responds to the other," says Groh. "They seem to be able to alternate between each one."
The team also applied the technique to visual stimuli, with an experiment that observed the firing rates of neurons in response to pictures of faces. Sure enough, when looking at two faces together, the neurons showed the same rapid-switching behavior.
Not only does this finding help reveal how we can pay attention to multiple stimuli simultaneously, the team says it might also explain some of our mental shortcomings – specifically, why we can only really hold five to seven things in our minds at a time, known as our working memory.
"Our working memory system is quite limited and no one really knows why," says Groh. "Perhaps that limit arises from some kind of cycling behavior where you are coding one thing at a time, and across a period of time, the number of things you can represent depends on how long you need to represent each one and how rapidly you can switch."
The research was published in the journal Nature Communications.
Source: Duke University