For decades, scientists have been discovering neurons with seemingly unique jobs. There are place cells that fire in response to a聽particular location, mirror neurons that may help us interpret behaviours and even neurons that respond to all things聽Jennifer Aniston. These discoveries have encouraged a picture of the brain as a collection of highly specialised cells.聽
But a new discovery suggests that most neurons are actually jacks-of-all-trades. 鈥淭hese [earlier] neurons are really the exception, not the rule,鈥� says聽聽at Columbia University in New York. And the more abundant, generalised cells 鈥渃an be as important or more important than the good-looking [specialist] neurons we isolated in the past鈥�, he says.
In a聽, Fusi noticed 鈥渕ixed selectivity鈥� neurons 鈥� cells that respond to multiple different task-related inputs at the same time 鈥� in the brains of monkeys. But it was unclear how widespread they are in the mammalian brain.
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Now, Fusi and his team have turned to the International Brain Laboratory. This holds information on the activity of individual neurons throughout the brains of more than 100 mice while they did a decision-making task.聽The task involved the mice looking for an image on the left or right side of a screen and moving it to the centre by quickly turning a wheel.
The researchers found that the only areas with specialised neurons were those in dedicated sensory cortices, such as the primary visual cortex. Instead, the neurons in most areas of the聽brain聽responded to various aspects of the task. For instance, those outside of the visual cortex also seemed to be involved in processing the image, as well as deciding which side of the screen it was on and generating the movement to turn the wheel. 鈥淭he important thing to stress is that there are specialised neurons [such as place cells],鈥� says Fusi. But these are not the norm, he adds.
The researchers also found that most neurons didn鈥檛 respond in the same way to the same stimulus. For example, the somatomotor areas, which are regions associated with planning movements, responded strongly to licks and whisker movements. Other areas still responded, but not as strongly. This suggests that while specialism exists, individual neurons generally aren鈥檛 myopically focused on one type of information.
鈥淭he problem is if you鈥檙e arguing there鈥檚 no [specialisation], somebody can always say: 鈥榊ou just needed [to study] a few more neurons [to uncover it],鈥欌�� says聽聽at Baylor College of Medicine in Texas. 鈥淭his [study] is a giant smack in the face saying: 鈥楩ine, we got 14,000 neurons and we still didn鈥檛 find it [for the most part].鈥欌��
Having a collection of neurons that respond to the same stimulus in slightly different ways may maximise the amount of information the brain can process. This may mean that these generalised neurons contribute to a plethora of tasks, including memory, cognition, decision-making and perception.
The researchers point out that the mice鈥檚 task wasn鈥檛 particularly complex. A complicated, multidimensional set of tasks could shed light on when individual neurons focus their responses on one aspect of such tasks versus another, says Fusi.聽
He and his team are now collaborating with researchers who study non-human primates and patients who are conscious during open brain surgeries, such as those treating epilepsy, to see if the findings in mice apply to people.
With all these projects, Fusi hopes to shift the emphasis from looking at the role of individual neurons to patterns of activity across many cells in the brain.聽鈥淚t鈥檚 very difficult to draw conclusions by looking at one neuron at the time,鈥� he says. 鈥淚nstead, we [should] look at all the neurons together. That鈥檚 the perspective of the brain because each neuron in the brain doesn鈥檛 [get input from] one single [other] neuron. Each neuron in the brain looks at 10,000 other neurons that are connected on its dendritic tree.鈥�
Nature
