Spend enough time in the Arctic, and the featureless vista of white snow can eventually make you go blind. The reason is that if you stare into blank nothingness long enough, your brain shuts down your visual processing. Your eyes simply cease to carry the unchanging input. You may even start to hallucinate. It’s called the Ganzfeld effect, and it highlights how changing and fluctuating external input — or input noise — plays a role in the functioning of the brain.
So what is this noise all about? Well, for starters, there’s noise in your brain. Just like a TV set that’s not tuned into a channel, your brain produces static when it’s got nothing to do. This intrinsic noise is actually your neurons firing at random. And when external noise (that is, input fluctuations) is introduced, there is an interaction between the two kinds of noise. Arvind Kumar, a researcher in computational biology at KTH Royal Institute of Technology, says that to some extent, this interaction holds the key to how our brains function.
“You have noise in the brain when you aren’t doing anything; and when you start doing something, this noise is reduced,” he explains. “But it is not well understood how this noise is reduced even though overall activity of the brain increases.”
This week, Kumar and his colleagues published a paper seeking to explain why that happens. Their report comes soon after Kumar’s recent study that suggests the human tendency to say ‘no’ may actually be hardwired into our brains.
But let’s return for a moment to our Arctic explorer. The neurons in his or her brain become elevated as they seek in vain for some correlation in the visual input. It’s a fruitless effort because, unlike in a forest or on a city street, there are no correlations in the visual stimulus you get while staring into a blank void. A flat wall has no structure the neurons can get meaningful input from. Look at it, and the result is random electrical fluctuations in your head, as the brain gives up on processing the input.
“We want meaningful inputs,” Kumar says. “And this will have fluctuations around an average value. But these fluctuations are structures. For instance, when you look at a landscape, different pixels have this structure in terms of their relationship to each other.”
Everywhere in our world, there is noise — or more accurately, correlated fluctuation —inside and outside of our brains. And when these two noises interact, a curious thing happens. Instead of elevating our intrinsic noise, noisy input appears to cancel it out.
The mystery is, why?
One theory is that there are some mechanisms within the brain that quiet down the noise internally. One such mechanism could lie in the brain’s wiring. “That explanation is compelling”, says Kumar, “but it’s incomplete. If the mechanism is hardwired, quenching of the noise would be independent of the stimulus,” he says. “The very cause of the change in the brain activity, that is, the stimulus, has to play a role in determining the response.”
In a new paper, published in the Journal of Neuroscience, Kumar argues that the fluctuations in the input (or the external noise) itself is what can cancel out the intrinsic noise of the brain.
“The input itself has properties that cancel the noise,” he says.
It goes something like this. If you look out at a river and there are waves, and you throw a small stone in, you wouldn’t see its effect on the wave structure. But if you throw a big enough stone, you see the change clearly. That is, the input must have features that are bigger than the noise in the system.
More specifically, Kumar and his colleagues point out, some input features mainly affect individual neurons, while others affect groups of neurons. Input features that affect individual neuron responses determine the variability and noise in the response. Input features that affect populations of neurons define the magnitude of the response.
Kumar says that with this study, there are two plausible mechanisms that could reduce the noise and variability in the brain – the external input, and the local connectivity in the brain. Kumar says that the ultimate explanation may be a combination of a noise dampening mechanism inside the brain coupled with the affect of external noise.
“I think this is what makes biological systems complex and interesting, that when you have multiple explanations, all are correct to some extent. The challenge is to find out the domain of their applicability.
“Fortunately, in this case the two explanations make different and testable predictions,” he says. “So, the ball is back in the court of the experimentalists to test these theoretical predictions.”
David Callahan
Bujan AF, Aertsen A, Kumar A (2015) Role of input correlations in shaping the variability and noise correlations of evoked activity in the neocortex. J Neurosci. 35(22):8611-8625
doi:10.1523/jneurosci.4536-14.2015
