, as measured by an electroencephalogram, or EEG, are much smaller than the amplitude of individual spikes, and were long thought to be epiphenomenal: pointers to various brain states, but not an information-carrying signal in itself. In other words, noise.
Slow oscillations in the brain are coherent over large anatomical distances, in the same way you can hear the bass from your neighbor’s sound system better than the treble.
Yet, as we saw with genetic expression, interactions among many discrete elements can be incredibly useful. It’s becoming clear that the so-called noise seen in an EEG is actually an important signal, and one that is crucial to neural computation. The fluctuation of the summed output of the brain’s neurons feeds back to affect the resting potential of those neurons, bringing them closer to or farther from spike threshold. Slow oscillations (as found in drowsy or sleeping animals) are coherent over large anatomical distances, in the same way you can hear the bass from your neighbor’s sound system better than the treble. This is how the brain may be facilitating communication between disparate brain areas: If an individual spike is timed with the crest of the slow amplitude fluctuation, it will have a bigger effect downstream than if it is timed with a trough. Neurons are somehow coordinating themselves to generate self-directed, global feedback that serves to adaptively boost even sub-detectable signals: No informational signal is wasted.
In fact, it is an open question whether the idea of “waste” can even be properly defined in the context of the brain. Theoretical neuroscientist Tony Bell argues that the hallmark of biological computation is that information flows across multiple hierarchies of complexity, bubbling up from microscopic to macroscopic levels and back down again.
Simpler structures (like neurons) communicate with more complex structures (like areas of the brain), and vice versa. Computations are occurring on and between every level, from molecules to cells to organisms as a whole. There is a good deal of evidence that even the subcellular compartments within neurons are doing their own computations. Most recently, a study1 published in Nature in October 2013 showed that activity within dendrites (a compartment representing a neuron’s “input”) shapes the information processed by that neuron. Given this multi-level information flow, Bell argues that noise simply cannot be defined: Deviations from expectation should be interpreted as meaningful communication at another level of the complexity hierarchy.
This information flow is not restricted to the brain, and can also be observed, for example, in epigenetics. Bell says this kind of dynamic multidirectional information flow is what defines life. Artificial intelligence, when it arrives, may not be built from silicon, but repurposed biology. Before we get there, though, we will need a vastly different approach to science in order to understand the higher-order, emergent capabilities of nature’s self-organizing structures. While biomimicry holds incredible promise, perhaps in so assiduously copying the physical design of systems we are missing the point: Form is transient in Nature, cast to meet shifting needs in a given niche. Determining the rules by which biology struck upon these ingenious solutions in the first place will spark the true revolution.
Kelly Clancy studied physics at MIT, then worked as an itinerant astronomer for several years before serving with the Peace Corps in Turkmenistan. She is currently a National Science Foundation fellow studying neuroscience at UC Berkeley.
1. Smith, S., et al Dendritic spikes enhance stimulus selectivity in cortical neurons in vivo. Nature 503, 115-120 (2013).