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New probe measures electrical activity of hundreds of individual neurons in living brains

It’s hard to take electrical measurements of what a single neuron is doing, especially while it’s still alive and wired up to its fellows. To move beyond historically difficult and damaging methods of taking these readings, researchers from Caltech have demonstrated a minimally invasive probe for doing in vivo electrophysiology: real-time electrical recordings of living nerves in the brain. The probe can measure electrical activity in three dimensions, which could be a huge breakthrough for our understanding of cognition and neural activity.

The physical build makes sense. Since it’s a probe, it has to gently, non-destructively enter living brain tissue, and it has to go through the skull to do so. Because neurons are so delicate and closely packed, you might imagine that this would require something extremely slender, something that wouldn’t disturb all those irreproducible synapses and support-cell connections. These are thin, elongated ovals with streamlined ends, meant to slide between the axons long-ways, minimizing the potential for damage.

If one probe looks like a needle, and several ganged together look like a comb, the device as a whole looks like several combs held together side-by-side so that their teeth form a grid. Probes are built on a slice of commercial silicon coated in a bio-inert polymer, and along the last millimeter of each, there are vapor-deposited gold electrodes that can be used both to record and stimulate. And the whole thing is mechanically decoupled from its associated circuit board by a flexible cable, which means it can be indwelling: used in living creatures’ fragile brains for a relatively long time.

The fact that the probe is meant to take recordings in three dimensions is also important. The human cortex is where most of the brain’s complex functions are managed, and it’s only about 2-4mm thick. That few millimeters is divided into up to six distinct strata of cells. Clustering more than a thousand electrode sites into a cubic millimeter would give unprecedented granularity for knowing which cells are doing precisely what, and when. While the number might seem a little excessive, it’s actually about a 10x10x10 grid of electrodes. Even so, this is the first such device to scale up individual probes and electrodes into such an array.

Innovations like this have obvious applications when it comes to more sophisticated recording and brain mapping, but they also lend themselves to research into brain-computer interfaces. The research and demonstrations that led to this report were done on living animals. Having in vivo recordings of a whole bank of neurons could go a long way toward making an actual, indwelling BCI, because we need good mathematical models of nerve function in order to make electronic interfaces. Since the voltage differential neurons need is on the order of tens of millivolts, it will be important to make indwelling interfaces that can faithfully reproduce brain activity. Real-time, single-neuron electrical measurements are the first concrete step on that road.

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