Researchers have persistently sought ways to eavesdrop on neuronal communication. With each new method our understanding of neuroscience improves, but each comes with drawbacks. A new study reports the development of a neural interface array (the NeuroGrid) to record brain activity without penetrating the brain surface.



Typically, we listen to neuronal communication in animals by sampling chemical transmission in the fluid surrounding cells with dialysis probes, or with other types of probes that either translate chemical signaling into electrical current or directly record electrical current. Some probes measure with low spatial resolution, such as a region that contains many neurons. Other techniques, such as patch clamp, measure activity in single neurons but give little information about the behavior of neuronal populations.



We can gather more information from brain by using multiple probes and probes with multiple recording sites, which can measure signals from single neurons and populations. All of these approaches, however, share a significant drawback – they are invasive. When a probe is dropped into brain tissue, it disrupts the very thing it is trying to measure. Not only does this confound measurement, but also the brain environment that is inhospitable to long term measurement - a glial scar forms around the surface of the probe to protect the brain from the foreign object and the disruption in its protective dura mater cover.



Glia act differently than neurons, with little electrical activity and release of few neurotransmitters. Additionally, glia that react to an incision and foreign object are reactive and create an inflammatory microenvironment that is characterized by excitatory neurotransmitters and different electrical dynamics.



Another approach is to visualize activity with optical methods and dyes, but this is also invasive because it adds chemicals directly into cells or the surrounding environment. Non-invasive approaches hold promise, because they are less likely to confound the measurements and more practical for use in humans.



Recently, Khodagholy and colleagues developed a new, non-invasive technique called NeuroGrid . Their findings are published in Nature Neuroscience. NeuroGrid lays on the surface of the brain and conforms exquisitely to its contours. Therefore, it does not disrupt the cytoarchitecture of the brain, nor directly interfere with the electrical or chemical activity of the underlying neurons. The research team began by demonstrating their apparatus gently hugging the curved delicate edges of an orchid petal.



The NeuroGrid has multiple recording sites which allows for measurement of local field potentials (LFPs) created by activity in populations of neurons, and also of action potentials from individual neurons. The density and spacing of its electrodes are similar to the density and spacing of neurons, which helps NeuroGrid to capture individual action potentials. Moreover, neuronal types could be differentiated by evaluating the waveform and duration of recorded signals, and this was confirmed by recording with standard silicone probes.



NeuroGrid is made from an organic polymer material that conducts both ionic and electronic current and limits the electrochemical impedance that more strongly effects other types of probes. Additionally, NeuroGrid can easily be made bigger to accommodate relatively large surface areas and record from more neuron populations.



Eavesdropping on neurotransmission in humans is also a realistic possibility. The authors began by testing NeuroGrid in awake moving rats, both on the surface of the cortex and on the underlying hippocampus after surgically retracting the cortex. The authors used NeuroGrid to record in rats over 10 days without changes in the detection threshold. More impressive than rats, the authors demonstrated the approach can be used to record neuronal activity in human patients who were undergoing brain surgery to reduce the source of epileptic seizures.



Despite overcoming hurdles, there are some drawbacks to this technique. For example, it can only measure superficial layers of cortical neurons, up to 200 µm in depth below the brain surface. This alone rules out recording from the majority of the brain with NeuroGrid. However, NeuroGrid can be applied to deeper brain regions if superficial regions are moved (the authors proved this in rats) and the authors argue that this recording from superficial cortical neurons in humans is still important. They cite that useful information can be gained even from the top layers of cortex. For example, others have captured the processing of speech phonemes and alphabet letters by recording high frequency field potentials from superficial neurons using other devices. NeuroGrid technology is uniquely poised to delve deeper into the complicated processes of neuronal communication.