Agree, fascinating question. To be precise, they used genetically modified neurons (aka optogenetics) to test if the device can deliver a signal into the brain. Optogenetics incorporates neurons modified with light-sensitive channel proteins, so the neuron activates when a precise wavelength of light is “seen” by the special protein. One of the coolest methods in neuroscience, in my opinion.

“To see if the idea works in practice they installed the device in mice, using neurons genetically modified to react to light. Three weeks after implantation, they carried out a series of experiments where they trained the mice to respond whenever a light was shone on the device. The mice were able to detect when this happened, suggesting the light-sensitive neurons had merged with their native brain cells.”