Extracellular Recording of Retinal Neurons Using Novel Multi-electrode Microarray System

1 The retina, which lines the inner surface of the eye, receives information from the visual world and then processes and transmits it to the central nervous system. Cells called neurons make up this retinal tissue, and send long wire-like projections to the brain to transport processed information in the form of electrical signals called action potentials, or spikes. These cells, however, do not act independently; it is now thought that neurons function with other neurons in a group. To study this, an extracellular recording method utilizing a 512 electrode microarray system was developed. Extracellular techniques involve placing electrodes near neurons and recording changes in membrane potential, indicating the presence of a spike. Single electrode extracellular recording has been done extensively in previous years, which can only measure the output activity of a single neuron. However, a multi-electrode extracellular approach can record activity from many neurons. In the Santa Cruz Institute of Particle Physics, a microarray consisting of 512 electrodes in rectangular geometry was fabricated, with electrodes spaced at 60 µm. These specifications are purposely congruous with that in the neuronal system of the retina; the spacing of neurons must match the spacing of electrodes in order to accurately identify neuronal patterns of spike activity. Also, neurons must fully occupy a local area to match the density of electrodes on the array system. Before performing retinal experiments with these arrays, the arrays themselves need to be tested. There were in total 99 arrays, and each was tested for the presence of shorts between pairs of electrodes. The threshold resistance between channels is ~10^7 Ω, any resistance found below this point was considered a short. The testing determined that approximately 25% of arrays contained zero shorts.