Analysis of Tetrode Recordings in Cat Visual System

From simultaneous recording of closely located neurons one can obtain information about local inter-neuronal connectivity and acquire fine-scale neuronal response mappings. A multitrode (by which we mean a tight bundle of traditional extracellular electrodes) (McNaughton et al., 1983) is a natural choice for this kind of recording, because it allows discrimination of multiple neurons at a single recording site. Separation of spikes from different neurons is based on the assumption that spike amplitude decreases with the distance between the neuron and the tip of the electrode. Thus, the ratio of spike amplitudes measured on different wires provides reliable information for spike discrimination. The tetrode, which is made of four very thin wires (typical wire diameter with the insulation is 20 μm) is the minimal set of electrodes that provides “triangulation” in the three- dimensional space. If tetrode tips are not located in a plane, and under the idealization that neurons are point sources in a homogenous medium, the ratios of spike amplitudes will be unique for each neuron. In contrast to the tetrode, a single electrode is “spatially blind”, and its use for multi-neuron recordings can easily lead to mixing of signals from different cells whose spike amplitudes as measured at the electrode site are equal.