Unit activity, evoked potentials and slow waves in the rat hippocampus and olfactory bulb recorded with a 24-channel microelectrode.

Activity from a number of neighboring neurons can be recorded simultaneously with multichannel microelectrodes. A new version of a 24-channel microelectrode system has been fabricated and used to record different types of neurophysiological data in the rat brain. The system called PRONG (Parallel Recording Of Neural Groups) includes a microelectrode, a lightweight reusable connector, a 24-channel FET-hybrid preamplifier, a 3-band 24-channel amplifier, a 24-channel spike monitor, high-speed digital and analog interfaces and a computer. The electrode-recording locations are arranged in 2 arrays of 12 sites. The arrays are spaced 100 micron apart along either edge of the recording section and the sites within each array are spaced 120 micron apart. The electrodes are fabricated using photolithography in patterned layers totaling 17 micron thick and 114 micron wide in the recording section. The recording sites are 20 micron2 and are plated with platinum black. Performance of the PRONG was compared with that of conventional single microelectrodes and with results in the literature on three kinds of extracellular activity in the rat hippocampal formation and olfactory bulb: action potentials, evoked field potentials and slow-wave activity. The selectivity and sensitivity of the PRONG compared favorably with characteristics of conventional electrodes. Background noise averaged 15 microV and no signal cross talk was observed between neighboring channels. Discriminable action potentials (signal-to-noise ratios of 2:1 to 15:1) were observed at 37-95% of the viable recording sites with a maximum of 19 units in one recording. Units were observed in waking animals for up to 4 days. The waveforms, firing repertoires and laminar distribution of units were the same as those recorded with conventional microelectrodes. This indicates that penetration by the PRONG spares tissue from functional damage. "Instant" laminar profiles were created for commissural and perforant path evoked potentials in the hippocampal formation. These profiles were nearly identical with those created by successive recordings made with conventional microelectrodes. Laminar profiles and behavioral activity appeared to be "normal" as collected with this electrode. These results set the foundation for use of the PRONG as a tool for the study of local neural interactions.