The feasibility of using thin-film recording arrays (multisite microprobes) to record cortical activity on a long-term basis has been assessed, using an in vitro turtle preparation and the somatosensory cortex of the cat When chronically implanted, the impedance of most sites remained stable with inter-measurement variability no larger than that reported for other chronically implanted electrodes. In one case, we observed a slow increase in impedance indicative of fibrous encapsulation. Microprobes were much less susceptible to artifact due to respiration, heart beat and head movements than were traditional rigidly-mounted metal electrodes. Stable cortical signals were recorded for periods as long as four weeks. Adhering the microprobes to the pia using cyanoacrylate glue appeared to lengthen significantly the longevity of the recordings. We were able to identify small groups of neurons that responded to a characteristic cutaneous receptive field over time, indicating that the microprobes likely maintain a constant position within the cortex. Using microprobes with multiple recording sites located along a single shank, it was possible to record several units within a single cortical column. These units nearly always modulated their discharge pattern in unison. Microprobes of this design may provide a degree of redundancy in the signals obtained from a given column. Using microprobes with multiple shanks, it was possible to record groups of neurons spaced as close as 400 fim parallel to the cortical surface that were related to specific, distinct cutaneous receptive fields. Microprobes of this design may be useful for obtaining multiple, independent signals from the cortex. Based on the results of this study, multisite microprobes appear to offer good recording stability and longevity, together with a dramatic increase in available recording sites.
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