Simultaneous Single Unit Recording in vitro with a Photoetched Laser Deinsulated Gold Multimicroelectrode Surface

Flat photoetched gold conductors 12 ¿m wide and 2 ¿m thick, situated on a glass plate and deinsulated at their tips with single 8 ns UV laser pulses, have been utilized to record single unit extra-cellular activity from brain ganglia of the snail Helix pomatia. A fixed array of 36 such conductors, terminating in six rows and six columns in a 0.5 mm x 1 mm area, is capable of monitoring simultaneous single unit activity from numerous neurons. Spike amplitudes of 300¿500 ¿V are generally observed from the predominant 40 ¿m diameter cells of these ganglia. Giant neurons usually produce signal amplitudes of over 3 mV that are simultaneously seen by many electrodes. Signals can be monitored by merely resting a locally desheathed ganglion on the recording area under its own weight in a shallow pool of Ringer. A 10 ¿m diameter crater in a 3¿4 ¿m thick insulation layer has an impedance of 2¿4 M¿at 1 kHz. The capacitance of this metal electrolyte interface is about 0.5 pF/¿m2, suggesting that the UV laser produces a partially colloidal gold surface. With this recessed tip design, simultaneous single unit recording from small neurons appears ensured if electrode impedances are below 4 M¿, shunt impedances are above 30 M¿, and glia cells are not allowed to reinsulate the end of the gold conductor.