Precise control of stimulation parameters in low impedance conditions

¹ Medical Electronics Department and Laboratory of Neurophysiology, Good Samaritan Hospital, Portland, Oregon

A system is described for accurate control of stimulation parameters in conditions where impedances between 100 and 30,000 ohms are encountered across the secondary of a stimulus isolation transformer. The dual-channel stimulator is constructed mainly from Tektronix 160 series pulse and waveform generators. The stimulator was designed primarily for evoked potential studies in the nervous system with peripheral nerve stimulation but will operate equally well with concentric or needle electrodes in deep brain structure, providing that total electrode-tissue impedances are kept within the stipulated limits. The unique feature of the stimulator is embodied in the operational amplifier interposed between the Tektronix stimulus pulse generator and the primary of the isolation transformer. This system is designed to read directly in stimulus intensities relative to any stipulated physiological threshold. Absolute stimulus voltages may be continuously monitored to ± 3% accuracy. The impedance matching characteristics of the described system will permit effective stimulation of excitable tissues while maintaining stimulus control by the operational amplifier settings within ± 1%. The large range of pulse durations, stimulus intensities, and the use of more than one type of stimulus waveform, have been sacrificed in favor of maintaining rigid control over pulse waveforms and amplitudes with passage of large stimulating currents at low intensities. This system comes close to achieving those qualities embodied in the ideal constant voltage stimulator.

Submitted on July 10, 1964