Electrical stimulation of brain localized without probes--theoretical analysis of a proposed method.

Abstract

A method of electrically stimulating neural elements confined within small volumes of brain at predetermined deep locations without employing probes is considered theoretically. The basic principle of the method is the partial rectification in the focal region of an ultrasonic field of the alternating current that flows in the entire brain, or a major portion thereof, in response to an externally applied electric field of the same frequency. Since the magnitude of the electrical conductivity of the tissue varies with the temperature the adiabatic temperature, changes produced by the acoustic disturbance cause a periodic variation in the conductivity that results in a net unidirectional transfer of charge when the frequenices of the two fields are equal. The condition for stimulation is expressed quantitatively by a relation involving the amplitudes of the electric and acoustic field parameters, the thermal coefficient of electric conductivity of the tissue, the threshold quantity of charge that must be transferred unidirectionally per unit area to result in stimulation, the pulse duration, and a parameter that is determined by the geometric orientation of the electric and ultrasonic field vectors at the acoustic focus. Numerical calculations suggest that stimulation can be produced in the absence of thermal or other mechanisms of damage to the tissue. The values of the charge-stimulation and electrical-conductivity parameters employed in the calculations are derived from available experimental data.