Volume of tissue activated (VTA)

The volume of tissue activated is the spatial region around a stimulating contact within which neural elements are driven to fire by the applied stimulation. It is estimated by coupling a finite-element model of the electric potential in tissue (accounting for conductivity, anisotropy from white-matter tracts, and the insulating encapsulation layer around a chronic electrode) with multicompartment axon cable models, or with a simpler threshold on the activating function (the second spatial derivative of the extracellular potential along an axon).

VTA models underpin patient-specific DBS programming and surgical targeting, letting a clinician visualize which structures a given contact and amplitude will recruit. Their limitations should be kept in mind: they typically predict axonal activation rather than the network or behavioral effect that actually matters, they are sensitive to assumed tissue properties and to the encapsulation state that changes over time, and the binary activated-or-not abstraction hides graded, frequency-dependent, and fiber-orientation-dependent effects.