1. A method for determining how to stimulate a target patch of neurons in a subject's brain and stimulating the same, comprising: obtaining an impedance model of the brain, and identifying within the impedance model a target location corresponding to the target of neurons; obtaining a device model of a predetermined device capable of producing electric or magnetic fields outside the brain; using the device model and the impedance model to compute a first virtual target stimulating current vector at the target location resulting from virtually using the device as disposed in a first trial disposition outside the brain to produce a virtual electric or magnetic field, and a second virtual target stimulating current vector at the target location resulting from virtually using the device as disposed in a second trial disposition outside the brain distinct from the first trial disposition to produce a virtual electric or magnetic field; obtaining an image of the cortical surface of the brain at the target location and using the image for computing a target normal vector normal to the cortical surface at the target location; and determining which of the first and second virtual target stimulating current vectors has a greater component of virtual target stimulating current projecting at a predetermined first polar angle between zero and 45 degrees with reference to a spherical coordinate system taking the direction of the target normal vector as the zenith direction; and selecting one of the first and second trial dispositions; and stimulating the target patch of neurons comprising actually using the device in the selected trial disposition to produce a real electric or magnetic field.