**7. Conclusion**

Magnetic neuroparticle solutions to brain-machine interface were predicted a long time ago and are under development today. As shown above, animal data show that nontoxic magnetic particles could be noninvasively directed to specific locations in the brain under realtime imaging guidance. Particles could be placed with high spatial resolution in focal regions for specific clinical indications (addiction, Parkinson's disease). Alternatively, the particles could be globally spread in the brain and selectively addressed for local stimulation and/

or readout with appropriate RF or magnetic tuning. Many of the particles listed above (e.g., magnetoelectric, electret-based particles) can both read and write electrically and therefore potentially fit the bill for high-speed bidirectionality. Building on the work of deep-brain stimulation, one might expect that the focal stimulation of specific brain nuclei would be the first clinical target for noninvasive or minimally invasive bidirectional BMI. The high temporal and spatial resolution of voltage-sensitive contrast media would likely shed additional light on large-scale brain processes (e.g., attractors [44]) that would be useful in building more eloquent BMIs. System architectures for reading from and writing to the brain would be similar to conventional MRI systems, preferably with the ability to rapidly turn off the static magnetic field in order to manipulate the magnetic particles with high flexibility [12]. Once the particles were placed in the appropriate location, stimulation could be implemented with a wearable coil. Readout with voltage-sensitive contrast media could be performed with conventional MRI systems.
