Advances in medical device engineering, as well as our continuously increasing understanding of brain function, has increased the safety, reliability, and efficacy of neuromodulation methods for regulating human behavior over the past couple decades. Clinical neuromodulation devices are becoming frequently used for treating neurological and psychiatric disorders. Most clinical methods however rely on surgically implanted electrodes, such as those incorporated by deep-brain stimulation systems to treat Parkinson’s disease. Several different methods of noninvasive neuromodulation also exist and have been used to alter brain activity for research applications or to treat some nervous systems disorders. Some noninvasive methods involve delivering electrical currents or electromagnetic pulses to specific peripheral nerves or brain targets in order to modulate nervous system activity. Other more recently developed methods use focused ultrasound to noninvasively modulate human brain circuits or nerves at high spatiotemporal resolutions. Beyond the utility of restoring loss of function in therapeutic embodiments, these safety benchmarks have encouraged the widespread investigation of several different types of neuromodulation methods for their potential to increase gains in the performance and function of healthy individuals. Noninvasive neuromodulation has been shown to enhance brain plasticity and physical performance, accelerate learning, increase attention, and improve sleep. Additive gains have been shown when combining neuromodulation devices with biosensors and virtual immersion gaming systems as human computer interfaces. An overview of how electrical and ultrasonic neuromodulation methods embodied as human computer interfaces are being developed for enhancing the performance of military operators and defense personnel is provided in the present paper.
We examine the potential for low-intensity focused ultrasound to non-invasively produce small (< 1mm<sup>3</sup>) focal acoustic fields for precise brain stimulation near the skull. Our goal is to utilize transcranial ultrasonic neuromodulation to transform communications and immersive gaming experiences and to optimize neuromodulation applications in medicine. To begin evaluating possible hardware design strategies for engineering ultrasonic brain interfaces, in the present study we evaluated the skull transmission properties of longitudinal and shear waves as a function of incidence angle for 0–2 MHz. We also employed K-wave and time-reversal numerical simulations to further inspect waveform interactions with modeled layers. Timereversal focusing for single-layer and three-layer skull cases were simulated for three different bandwidth ranges (MHz): Broadband(0–2), 1 MHz(0.4–1.4), and 0.2 MHz(0.4–0.6). Broadband and 1 MHz bandwidths emulate the performance of micromachined or piezo membrane ultrasonic arrays, while 0.2 MHz bandwidth is representative of the performance of conventional piezoelectric ultrasonic transducer. We found the 3dB focal volume was ~0.6 mm for broadband and 1 MHz, with the latter showing a slightly larger sidelobe. In contrast, 0.2 MHz nearly doubled the size of the 3dB focal volume while producing prominent sidelobes. Our results provide initial confirmation that a broadband, ultrasonic, linear array can access the first 15 mm of the human brain, which contains circuitry essential to sensory processing including pre-motor and motor planning, somatosensory feedback, and visual attention. These areas are critical targets for providing haptic feedback via non-invasive neural stimulation.
Neurotechnologies for non-invasively interfacing with neural circuits have been evolving from those capable of sensing neural activity to those capable of restoring and enhancing human brain function. Generally referred to as non-invasive neural stimulation (NINS) methods, these neuromodulation approaches rely on electrical, magnetic, photonic, and acoustic or ultrasonic energy to influence nervous system activity, brain function, and behavior. Evidence that has been surmounting for decades shows that advanced neural engineering of NINS technologies will indeed transform the way humans treat diseases, interact with information, communicate, and learn. The physics underlying the ability of various NINS methods to modulate nervous system activity can be quite different from one another depending on the energy modality used as we briefly discuss. For members of commercial and defense industry sectors that have not traditionally engaged in neuroscience research and development, the science, engineering and technology required to advance NINS methods beyond the state-of-the-art presents tremendous opportunities. Within the past few years alone there have been large increases in global investments made by federal agencies, foundations, private investors and multinational corporations to develop advanced applications of NINS technologies. Driven by these efforts NINS methods and devices have recently been introduced to mass markets via the consumer electronics industry. Further, NINS continues to be explored in a growing number of defense applications focused on enhancing human dimensions. The present paper provides a brief introduction to the field of non-invasive neural stimulation by highlighting some of the more common methods in use or under current development today.