Various neuromodulation approaches have been employed to alter neuronal spiking activity and thus regulate brain functions and alleviate neurological disorders. Infrared neural stimulation (INS) could be a potential approach for neuromodulation because it requires no tissue contact and possesses a high spatial resolution. However, the risk of overheating and an unclear mechanism hamper its application. Here we show that midinfrared stimulation (MIRS) with a specific wavelength exerts nonthermal, long-distance, and reversible modulatory effects on ion channel activity, neuronal signaling, and sensorimotor behavior. Patch-clamp recording from mouse neocortical pyramidal cells revealed that MIRS readily provides gain control over spiking activities, inhibiting spiking responses to weak inputs but enhancing those to strong inputs. MIRS also shortens action potential (AP) waveforms by accelerating its repolarization, through an increase in voltage-gated K+ (but not Na+) currents. Molecular dynamics simulations further revealed that MIRS-induced resonance vibration of –C=O bonds at the K+ channel ion selectivity filter contributes to the K+ current increase. Importantly, these effects are readily reversible and independent of temperature increase. At the behavioral level in larval zebrafish, MIRS modulates startle responses by sharply increasing the slope of the sensorimotor input–output curve. Therefore, MIRS represents a promising neuromodulation approach suitable for clinical application.

The myelin sheath enables dramatic speed enhancement for signal propagation in nerves. In this work, myelinated nerve structure is experimentally and theoretically studied using synchrotron-radiation-based Fourier-transform infrared microspectroscopy. It is found that, with a certain mid-infrared to terahertz spectral range, the myelin sheath possesses a ≈2-fold higher refraction index compared to the outer medium or the inner axon, suggesting that myelin can serve as an infrared dielectric waveguide. By calculating the correlation between the material characteristics of myelin and the radical energy distribution in myelinated nerves, it is demonstrated that the sheath, with a normal thickness (≈2 µm) and dielectric constant in nature, can confine the infrared field energy within the sheath and enable the propagation of an infrared signal at the millimeter scale without dramatic energy loss. The energy of signal propagation is supplied and amplified when crossing the nodes of Ranvier via periodic relay. These findings provide the first model for explaining the mechanism of infrared and terahertz neurotransmission through myelinated nerves, which may promote the development of biological-tissue label-free detection, biomaterial-based sensors, neural information, and noninvasive brain-machine interfaces.

Neurostimulant drugs or magnetic/electrical stimulation techniques can overcome attention deficits, but these drugs or techniques are weakly beneficial in boosting the learning capabilities of healthy subjects. Here, we report a stimulation technique, mid-infrared modulation (MIM), that delivers mid-infrared light energy through the opened skull or even non-invasively through a thinned intact skull and can activate brain neurons in vivo without introducing any exogeneous gene. Using c-Fos immunohistochemistry, in vivo single-cell electrophysiology and two-photon Ca2+ imaging in mice, we demonstrate that MIM significantly induces firing activities of neurons in the targeted cortical area. Moreover, mice that receive MIM targeting to the auditory cortex during an auditory associative learning task exhibit a faster learning speed (~50% faster) than control mice. Together, this non-invasive, opsin-free MIM technique is demonstrated with potential for modulating neuronal activity.

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A phase-interrogation approach for the bulk refractive index sensing based on a silicon-on-insulator (SOI) ring resonator is introduced. The rapid phase variation around the resonance of the ring resonator is interrogated, and a single-sideband generation and coherent detection technology is adopted for the phase measurement. In the proposed approach, most of the intensity noise can be shielded, which leads to an ultrastable reading for the phase signals. A sensitivity of 6 Â 10 3 rad/RIU and a detection limit of 2:5 Â 10 À6 RIU are demonstrated.

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