Jonathan M. Cayce

Pulsed infrared light alters neural activity in rat somatosensory cortex in vivo

Pulsed infrared light has shown promise as an alternative to electrical stimulation in applications where contact free or high spatial precision stimulation is desired. Infrared neural stimulation (INS) is well characterized in the peripheral nervous system; however, to date, research has been limited in the central nervous system. In this study, pulsed infrared light (λ=1.875 μm, pulse width=250 μs, radiant exposure=0.01-0.55 J/cm(2), fiber size=400 μm, repetition rate=50-200 Hz) was used to stimulate the somatosensory cortex of anesthetized rats, and its efficacy was assessed using intrinsic optical imaging and electrophysiology techniques. INS was found to evoke an intrinsic response of similar magnitude to that evoked by tactile stimulation (0.3-0.4% change in intrinsic signal magnitude). A maximum deflection in the intrinsic signal was measured to range from 0.05% to 0.4% in response to INS, and the activated region of cortex measured approximately 2mm in diameter. The intrinsic signal magnitude increased with faster laser repetition rates and increasing radiant exposures. Single unit recordings indicated a statistically significant decrease in neuronal firing that was observed at the onset of INS stimulation (0.5s stimulus) and continued up to 1s after stimulation onset. The pattern of neuronal firing differed from that observed during tactile stimulation, potentially due to a different spatial integration field of the pulsed infrared light compared to tactile stimulation. The results demonstrate that INS can be used safely and effectively to manipulate neuronal firing.

Infrared neural stimulation of primary visual cortex in non-human primates.

Infrared neural stimulation (INS) is an alternative neurostimulation modality that uses pulsed infrared light to evoke spatially precise neural activity that does not require direct contact with neural tissue. With these advantages INS has the potential to increase our understanding of specific neural pathways and impact current diagnostic and therapeutic clinical applications. In order to develop this technique, we investigate the feasibility of INS (λ=1.875μm, fiber diameter=100-400μm) to activate and modulate neural activity in primary visual cortex (V1) of Macaque monkeys. Infrared neural stimulation was found to evoke localized neural responses as evidenced by both electrophysiology and intrinsic signal optical imaging (OIS). Single unit recordings acquired during INS indicated statistically significant increases in neuron firing rates that demonstrate INS evoked excitatory neural activity. Consistent with this, INS stimulation led to focal intensity-dependent reflectance changes recorded with OIS. We also asked whether INS is capable of stimulating functionally specific domains in visual cortex and of modulating visually evoked activity in visual cortex. We found that application of INS via 100μm or 200μm fiber optics produced enhancement of visually evoked OIS response confined to the eye column where INS was applied and relative suppression of the other eye column. Stimulating the cortex with a 400μm fiber, exceeding the ocular dominance width, led to relative suppression, consistent with involvement of inhibitory surrounds. This study is the first to demonstrate that INS can be used to either enhance or diminish visual cortical response and that this can be done in a functional domain specific manner. INS thus holds great potential for use as a safe, non-contact, focally specific brain stimulation technology in primate brains.

Combined optical and electrical stimulation of neural tissue in vivo

Low-intensity, pulsed infrared light provides a novel nerve stimulation modality that avoids the limitations of traditional electrical methods such as necessity of contact, presence of a stimulation artifact, and relatively poor spatial precision. Infrared neural stimulation (INS) is, however, limited by a 2:1 ratio of threshold radiant exposures for damage to that for stimulation. We have shown that this ratio is increased to nearly 6:1 by combining the infrared pulse with a subthreshold electrical stimulus. Our results indicate a nonlinear relationship between the subthreshold depolarizing electrical stimulus and additional optical energy required to reach stimulation threshold. The change in optical threshold decreases linearly as the delay between the electrical and optical pulses is increased. We have shown that the high spatial precision of INS is maintained for this combined stimulation modality. Results of this study will facilitate the development of applications for infrared neural stimulation, as well as target the efforts to uncover the mechanism by which infrared light activates neural tissue.

Infrared Neural Stimulation of Thalamocortical Brain Slices

Infrared neural stimulation (INS) has been well characterized in the peripheral nervous system, and has been shown to enable stimulation with high spatial precision and without causing the typical electrical stimulation artifact on the recording electrode. The next step in the development of INS is to demonstrate feasibility to stimulate neurons located in the central nervous system (CNS). Thalamocortical brain slices were used to establish feasibility of INS in the CNS and to optimize laser parameters. Infrared light was used to evoke action potentials in the brain slice with no electrical stimulation artifact. This response was blocked by the application of tetrodotoxin demonstrating neurological origin of the recorded signal. Threshold radiant exposure decreased as the absorption coefficient of the wavelength of light increased. Higher repetition rates lead to a decrease in threshold radiant exposure, and threshold radiant exposure was found to decrease as the spot size diameter increased. Additionally, neuronal responses to INS were intracellularly recorded demonstrating artifact free electrical recordings. The results from this paper lay the foundation for future in vivo studies to develop INS for CNS stimulation.

Calcium imaging of infrared-stimulated activity in rodent brain

Infrared neural stimulation (INS) is a promising neurostimulation technique that can activate neural tissue with high spatial precision and without the need for exogenous agents. However, little is understood about how infrared light interacts with neural tissue on a cellular level, particularly within the living brain. In this study, we use calcium sensitive dye imaging on macroscopic and microscopic scales to explore the spatiotemporal effects of INS on cortical calcium dynamics. The INS-evoked calcium signal that was observed exhibited a fast and slow component suggesting activation of multiple cellular mechanisms. The slow component of the evoked signal exhibited wave-like properties suggesting network activation, and was verified to originate from astrocytes through pharmacology and 2-photon imaging. We also provide evidence that the fast calcium signal may have been evoked through modulation of glutamate transients. This study demonstrates that pulsed infrared light can induce intracellular calcium modulations in both astrocytes and neurons, providing new insights into the mechanisms of action of INS in the brain.

Infrared neural stimulation of human spinal nerve roots in vivo

Abstract. Infrared neural stimulation (INS) is a neurostimulation modality that uses pulsed infrared light to evoke artifact-free, spatially precise neural activity with a noncontact interface; however, the technique has not been demonstrated in humans. The objective of this study is to demonstrate the safety and efficacy of INS in humans in vivo. The feasibility of INS in humans was assessed in patients (n=7) undergoing selective dorsal root rhizotomy, where hyperactive dorsal roots, identified for transection, were stimulated in vivo with INS on two to three sites per nerve with electromyogram recordings acquired throughout the stimulation. The stimulated dorsal root was removed and histology was performed to determine thermal damage thresholds of INS. Threshold activation of human dorsal rootlets occurred in 63% of nerves for radiant exposures between 0.53 and 1.23  J/cm2. In all cases, only one or two monitored muscle groups were activated from INS stimulation of a hyperactive spinal root identified by electrical stimulation. Thermal damage was first noted at 1.09  J/cm2 and a 2∶1 safety ratio was identified. These findings demonstrate the success of INS as a fresh approach for activating human nerves in vivo and providing the necessary safety data needed to pursue clinically driven therapeutic and diagnostic applications of INS in humans.

Infrared Nerve Stimulation: A Novel Therapeutic Laser Modality

Neural stimulation is the process of activating neurons using an external source to evoke action potential propagation down an axon. Electrical, chemical, thermal, optical, and mechanical methods have all been reported to stimulate neurons in both the central nervous system (CNS) and the peripheral nervous system (PNS) [1]. For nearly 2 centuries electrical stimulation has been the gold standard for the stimulation of neurons and other excitable tissues. It functions by increasing the transmembrane potential to activate voltage-gated ion channels which induce action potential propagation down the axon of a neuron [2–5]. However, electrical stimulation lacks spatial precision due to the inherent electrical field propagation which results in the recruiting of multiple (unwanted) neuronal fibers. Additionally, electrical stimulation induces a stimulation artifact which can mask neuronal signals resulting from the simulation [6, 7].

Imaging optically induced neural activity in the brain

Infrared neural stimulation (INS) is well characterized for the peripheral nervous system; however, translation to the central nervous system (CNS) presents a new set of challenges which require us to consider different anatomy, multiple cell types, and the physiology associated with structures in the CNS. This study presents our first attempt to translate INS to in vivo stimulation of the CNS and to image the related response. The results from this study show that INS generates intrinsic optical signals of similar magnitude and shape associated with well characterized mechanical stimuli. The implications of this work could lead to neural implants which allows for single cell stimulation making it possible to design closed loop neural prosthetics.

Relating optical signals induced by infrared neural stimulation to electrophysiology

Infrared neural stimulation (INS) is well characterized for the peripheral nervous system; however, translation to the central nervous system (CNS) presents a new set of challenges which require us to consider different anatomy, multiple cell types, and the physiology associated with structures in the CNS. This study represents the first attempt to translate INS to in vivo stimulation of the CNS. The results from this study show that INS generates intrinsic optical signals of similar magnitude and shape associated with well characterized mechanical stimuli. Electrophysiology analysis indicates INS evokes inhibitory responses in rat somatosensory cortex. The implications of this work could lead to neural implants which allows for single cell stimulation making it possible to design closed loop neural prosthetics.

Neural responses of rat cortical layers due to infrared neural modulation and photoablation of thalamocortical brain slices

Infrared neural modulation (INM) is a label-free method for eliciting neural activity with high spatial selectivity in mammalian models. While there has been an emphasis on INM research towards applications in the peripheral nervous system and the central nervous system (CNS), the biophysical mechanisms by which INM occurs remains largely unresolved. In the rat CNS, INM has been shown to elicit and inhibit neural activity, evoke calcium signals that are dependent on glutamate transients and astrocytes, and modulate inhibitory GABA currents. So far, in vivo experiments have been restricted to layers I and II of the rat cortex which consists mainly of astrocytes, inhibitory neurons, and dendrites from deeper excitatory neurons owing to strong absorption of light in these layers. Deeper cortical layers (III-VI) have vastly different cell type composition, consisting predominantly of excitatory neurons which can be targeted for therapies such as deep brain stimulation. The neural responses to infrared light of deeper cortical cells have not been well defined. Acute thalamocortical brain slices will allow us to analyze the effects of INS on various components of the cortex, including different cortical layers and cell populations. In this study, we present the use of photoablation with an erbium:YAG laser to reduce the thickness of the dead cell zone near the cutting surface of brain slices. This technique will allow for more optical energy to reach living cells, which should contribute the successful transduction of pulsed infrared light to neural activity. In the future, INM-induced neural responses will lead to a finer characterization of the parameter space for the neuromodulation of different cortical cell types and may contribute to understanding the cell populations that are important for allowing optical stimulation of neurons in the CNS.