Galvanic vestibular stimulation (GVS) uses the external application of electrical current to selectively target the vestibular system in humans. Despite its recent popularity for the assessment/treatment of clinical conditions, exactly how this non-invasive tool activates the vestibular system remains an open question. Here we directly investigate single vestibular afferent responses to GVS applied to the mastoid processes of awake-behaving monkeys. Transmastoid GVS produces robust and parallel activation of both canal and otolith afferents. Notably, afferent activation increases with intrinsic neuronal variability resulting in constant GVS-evoked neuronal detection thresholds across all afferents. Additionally, afferent tuning differs for GVS versus natural self-motion stimulation. Using a stochastic model of repetitive activity in afferents, we largely explain the main features of GVS-evoked vestibular afferent dynamics. Taken together, our results reveal the neural substrate underlying transmastoid GVS-evoked perceptual, ocular and postural responses—information that is essential to advance GVS applicability for biomedical uses in humans.

A simple, fast, and reproducible method for the fabrication of disk ultramicroelectrodes (UMEs) with controlled geometry is reported. The use of prepulled soda-lime glass capillaries allows one to bypass the irreproducible torch-sealing and experimentally challenging tip-sharpening steps used in conventional fabrication protocols. A micron-sized electroactive wire is sealed inside this capillary producing UMEs with a highly reproducible geometry. Total fabrication time (1 h) and experimental difficulty are significantly reduced. Disk UMEs with various diameters and cores were fabricated, including carbon fiber (7 and 11 μm), gold (10 and 25 μm), platinum (10 and 25 μm), silver (25 μm), and mercury (25 μm). The ratio of the insulating sheath to the electroactive core of the UMEs was 2.5-3.6. Silver UMEs were also used to produce a Ag/AgCl microreference electrode. This general fabrication method can readily be applied to other electroactive cores and could allow any research group to produce high quality disk UMEs, which are a prerequisite for quantitative scanning electrochemical microscopy.

It is commonly assumed that the brain’s neural coding strategies are adapted to the statistics of natural stimuli. Specifically, to maximize information transmission, a sensory neuron’s tuning function should effectively oppose the decaying stimulus spectral power, such that the neural response is temporally decorrelated (i.e. ‘whitened’). However, theory predicts that the structure of neuronal variability also plays an essential role in determining how coding is optimized. Here, we provide experimental evidence supporting this view by recording from neurons in early vestibular pathways during naturalistic self-motion. We found that central vestibular neurons displayed temporally whitened responses that could not be explained by their tuning alone. Rather, computational modeling and analysis revealed that neuronal variability and tuning were matched to effectively complement natural stimulus statistics, thereby achieving temporal decorrelation and optimizing information transmission. Taken together, our findings reveal a novel strategy by which neural variability contributes to optimized processing of naturalistic stimuli.

At the synapse, D-serine is an endogenous coagonist for the N-methyl-D-aspartate receptor (NMDAR). It plays an important role in synaptic transmission and plasticity and has also been linked to several pathological diseases such as schizophrenia and Huntington's. The quantification of local changes in D-serine concentration is essential to further understanding these processes. We report herein the development of a disk-shaped amperometric enzymatic biosensor for detection of D-serine based on a 25 μm diameter platinum disk microelectrode with an electrodeposited poly-m-phenylenediamine (PPD) layer and an R. gracilis D-amino acid oxidase (RgDAAO) layer. The disk-shaped D-serine biosensor is 1−5 orders of magnitude smaller than previously reported probes and exhibits a sensitivity of 276 μA cm −2 mM −1 with an in vitro detection limit of 0.6 μM. We demonstrate its usefulness for in vivo applications by measuring the release of endogenous D-serine in the brain of Xenopus laevis tadpoles.