Charge Transition of Oxygen Vacancies during Resistive Switching in Oxide-Based RRAM.

Resistive random-access memory (RRAM) devices have attracted broad interest as promising building blocks for high-density nonvolatile memory and neuromorphic computing applications. Atomic level thermodynamic and kinetic descriptions of resistive switching (RS) processes are essential for continued device design and optimization but are relatively lacking for oxide-based RRAMs. It is generally accepted that RS occurs due to the redistribution of charged oxygen vacancies driven by an external electric field. However, this assumption contradicts the experimentally observed stable filaments, where the high vacancy concentration should lead to a strong Coulomb repulsion and filament instability. In this work, through predictive atomistic calculations in combination with experimental measurements, we attempt to understand the interactions between oxygen vacancies and the microscopic processes that are required for stable RS in a Ta2O5-based RRAM. We propose a model based on a series of charge transition processes that explains the drift and aggregation of vacancies during RS. The model was validated by experimental measurements where illuminated devices exhibit accelerated RS behaviors during SET and RESET. The activation energies of ion migration and charge transition were further experimentally determined through a transient current measurement, consistent with the modeling results. Our results help provide comprehensive understanding on the internal dynamics of RS and will benefit device optimization and applications.

K-means Data Clustering with Memristor Networks.

Memristor-based neuromorphic networks have been actively studied as a promising candidate to overcome the von-Neumann bottleneck in future computing applications. Several recent studies have demonstrated memristor network's capability to perform supervised as well as unsupervised learning, where features inherent in the input are identified and analyzed by comparing with features stored in the memristor network. However, even though in some cases the stored feature vectors can be normalized so that the winning neurons can be directly found by the (input) vector-(stored) vector dot-products, in many other cases, normalization of the feature vectors is not trivial or practically feasible, and calculation of the actual Euclidean distance between the input vector and the stored vector is required. Here we report experimental implementation of memristor crossbar hardware systems that can allow direct comparison of the Euclidean distances without normalizing the weights. The experimental system enables unsupervised K-means clustering algorithm through online learning, and produces high classification accuracy (93.3%) for the standard IRIS data set. The approaches and devices can be used in other unsupervised learning systems, and significantly broaden the range of problems a memristor-based network can solve.

On-Demand Reconfiguration of Nanomaterials: When Electronics Meets Ionics.

Rapid advances in the semiconductor industry, driven largely by device scaling, are now approaching fundamental physical limits and face severe power, performance, and cost constraints. Multifunctional materials and devices may lead to a paradigm shift toward new, intelligent, and efficient computing systems, and are being extensively studied. Herein examines how, by controlling the internal ion distribution in a solid-state film, a material's chemical composition and physical properties can be reversibly reconfigured using an applied electric field, at room temperature and after device fabrication. Reconfigurability is observed in a wide range of materials, including commonly used dielectric films, and has led to the development of new device concepts such as resistive random-access memory. Physical reconfigurability further allows memory and logic operations to be merged in the same device for efficient in-memory computing and neuromorphic computing systems. By directly changing the chemical composition of the material, coupled electrical, optical, and magnetic effects can also be obtained. A survey of recent fundamental material and device studies that reveal the dynamic ionic processes is included, along with discussions on systematic modeling efforts, device and material challenges, and future research directions.

Iodine Vacancy Redistribution in Organic-Inorganic Halide Perovskite Films and Resistive Switching Effects.

Organic-inorganic halide perovskite (OHP) materials, for example, CH3 NH3 PbI3 (MAPbI3 ), have attracted significant interest for applications such as solar cells, photodectors, light-emitting diodes, and lasers. Previous studies have shown that charged defects can migrate in perovskites under an electric field and/or light illumination, potentially preventing these devices from practical applications. Understanding and control of the defect generation and movement will not only lead to more stable devices but also new device concepts. Here, it is shown that the formation/annihilation of iodine vacancies (VI 's) in MAPbI3 films, driven by electric fields and light illumination, can induce pronounced resistive switching effects. Due to a low diffusion energy barrier (≈0.17 eV), the VI 's can readily drift under an electric field, and spontaneously diffuse with a concentration gradient. It is shown that the VI diffusion process can be suppressed by controlling the affinity of the contact electrode material to I- ions, or by light illumination. An electrical-write and optical-erase memory element is further demonstrated by coupling ion migration with electric fields and light illumination. These results provide guidance toward improved stability and performance of perovskite-based optoelectronic systems, and can lead to the development of solid-state devices that couple ionics, electronics, and optics.

Benefits of using transcranial magnetic stimulation as a tool to facilitate the chronic knee injury rehabilitation.

[Purpose] While primary motor cortex activation has been implicated as a key factor in the arthrogenic muscle inhibition after knee joint injury, no viable rehabilitation protocol has been developed to accommodate this factor. In this study, transcranial magnetic stimulation was applied as a means of dissipating arthrogenic muscle inhibition by introducing temporary motor cortex excitation prior to the rehabilitation. [Subjects and Methods] Twenty-four subjects who have underwent the surgery due to knee injury were recruited, and randomly assigned to the control or the simulation groups. The levels of electromyography signals during the maximum voluntary contraction of the quadriceps muscle before, during, and after training designed for the quadriceps strength rehabilitation were measured. [Results] When compared to controls, subjects who received the transcranial magnetic stimulations showed significantly increased levels of voluntary muscle contraction after the training. Moreover, the beneficial effect of the stimulation increased as the rehabilitation progressed. [Conclusion] Transcranial magnetic stimulation itself does not directly improve the symptoms related to knee injuries. However, the use of this technique can provide a time window for effective intervention by dissipating the unwanted effect of the arthrogenic muscle inhibition during rehabilitation.

Experimental Demonstration of Feature Extraction and Dimensionality Reduction Using Memristor Networks.

Memristors have been considered as a leading candidate for a number of critical applications ranging from nonvolatile memory to non-Von Neumann computing systems. Feature extraction, which aims to transform input data from a high-dimensional space to a space with fewer dimensions, is an important technique widely used in machine learning and pattern recognition applications. Here, we experimentally demonstrate that memristor arrays can be used to perform principal component analysis, one of the most commonly used feature extraction techniques, through online, unsupervised learning. Using Sanger's rule, that is, the generalized Hebbian algorithm, the principal components were obtained as the memristor conductances in the network after training. The network was then used to analyze sensory data from a standard breast cancer screening database with high classification success rate (97.1%).

Electronic and optical properties of oxygen vacancies in amorphous Ta2O5 from first principles.

Oxygen vacancies are important defects considered to play a central role in the electronic and optical properties of tantalum pentoxide (Ta2O5) films and devices. Despite extensive experimental studies on oxygen vacancies in Ta2O5, the reported defect states are ambiguously identified due to the absence of accurate and conclusive theoretical evidence. Here we investigate the thermodynamic, electronic, and optical properties of oxygen vacancies in amorphous Ta2O5 by first-principles calculations based on hybrid-functional density functional theory (DFT). The calculated thermodynamic and optical transition levels are in good agreement with a broad range of diverse measured properties with various experimental methods, providing conclusive evidence for the identification of the defect states observed in experiments as originating from oxygen vacancies. Our calculations also predict the formation of spin-polarized polarons. Our results elucidate the fundamental atomistic properties of oxygen vacancies in various oxidation states as a function of growth conditions and provide guidance to control the properties of Ta2O5 films/devices.

Motor Planning Influences the Perceived Timing of Vibrotactile Stimuli in an Amplitude-Dependent Manner.

The authors characterized how motor planning influences temporal order judgment (TOJ) tasks. They examined this by applying vibrotactile stimulation during the planning stages of a bimanual arm movement that would bring the arms into a crossed configuration. The authors have previously shown that planning to cross the arms induces a subjective reversal of spatially defined temporal order judgments that evolves over the course of the planning period. It was unclear, however, whether this effect is modulated by the extent to which the arms would be crossed after movement. The authors examined this issue by having participants plan to move to 4 different targets that would leave the arms in crossed configurations of varying extents. The results demonstrate that even though cutaneous stimuli were applied before the movements, if participants were planning to move into a more crossed configuration, performance on the TOJ task worsened depending on where they were in the planning process. This data suggest the brain uses planning signals to predict sensations from impending movements in a context-dependent manner.

Availability of vision and tactile gating: vision enhances tactile sensitivity.

A multitude of events bombard our sensory systems at every moment of our lives. Thus, it is important for the sensory and motor cortices to gate unimportant events. Tactile suppression is a well-known phenomenon defined as a reduced ability to detect tactile events on the skin before and during movement. Previous experiments (Buckingham et al. in Exp Brain Res 201(3):411-419, 2010; Colino et al. in Physiol Rep 2(3):e00267, 2014) found detection rates decrease just prior to and during finger abduction and decrease according to the proximity of the moving effector. However, what effect does vision have on tactile gating? There is ample evidence (see Serino and Haggard in Neurosci Biobehav Rev 34:224-236, 2010) observing increased tactile acuity when participants see their limbs. The present study examined how tactile detection changes in response to visual condition (vision/no vision). Ten human participants used their right hand to reach and grasp a cylinder. Tactors were attached to the index finger and the forearm of both the right and left arm and vibrated at various epochs relative to a "go" tone. Results replicate previous findings from our laboratory (Colino et al. in Physiol Rep 2(3):e00267, 2014). Also, tactile acuity decreased when participants did not have vision. These results indicate that the vision affects the somatosensation via inputs from parietal areas (Konen and Haggard in Cereb Cortex 24(2):501-507, 2014) but does so in a reach-to-grasp context.

Tuning Ionic Transport in Memristive Devices by Graphene with Engineered Nanopores.

Memristors, based on inherent memory effects in simple two-terminal structures, have attracted tremendous interest recently for applications ranging from nonvolatile data storage to neuromorphic computing based on non-von Neumann architectures. In a memristor, the ability to modulate and retain the state of an internal variable leads to experimentally observed resistive switching (RS) effects. Such phenomena originate from internal, microscopic ionic migration and associated electrochemical processes that modify the materials' electrical and other physical properties. To optimize the device performance for practical applications with large-size arrays, controlling the internal ionic transport and redox reaction processes thus becomes a necessity, ideally at the atomic scale. Here we show that the RS characteristics in tantalum-oxide-based memristors can be systematically tuned by inserting a graphene film with engineered nanopores. Graphene, with its atomic thickness and excellent impermeability and chemical stability, can be effectively integrated into the device stack and can offer unprecedented capabilities for the control of ionic dynamics at the nanoscale. In this device structure, the graphene film effectively blocks ionic transport and redox reactions; thereby the oxygen vacancies required during the RS process are allowed to transport only through the engineered nanosized openings in the graphene layer, leading to effective modulation of the device performance by controlling the nanopore size in graphene. The roles of graphene as an ion-blocking layer in the device structure were further supported by transmission electron microscopy, energy-dispersive X-ray spectroscopy, and atomistic simulations based on first-principles calculations.

Tuning resistive switching characteristics of tantalum oxide memristors through Si doping.

An oxide memristor device changes its internal state according to the history of the applied voltage and current. The principle of resistive switching (RS) is based on ion transport (e.g., oxygen vacancy redistribution). To date, devices with bi-, triple-, or even quadruple-layered structures have been studied to achieve the desired switching behavior through device structure optimization. In contrast, the device performance can also be tuned through fundamental atomic-level design of the switching materials, which can directly affect the dynamic transport of ions and lead to optimized switching characteristics. Here, we show that doping tantalum oxide memristors with silicon atoms can facilitate oxygen vacancy formation and transport in the switching layer with adjustable ion hopping distance and drift velocity. The devices show larger dynamic ranges with easier access to the intermediate states while maintaining the extremely high cycling endurance (>10(10) set and reset) and are well-suited for neuromorphic computing applications. As an example, we demonstrate different flavors of spike-timing-dependent plasticity in this memristor system. We further provide a characterization methodology to quantitatively estimate the effective hopping distance of the oxygen vacancies. The experimental results are confirmed through detailed ab initio calculations which reveal the roles of dopants and provide design methodology for further optimization of the RS behavior.

Effects of repetitive transcranial magnetic stimulation on the somatosensory cortex during prism adaptation.

Although the behavioral characteristics and the neural correlates of prism adaptation processes have been studied extensively, the underlying mechanism is yet to be investigated. Recently, somatosensory suppression was heralded as a mechanism for the sensory re-alignment process accompanying the adaptation. Somatosensory suppression should facilitate the re-alignment process in the proprioceptive system. The shift in the proprioceptive system takes place mostly during a concurrent visual feedback (CVF) condition; during a terminal visual feedback (TVF) condition, the visual system experiences significant adaptation (visual shift), so somatosensory suppression should have minimal functional consequences under TVF. To test this hypothesis, a repetitive transcranial magnetic stimulation (rTMS) was applied to the primary somatosensory cortex as an artificial somatosensory suppression right after the reaching initiation in CVF and TVF conditions, and changes in adaptation were observed. Because somatosensory suppression is already in effect during CVF, rTMS would cause no significant changes. During TVF with rTMS, however, significantly different patterns of adaptation could be expected when compared to a sham rTMS condition. Young adults (N = 12) participated in 4 sessions (CVF/ TVF, real/sham rTMS); visual proprioceptive, and total shifts were measured. Movement time and curvature of the reaching movement were measured during the adaptation phase. Results showed that while the total shift was unchanged, the proprioceptive shift increased and the visual shift decreased in the TVF condition when rTMS was delivered. However, the total, proprioceptive, and visual shifts were not influenced by rTMS in the CVF condition. Suppression of proprioception induced by the rTMS could be one of the requisites for successful proprioceptive shift during prism adaptation.

The human dorsal premotor cortex generates on-line error corrections during sensorimotor adaptation.

A number of different sites in the human brain have been shown to play a role in sensorimotor adaptation. However, the specific role played by each of these structures in the learning process is poorly understood. In the present study, the contribution of the dorsal aspect of the premotor cortex was examined by disrupting activity at this site using transcranial magnetic stimulation (TMS) while subjects wearing prism goggles pointed at visual targets. This manipulation slowed down the rate of adaptation when vision of the hand was available throughout the movement and reduced the presence of on-line trajectory corrections. This was accompanied by a reduced shift in the felt position of the arm. In contrast, TMS did not cause any alteration in the performance of this task when vision of the hand was available only at the end of the movement. Thus, we infer from this pattern of results that the human dorsal premotor cortex contributes to the generation of the visually based on-line error corrections that are responsible for the remapping of arm position sense underlying sensorimotor adaptation.

Enhanced accuracy in novel mirror drawing after repetitive transcranial magnetic stimulation-induced proprioceptive deafferentation.

When performing visually guided actions under conditions of perturbed visual feedback, e.g., in a mirror or a video camera, there is a spatial conflict between visual and proprioceptive information. Recent studies have shown that subjects without proprioception avoid this conflict and show a performance benefit. In this study, we tested whether deafferentation induced by repetitive transcranial magnetic stimulation (rTMS) can improve mirror tracing skills in normal subjects. Hand trajectory error during novel mirror drawing was compared across two groups of subjects that received either 1 Hz rTMS over the somatosensory cortex contralateral to the hand or sham stimulation. Mirror tracing was more accurate after rTMS than after sham stimulation. Using a position-matching task, we confirmed that rTMS reduced proprioceptive acuity and that this reduction was largest when the coil was placed at an anterior parietal site. It is thus possible, with rTMS, to enhance motor performance in tasks involving a visuoproprioceptive conflict, presumably by reducing the excitability of somatosensory cortical areas that contribute to the sense of hand position.

Bilateral accommodations to anterior cruciate ligament deficiency and surgery.

UNLABELLED: OBJECTIVE To determine bilateral lower extremity joint accommodations during gait in anterior cruciate ligament deficient subjects and uninjured controls. DESIGN: Gait testing of 10 chronic anterior cruciate ligament deficient subjects prior to and 3 months following reconstructive surgery, and 10 uninjured controls. BACKGROUND: It is possible that bilateral joint accommodations could occur as a result of anterior cruciate ligament injury and in response to surgical repair. Few studies have investigated bilateral joint accommodations to anterior cruciate ligament injury and there is little consistency in the reported results. METHODS: Bilateral lower extremity kinematic and kinetic data were collected from 12 walking trials and inverse dynamics calculations were made to estimate bilateral knee and hip joint angle, moment, and power patterns during the stance phase of gait. RESULTS: Control subjects exhibited asymmetrical hip but symmetrical knee joint moment and power patterns. In contrast, the anterior cruciate ligament deficient subjects exhibited symmetrical hip and asymmetrical knee joint moment and power patterns prior to and following reconstructive surgery. CONCLUSIONS: Gait asymmetry in healthy subjects should not be considered pathological. In addition, chronic anterior cruciate ligament injury results in joint specific, bilateral lower extremity accommodations in gait mechanics. These accommodations persist 3 months following surgical repair.

Reactive balance adjustments to unexpected perturbations during human walking.

The purpose of this investigation was to determine the effect of unexpected forward perturbations (FP) during gait on lower extremity joint mechanics and muscle Electromyographic (EMG) patterns in healthy adults. The muscles surrounding the hip were found to be most important in maintaining control of the trunk and preventing collapse in response to the FP. Distinct lower extremity joint moment and power patterns were observed in response to the FP but an overall positive moment of support (M(s)) was maintained. Therefore, reactive balance control was a synchronized effort of the lower extremity joints to prevent collapse during the FP.

Eye-hand interactions differ in the human premotor and parietal cortices.

In order to successfully look at and reach for a visual target the central nervous system must perform a complex sensorimotor transformation. How this transformation is mapped onto relevant brain structures has become the subject of much recent investigation. In the present paper we examined the contribution of the human premotor cortex (PMC) to this transformation process during a task requiring coordinated eye and hand movements. For this purpose, we made use of single-pulse transcranial magnetic stimulation (TMS) to temporarily disrupt the processing occurring in the PMC during task performance. Subjects made open-loop pointing movements accompanied by saccades of the same size or two or three times larger. Under normal circumstances without TMS, the pointing movement amplitude increased with saccade amplitude. When TMS was applied over the PMC 100-200 ms after target presentation, the influence of saccade amplitude on the pointing movement amplitude was increased. This is the opposite effect to that observed in a previous study [Journal of Neurophysiology 84 (200) 1677-1680] when TMS was applied over the posterior parietal cortex (PPC) during the same task. We suggest that this pattern of results is consistent with the coding of the reach plan in eye-centered coordinates in the PPC and limb-centered coordinates in the PMC.

Gait mechanics in chronic ACL deficiency and subsequent repair.

OBJECTIVE: To determine how normal gait patterns may change as a result of chronic anterior cruciate ligament deficiency and subsequent reconstructive surgery. DESIGN: Gait testing of 10 chronic anterior cruciate ligament deficient subjects prior to and 3 months following reconstructive surgery, and 10 uninjured controls. BACKGROUND: There is controversy whether persons with chronic anterior cruciate ligament deficiency develop a "quadriceps avoidance" pattern and how anterior cruciate ligament reconstructive surgery influences gait mechanics in these same individuals. METHODS: Gait analysis was employed to determine kinematic, kinetic, and muscle Electromyographic data. RESULTS: Prior to surgery, no anterior cruciate ligament deficient subject exhibited a quadriceps avoidance pattern. Following surgery, the subjects exhibited a significantly greater knee extensor moment during early stance as compared to the control group. Prior to and following surgery, anterior cruciate ligament deficient subjects demonstrated a significantly greater hip extensor moment possibly to reduce anterior tibial translation. CONCLUSIONS; These data suggest that (1) development of a quadriceps avoidance pattern is less common than previously reported, (2) anterior cruciate ligament deficient subjects accommodate through alterations of hip joint mechanics, (3) surgical repair significantly alters lower extremity gait patterns, and (4) re-establishment of pre-injury gait patterns takes longer than 3 months to occur. RELEVANCE: The results suggest that chronic anterior cruciate ligament deficient subjects do not exhibit a quadriceps avoidance gait pattern. Surgical intervention significantly alters lower extremity gait mechanics in a population that has accommodated to anterior cruciate ligament deficiency.

Dorsal and ventral visual stream contributions to perception-action interactions during pointing.

The Ebbinghaus illusion, in which a central circle surrounded by large circles appears to be smaller than a central circle surrounded by small circles, affects the speed of pointing movements. When the central circle appears to be big, pointing movements directed towards it are faster than when the central circle appears to be small. This effect could be due to an interaction between ventral stream processing associated with determining relative object size and dorsal stream processing associated with sensorimotor output. Alternatively, the dorsal stream alone could mediate the effect via the transformation of object shape representations into motor output within the parietal lobe. Finally, ventral stream processing could be integrated into motor output through projections to the prefrontal cortex and subsequently to the motor areas of the cortex, thus bypassing the dorsal stream. These three alternatives were tested by disrupting either the ventral or dorsal stream processing using transcranial magnetic stimulation (TMS) while subjects made pointing movements as quickly and accurately as possible to the central target circles within the Ebbinghaus illusion display. The relative changes in reaction time, movement speed, and movement accuracy for small versus large appearing target circles were compared when TMS was delivered over each site as well as at a control site (SMA). The results showed that TMS over either the dorsal or ventral stream but not the SMA reduced the influence of the illusion on the pointing movement speed but did not affect reaction time or movement accuracy. A second control experiment was completed in which TMS was delivered during pointing movements to target circles of physically different sizes that were not surrounded by either large or small circles. This allowed us to determined whether the effect we observed in the main experiment was due specifically to the relative size information contained within the illusory display and the effect this has on the preparation of pointing responses or to an influence on basic perceptual and sensorimotor processes occurring within the ventral and dorsal streams, respectively. The results showed that the affect on pointing movement speed was still present with dorsal but not ventral stream stimulation. Taken together, this evidence suggests that the ventral stream contributes to pointing movements based on relative object size information via its projections to the prefrontal areas and not necessarily through interactions with the dorsal stream.