Exploring the Influence of Energy Constraints on Liquid State Machines

Biological organisms operate under severe energy constraints but are still the most powerful computational systems that we know of. In contrast, modern AI algorithms are generally implemented on power-hungry hardware resources such as GPUs, limiting their use at the edge. This work explores the application of biologically-inspired energy constraints to spiking neural networks to better understand their effects on network dynamics and learning and to gain insight into the creation of more energy-efficient AI. Energy constraints are modeled by abstracting the role of astrocytes in metabolizing glucose and regulating the activity-driven distribution of ATP molecules to “pools” of neurons and synapses. First, energy constraints are applied to the fixed recurrent part (a.k.a. reservoir) of liquid state machines (LSM)—a type of recurrent spiking neural network—in order to analyze their effects on both the network’s computational performance and ability to learn. Energy constraints were observed to have a significant influence on the dynamics of the network based on metrics such as Lyapunov exponent and separation ratio. In several cases the energy constraints also led to an increase in the LSM’s classification accuracy (up to 6.17\% improvement over baseline) when applied to two time series classification tasks: epileptic seizure detection and gait recognition. This improvement in classification accuracy was also typically correlated with the LSM separation metric (Pearson correlation coefficient of 0.9 for seizure detection task). However, the increased classification accuracy was generally not observed in LSMs with sparse connectivity, highlighting the role of energy constrains in sparsifying the LSM’s spike activity, which could lead to real-world energy savings in hardware implementations. In addition to the fixed LSM reservoir, the impact of energy constraints was also explored in the context of unsupervised learning with spike-timing dependent plasticity (STDP). It was observed that energy constraints can have the effect of decreasing the magnitude of the update of synaptic weights by up to 72.4\%, on average, depending on factors such as the energy cost of neuron spikes and energy pool regeneration rate. Energy constraints under certain conditions were also seen to modify which input frequencies the synapses respond to, tending to attenuate or eliminate weight updates from high frequency inputs. The effects of neuronal energy constraints on STDP learning were also studied at the network level to determine their effects on classification task performance. The final part of this work attempts to co-optimize an LSM’s energy consumption and performance through reinforcement learning. A proximal policy optimization (PPO) agent is introduced into the LSM reservoir to control the level of neuronal spiking. This was done by allowing it to modify individual energy constraint parameters. The agent is rewarded based on the separation of the reservoir and additionally rewarded for the reduction of reservoir energy consumption

Introduction to Special Section on Microcosms in Ice: The Biogeochemistry of Cryoconite Holes

Cryoconite holes are small, water filled, cylindrical melt-holes on glacial ice surface. Cryoconite, \u27cold dust,\u27 refers to the thin layer of sediment at the hole bottom. The holes form from surficial sediment patches that absorbs more solar radiation than the surrounding ice and which preferentially melt into the glacier forming a cylindrical water-filled hole. These holes form on the ice-covered, as opposed to snow covered, parts of glaciers world-wide, wherever there is sufficient energy for melting. Biogeochemically, cryoconite holes are interesting because the sediment is inncoculated with biologic material, a fraction of which thrives in the cryoconite environment of near-freezing waters and limited nutrient supply. The holes are thus oases for microbial life and biologically mediated chemical reactions on otherwise relatively inert glacier surfaces. Examining the chemical evolution of waters in cryoconite holes, showing how biogeochemical processes in cryoconite holes lead to increasing concentrations of dissolved organic carbon over time, which in may enhance adsorption of solar radiation by the water, aiding the development of deeper holes. If this is true, it suggests that there are a number of complex interactions between the biology, chemistry and biology of cryoconite holes, which act in concert to maintain life on glacier surfaces

The arctic circle boundary and the Airy process

We prove that the, appropriately rescaled, boundary of the north polar region in the Aztec diamond converges to the Airy process. The proof uses certain determinantal point processes given by the extended Krawtchouk kernel. We also prove a version of Propp's conjecture concerning the structure of the tiling at the center of the Aztec diamond.Comment: Published at http://dx.doi.org/10.1214/009117904000000937 in the Annals of Probability (http://www.imstat.org/aop/) by the Institute of Mathematical Statistics (http://www.imstat.org

Glaciers in Equilibrium, McMurdo Dry Valleys, Antarctica

The McMurdo Dry Valleys are a cold, dry polar desert and the alpine glaciers therein exhibit small annual and seasonal mass balances, ofte

Distributed Modeling of Ablation (1996–2011) and Climate Sensitivity on the Glaciers of Taylor Valley, Antarctica

The McMurdo Dry Valleys of Antarctica host the coldest and driest ecosystem on Earth, which is acutely sensitive to the availability of water coming from glacial runoff. We modeled the spatial variability in ablation and assessed climate sensitivity of the glacier ablation zones using 16 years of meteorological and surface mass-balance observations collected in Taylor Valley. Sublimation was the primary form of mass loss over much of the ablation zones, except for near the termini where melt, primarily below the surface, dominated. Microclimates in ~10 m scale topographic basins generated melt rates up to ten times higher than over smooth glacier surfaces. In contrast, the vertical terminal cliffs on the glaciers can have higher or lower melt rates than the horizontal surfaces due to differences in incoming solar radiation. The model systematically underpredicted ablation for the final 5 years studied, possibly due to an increase of windblown sediment. Surface mass-balance sensitivity to temperature was ~−0.02 m w.e. K−1, which is among the smallest magnitudes observed globally. We also identified a high sensitivity to ice albedo, with a decrease of 0.02 having similar effects as a 1 K increase in temperature, and a complex sensitivity to wind speed

Do Patients Taking Warfarin Experience Delays to Theatre, Longer Hospital Stay, and Poorer Survival After Hip Fracture?

BACKGROUND: Patients sustaining a fractured neck of the femur are typically of advanced age with multiple comorbidities. As a consequence, the proportion of these patients receiving warfarin therapy is approximately 10%. There are currently few studies investigating outcomes in this subset of patients. QUESTIONS/PURPOSES: The purpose of this study was to assess the association between warfarin therapy and time to surgery, length of hospital stay, and survival in patients sustaining a fractured neck of the femur. METHODS: Data for 2036 patients admitted to our center between July 2009 and July 2014 with a fractured neck of the femur were extracted from the National Hip Fracture Database. Fifty-seven patients received no surgical treatment and were excluded from analysis. Multivariable ordinary least squares regression was performed to test the association between warfarin treatment on time to surgery and length of stay, and Cox proportional hazards to test followup survival. Variables included in the regression model were age, sex, American Society of Anesthesiologists (ASA) score, admission Abbreviated Mental Test Score (AMTS), fracture type, operation type, and premorbid Work Ability Index (WAI). One hundred fifty-two of 1979 surgically treated patients (8%) were receiving warfarin therapy at the time of admission. RESULTS: After controlling for age, sex, ASA score, AMTS, fracture type, operation type, and WAI, we found that patients taking warfarin were less likely to go to surgery by 36 hours (odds ratio [OR], 0.20; 95% CI, 0.14-0.30), and less likely to go to surgery by 48 hours (OR, 0.17; 95% CI, 0.11-0.24). Patients taking warfarin had a longer length of stay (median, 15 days; interquartile range [IQR], 12-22 days) compared with patients not taking warfarin (median, 13 days; IQR, 9-20 days; p < 0.001). Survival analysis to June 2015 showed a higher mortality for patients taking warfarin (12-month survival, 66% vs 76%; hazard ratio, 1.57; 95% CI, 1.21-2.04; p < 0.001). CONCLUSIONS: After controlling for multiple prognostic factors such as age, ASA score, AMTS, and WAI, warfarin therapy at the time of injury is associated with increased time to surgery, length of stay, and decreased survival. This study highlights the need to view warfarin therapy as a 'red flag' in patients presenting with a fractured neck of the femur. Preoperatively, prompt warfarin reversal together with adequate investigation and optimization of the patient should ensure timely, safe surgery. Early involvement of the anesthesia team should ensure an appropriate level of postoperative care for these patients. LEVEL OF EVIDENCE: Level III, therapeutic study.This is the final version of the article. It first appeared from Springer via http://dx.doi.org/10.1007/s11999-016-5056-

A digital archive of human activity in the McMurdo Dry Valleys, Antarctica

Over the last half century, the McMurdo Dry Valleys (MDV) of East Antarctica have become a globally important site for scientific research and environmental monitoring. Historical data can make important contributions to current research activities and environmental management in Antarctica but tend to be widely scattered and difficult to access. We address this need in the MDV by compiling over 5000 historical photographs, sketches, maps, oral interviews, publications, and other archival resources into an online digital archive. The data have been digitized and georeferenced using a standardized metadata structure, which enables intuitive searches and data discovery via an online interface. The ultimate aim of the archive is to create as comprehensive as possible a record of human activity in the MDV to support ongoing research, management, and conservation efforts. This is a valuable tool for scientists seeking to understand the dynamics of change in lakes, glaciers, and other physical systems, as well as humanistic inquiry into the history of the Southern Continent. In addition to providing benchmarks for understanding change over time, the data can help target field sampling for studies working under the assumption of a pristine landscape by enabling researchers to identify the date and extent of past human activities