Extrapolated High-Order Propagators for Path Integral Monte Carlo Simulations

We present a new class of high-order imaginary time propagators for path-integral Monte Carlo simulations by subtracting lower order propagators. By requiring all terms of the extrapolated propagator be sampled uniformly, the subtraction only affects the potential part of the path integral. The negligible violation of positivity of the resulting path integral at small time steps has no discernable affect on the accuracy of our method. Thus in principle arbitrarily high order algorithms can be devised for path-integral Monte Carlo simulations. We verify this claim is by showing that fourth, sixth, and eighth order convergence can indeed be achieved in solving for the ground state of strongly interacting quantum many-body systems such as bulk liquid $^4$He.Comment: 9 pages and 3 figures. Submitted to J. Chem. Phy

Sparse, reliable, and long-term stable representation of periodic whisker deflections in the mouse barrel cortex

The rodent whisker system is a preferred model for studying plasticity in the somatosensory cortex (barrel cortex). Contrarily, only a small amount of research has been conducted to characterize the stability of neuronal population activity in the barrel cortex. We used the mouse whisker system to address the neuronal basis of stable perception in the somatosensory cortex. Cortical representation of periodic whisker deflections was studied in populations of neurons in supragranular layers over extended time periods (up to 3 months) with long-term two-photon Ca(2+) imaging in anesthetized mice. We found that in most of the neurons (87%), Ca(2+) responses increased sublinearly with increasing number of contralateral whisker deflections. The imaged population of neurons was activated in a stereotypic way over days and for different deflection rates (pulse frequencies). Thus, pulse frequencies are coded by response strength rather than by distinct neuronal sub-populations. A small population of highly responsive neurons (~3%) was sufficient to decode the whisker stimulus. This conserved functional map, led by a small set of highly responsive neurons, might form the foundation of stable sensory percepts

Distinct Contributions of Whisker Sensory Cortex and Tongue-Jaw Motor Cortex in a Goal-Directed Sensorimotor Transformation

The neural circuits underlying goal-directed sensorimotor transformations in the mammalian brain are incompletely understood. Here, we compared the role of primary tongue-jaw motor cortex (tjM1) and primary whisker sensory cortex (wS1) in head-restrained mice trained to lick a reward spout in response to whisker deflection. Two-photon microscopy combined with microprisms allowed imaging of neuronal network activity across cortical layers in transgenic mice expressing a genetically encoded calcium indicator. Early-phase activity in wS1 encoded the whisker sensory stimulus and was necessary for detection of whisker stimuli. Activity in tjM1 encoded licking direction during task execution and was necessary for contralateral licking. Pre-stimulus activity in tjM1, but not wS1, was predictive of lick direction and contributed causally to small preparatory jaw movements. Our data reveal a shift in coding scheme from wS1 to tjM1, consistent with the hypothesis that these areas represent cortical start and end points for this goal-directed sensorimotor transformation

Novel two-alternative forced choice paradigm for bilateral vibrotactile whisker frequency discrimination in head-fixed mice and rats

Rats and mice receive a constant bilateral stream of tactile information with their large mystacial vibrissae when navigating in their environment. In a two-alternative forced choice paradigm (2-AFC), head-fixed rats and mice learned to discriminate vibrotactile frequencies applied simultaneously to individual whiskers on the left and right sides of the snout. Mice and rats discriminated 90-Hz pulsatile stimuli from pulsatile stimuli with lower repetition frequencies (10-80 Hz) but with identical kinematic properties in each pulse. Psychometric curves displayed an average perceptual threshold of 50.6-Hz and 53.0-Hz frequency difference corresponding to Weber fractions of 0.56 and 0.58 in mice and rats, respectively. Both species performed >400 trials a day (>200 trials per session, 2 sessions/day), with a peak performance of >90% correct responses. In general, rats and mice trained in the identical task showed comparable psychometric curves. Behavioral readouts, such as reaction times, learning rates, trial omissions, and impulsivity, were also very similar in the two species. Furthermore, whisking of the animals before stimulus presentation reduced task performance. This behavioral paradigm, combined with whisker position tracking, allows precise stimulus control in the 2-AFC task for head-fixed rodents. It is compatible with state-of-the-art neurophysiological recording techniques, such as electrophysiology and two-photon imaging, and therefore represents a valuable framework for neurophysiological investigations of perceptual decision-making

Shape-Controlled Nanoparticles in Pore-Confined Space

Increasing the catalyst’s stability and activity are one of the main quests in catalysis. Tailoring crystal surfaces to a specific reaction has demonstrated to be a very effective way in increasing the catalyst’s specific activity. Shape controlled nanoparticles with specific crystal facets are usually grown kinetically and are highly susceptible to morphological changes during the reaction due to agglomeration, metal dissolution, or Ostwald ripening. A strong interaction of the catalytic material to the support is thus crucial for successful stabilization. Taken both points into account, a general catalyst design is proposed, combining the enhanced activity of shape-controlled nanoparticles with a pore-confinement approach for high stability. Hollow graphitic spheres with narrow and uniform bimodal mesopores serve as model system and were used as support material. As catalyst, different kinds of particles, such as pure platinum (Pt), platinum/nickel (Pt3Ni) and Pt3Ni doped with molybdenum (Pt3Ni–Mo), have exemplarily been synthesized. The advantages, limits and challenges of the proposed concept are discussed and elaborated by means of time-resolved, in and ex situ measurements. It will be shown that during catalysis, the potential boundaries are crucial especially for the proposed catalyst design, resulting in either retention of the initial activity or drastic loss in shape, size and elemental composition. The synthesis and catalyst design can be adapted to a wide range of catalytic reactions where stabilization of shape-controlled particles is a focus

Design and performance of an ultra-flexible two-photon microscope for in vivo research

We present a cost-effective in vivo two-photon microscope with a highly flexible frontend for in vivo research. Our design ensures fast and reproducible access to the area of interest, including rotation of imaging plane, and maximizes space for auxiliary experimental equipment in the vicinity of the animal. Mechanical flexibility is achieved with large motorized linear stages that move the objective in the X, Y, and Z directions up to 130 mm. 360° rotation of the frontend (rotational freedom for one axis) is achieved with the combination of a motorized high precision bearing and gearing. Additionally, the modular design of the frontend, based on commercially available optomechanical parts, allows straightforward updates to future scanning technologies. The design exceeds the mobility of previous movable microscope designs while maintaining high optical performance

Tactile frequency discrimination is enhanced by circumventing neocortical adaptation

Neocortical responses typically adapt to repeated sensory stimulation, improving sensitivity to stimulus changes, but possibly also imposing limitations on perception. For example, it is unclear whether information about stimulus frequency is perturbed by adaptation or encoded by precise response timing. We addressed this question in rat barrel cortex by comparing performance in behavioral tasks with either whisker stimulation, which causes frequency-dependent adaptation, or optical activation of cortically expressed channelrhodopsin-2, which elicits non-adapting neural responses. Circumventing adaption by optical activation substantially improved cross-hemispheric discrimination of stimulus frequency. This improvement persisted when temporal precision of optically evoked spikes was reduced. We were able to replicate whisker-driven behavior only by applying adaptation rules mimicking sensory-evoked responses to optical stimuli. Conversely, in a change-detection task, animals performed better with whisker than optical stimulation. Our results directly demonstrate that sensory adaptation critically governs the perception of stimulus patterns, decreasing fidelity under steady-state conditions in favor of change detection