Optimal neural population coding of an auditory spatial cue

A sound, depending on the position of its source, can take more time to reach one ear than the other. This interaural (between the ears) time difference (ITD) provides a major cue for determining the source location. Many auditory neurons are sensitive to ITDs, but the means by which such neurons represent ITD is a contentious issue. Recent studies question whether the classical general model (the Jeffress model) applies across species. Here we show that ITD coding strategies of different species can be explained by a unifying principle: that the ITDs an animal naturally encounters should be coded with maximal accuracy. Using statistical techniques and a stochastic neural model, we demonstrate that the optimal coding strategy for ITD depends critically on head size and sound frequency. For small head sizes and/or low-frequency sounds, the optimal coding strategy tends towards two distinct sub-populations tuned to ITDs outside the range created by the head. This is consistent with recent observations in small mammals. For large head sizes and/or high frequencies, the optimal strategy is a homogeneous distribution of ITD tunings within the range created by the head. This is consistent with observations in the barn owl. For humans, the optimal strategy to code ITDs from an acoustically measured distribution depends on frequency; above 400 Hz a homogeneous distribution is optimal, and below 400 Hz distinct sub-populations are optimal.     

Comprehensive list by habitat of algae of Utah, USA.

A list of the algal species that have been reported from the state of Utah is presented. Also listed are the habitats from which these algae were collected. A total of nearly 1,900 taxa have been identified to the species level or below. Diatoms comprise the largest group, with nearly 1,000 taxa, followed by the green algae with over 550 taxa.     

基于科学思维的英语文学教学:跨学科的视角

本文阐述基于科学思维过程的英语文学教学策略,即英语文学作品以综合课程的形式应用于教学实践活动.其要点是从优秀文学作品中选取需要解决问题的情境,从跨学科的视角来审视作品中的矛盾和冲突.教师们通过分析这些问题情境教给学生科学的思维方法.其教学策略的重点在于提出问题和做出决策.这些用于跨学科教学策略的文学作品是关注真实世界的起点.生活中面临的两难问题通过基于科学思维的文学教学而获得理性的解释或合理的解决.     

A mechanistic principle for proton pumping by cytochrome c oxidase

In aerobic organisms, cellular respiration involves electron transfer to oxygen through a series of membrane-bound protein complexes. The process maintains a transmembrane electrochemical proton gradient that is used, for example, in the synthesis of ATP. In mitochondria and many bacteria, the last enzyme complex in the electron transfer chain is cytochrome c oxidase (CytcO), which catalyses the four-electron reduction of O2 to H2O using electrons delivered by a water-soluble donor, cytochrome c. The electron transfer through CytcO, accompanied by proton uptake to form H2O drives the physical movement (pumping) of four protons across the membrane per reduced O2. So far, the molecular mechanism of such proton pumping driven by electron transfer has not been determined in any biological system. Here we show that proton pumping in CytcO is mechanistically coupled to proton transfer to O2 at the catalytic site, rather than to internal electron transfer. This scenario suggests a principle by which redox-driven proton pumps might operate and puts considerable constraints on possible molecular mechanisms by which CytcO translocates protons.     

Nanotechnology: high-speed integrated nanowire circuits

Macroelectronic circuits made on substrates of glass or plastic could one day make computing devices ubiquitous owing to their light weight, flexibility and low cost. But these substrates deform at high temperatures so, until now, only semiconductors such as organics and amorphous silicon could be used, leading to poor performance. Here we present the use of low-temperature processes to integrate high-performance multi-nanowire transistors into logical inverters and fast ring oscillators on glass substrates. As well as potentially enabling powerful electronics to permeate all aspects of modern life, this advance could find application in devices such as low-cost radio-frequency tags and fully integrated high-refresh-rate displays.     

Transverse and longitudinal variation in woody riparian vegetation along a montane river

This study explores how the relationship between flow and riparian vegetation varies along a montane river. We mapped occurrence of woody riparian plant communities along 58 km of the San Miguel River in southwestern Colorado. We determined the recurrence interval of inundation for each plant community by combining step-backwater hydraulic modeling at 4 representative reaches with Log-Pearson analysis of 4 stream gaging stations. Finally, we mapped bottomland surficial geology and used a Geographic Information System to overlay the coverages of geology and vegetation. Plant communities were distinctly arrayed along the hydrologic gradient. The Salix exigua Nuttall (sandbar willow) community occurred mostly on surfaces with a recurrence interval of inundation shorter than 2.2 years; the Betula occidentalis Hooker (river birch) community peaked on sites with recurrence intervals of inundation between 2.2 and 4.6 years. The hydrologic position occupied by communities dominated by Populus angustifolia James (narrowleaf cottonwood) was strongly related to age of trees and species composition of understory shrubs. The fraction of riparian vegetation on surfaces historically inundated by the river decreased in the upstream direction from almost 100% near Uravan to <50% along the South Fork of the San Miguel River. In upstream reaches much of the physical disturbance necessary to maintain riparian vegetation is provided by valley-side processes including debris flows, floods from minor tributaries, landslides, and beaver activity. Where valley-side processes are important, prediction of riparian vegetation change based on alterations of river flow will be incomplete.     

Precise inhibition is essential for microsecond interaural time difference coding

Microsecond differences in the arrival time of a sound at the two ears (interaural time differences, ITDs) are the main cue for localizing low-frequency sounds in space. Traditionally, ITDs are thought to be encoded by an array of coincidence-detector neurons, receiving excitatory inputs from the two ears via axons of variable length ('delay lines'), to create a topographic map of azimuthal auditory space. Compelling evidence for the existence of such a map in the mammalian lTD detector, the medial superior olive (MSO), however, is lacking. Equally puzzling is the role of a--temporally very precise glycine--mediated inhibitory input to MSO neurons. Using in vivo recordings from the MSO of the Mongolian gerbil, we found the responses of ITD-sensitive neurons to be inconsistent with the idea of a topographic map of auditory space. Moreover, local application of glycine and its antagonist strychnine by iontophoresis (through glass pipette electrodes, by means of an electric current) revealed that precisely timed glycine-controlled inhibition is a critical part of the mechanism by which the physiologically relevant range of ITDs is encoded in the MSO. A computer model, simulating the response of a coincidence-detector neuron with bilateral excitatory inputs and a temporally precise contralateral inhibitory input, supports this conclusion.