Point Process Analysis of Noise in Early Invertebrate Vision

Noise is a prevalent and sometimes even dominant aspect of many biological processes. While many natural systems have adapted to attenuate or even usefully integrate noise, the variability it introduces often still delimits the achievable precision across biological functions. This is particularly so for visual phototransduction, the process responsible for converting photons of light into usable electrical signals (quantum bumps). Here, randomness of both the photon inputs (regarded as extrinsic noise) and the conversion process (intrinsic noise) are seen as two distinct, independent and significant limitations on visual reliability. Past research has attempted to quantify the relative effects of these noise sources by using approximate methods that do not fully account for the discrete, point process and time ordered nature of the problem. As a result the conclusions drawn from these different approaches have led to inconsistent expositions of phototransduction noise performance. This paper provides a fresh and complete analysis of the relative impact of intrinsic and extrinsic noise in invertebrate phototransduction using minimum mean squared error reconstruction techniques based on Bayesian point process (Snyder) filters. An integrate-fire based algorithm is developed to reliably estimate photon times from quantum bumps and Snyder filters are then used to causally estimate random light intensities both at the front and back end of the phototransduction cascade. Comparison of these estimates reveals that the dominant noise source transitions from extrinsic to intrinsic as light intensity increases. By extending the filtering techniques to account for delays, it is further found that among the intrinsic noise components, which include bump latency (mean delay and jitter) and shape (amplitude and width) variance, it is the mean delay that is critical to noise performance. Consequently, if one wants to increase visual fidelity, reducing the photoconversion lag is much more important than improving the regularity of the electrical signal.This work was supported by the Gates Cambridge Trust (PhD studentship for research) https://www.gatescambridge.org/. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript

Why <i>trans</i>- or <i>cis</i>-Dimethyl Fumarate Addition to 2,5-Dimethylpyrrole Gives Exclusively <i>trans</i>-7-Azanorbornane

The addition mechanism of dimethyl fumarate into 2,5-dimethylpyrrole is explored using density functional theory (DFT) methods. Our calculations find that TpW­(NO)­(PMe₃)­(η²-3<i>H</i>-2,5-di­methyl­pyrrole) prefers to undergo two TpW­(NO)­(PMe₃) migrations, two 1,5-hydride migrations, and one reductive elimination to isomerize into TpW­(NO)­(PMe₃)­(η²-1<i>H</i>-2,5-dimethylpyrrole), in which TpW­(NO)­(PMe₃) plays a proton-transfer role. <i>trans</i>-Dimethyl fumarate and TpW­(NO)­(PMe₃)­(η²-1<i>H</i>-2,5-dimethylpyrrole) tend to adopt a concerted cycloaddition manner to afford <i>trans</i>-7-azanorbornane with a free-energy barrier of 21.8 kcal/mol. <i>cis</i>-Dimethyl fumarate and TpW­(NO)­(PMe₃)­(η²-1<i>H</i>-2,5-di­methyl­pyrrole) are the most likely to experience a concerted cycloaddition → ring opening → ring closing process to provide <i>trans</i>-7-azanorbornane in which the concerted cycloaddition and the ring-opening process are in dynamic equilibrium (with similar energy barriers of 21.5 and 21.9 kcal/mol, respectively). The presence of TpW­(NO)­(PMe₃) not only promotes the cycloaddition of <i>trans</i>- or <i>cis</i>-dimethyl fumarate with 2,5-di­methyl­pyrrole by donating d-electrons of the W atom into the diene system of the Diels–Alder reaction, but also is favorable for the ring-opening process of the formed <i>cis</i>-7-azanorbornane. Furthermore, <i>trans</i>-azanorbornane is 7.4 kcal/mol more stable than <i>cis</i>-azanorbornane. Our calculations provide a new explanation of the addition of dimethyl fumarate with 2,5-dimethylpyrrole exclusively giving <i>trans</i>-7-azanorbornane

The relationship of the shear strength parameter and strength damage coefficient with the number of freeze‒thaw cycles.

(a) cohesion; (b) internal friction angle; (c) Kc; and (d) Kφ.</p

Original data.

Natural disasters such as landslides often occur on soil slopes in seasonally frozen areas that undergo freeze‒thaw cycling. Ecological slope protection is an effective way to prevent such disasters. To explore the change in the mechanical properties of soil under the influence of both root reinforcement and freeze‒thaw cycles and its influence on slope stability, the Baijiabao landslide in the Three Gorges Reservoir area was taken as an example. The mechanical properties of soil under different confining pressures, vegetation coverages (VCs) and numbers of freeze‒thaw cycles were studied via mechanical tests, such as triaxial compression tests, wave velocity tests and FLAC3D simulations. The results show that the shear strength of a root–soil composite increases with increasing confining pressure and VC and decreases with increasing number of freeze‒thaw cycles. Bermuda grass roots and confining pressure jointly improve the durability of soil under freeze‒thaw conditions. However, with an increase in the number of freeze‒thaw cycles, the resistance of root reinforcement to freeze‒thaw action gradually decreases. The observed effect of freeze‒thaw cycles on soil degradation was divided into three stages: a significant decrease in strength, a slight decrease in strength and strength stability. Freeze‒thaw cycles and VC mainly affect the cohesion of the soil and have little effect on the internal friction angle. Compared with that of a bare soil slope, the safety factor of a slope covered with plants is larger, the maximum displacement of a landslide is smaller, and it is less affected by freezing and thawing. These findings can provide a reference for research on ecological slope protection technology.</div

The <i>λ</i>_<i>i−j</i> values for different VC and freeze‒thaw cycle test conditions.

The λi−j values for different VC and freeze‒thaw cycle test conditions.</p

The vegetation coverage (VC) and root content.

Natural disasters such as landslides often occur on soil slopes in seasonally frozen areas that undergo freeze‒thaw cycling. Ecological slope protection is an effective way to prevent such disasters. To explore the change in the mechanical properties of soil under the influence of both root reinforcement and freeze‒thaw cycles and its influence on slope stability, the Baijiabao landslide in the Three Gorges Reservoir area was taken as an example. The mechanical properties of soil under different confining pressures, vegetation coverages (VCs) and numbers of freeze‒thaw cycles were studied via mechanical tests, such as triaxial compression tests, wave velocity tests and FLAC3D simulations. The results show that the shear strength of a root–soil composite increases with increasing confining pressure and VC and decreases with increasing number of freeze‒thaw cycles. Bermuda grass roots and confining pressure jointly improve the durability of soil under freeze‒thaw conditions. However, with an increase in the number of freeze‒thaw cycles, the resistance of root reinforcement to freeze‒thaw action gradually decreases. The observed effect of freeze‒thaw cycles on soil degradation was divided into three stages: a significant decrease in strength, a slight decrease in strength and strength stability. Freeze‒thaw cycles and VC mainly affect the cohesion of the soil and have little effect on the internal friction angle. Compared with that of a bare soil slope, the safety factor of a slope covered with plants is larger, the maximum displacement of a landslide is smaller, and it is less affected by freezing and thawing. These findings can provide a reference for research on ecological slope protection technology.</div

Shear wave velocity test results.

Natural disasters such as landslides often occur on soil slopes in seasonally frozen areas that undergo freeze‒thaw cycling. Ecological slope protection is an effective way to prevent such disasters. To explore the change in the mechanical properties of soil under the influence of both root reinforcement and freeze‒thaw cycles and its influence on slope stability, the Baijiabao landslide in the Three Gorges Reservoir area was taken as an example. The mechanical properties of soil under different confining pressures, vegetation coverages (VCs) and numbers of freeze‒thaw cycles were studied via mechanical tests, such as triaxial compression tests, wave velocity tests and FLAC3D simulations. The results show that the shear strength of a root–soil composite increases with increasing confining pressure and VC and decreases with increasing number of freeze‒thaw cycles. Bermuda grass roots and confining pressure jointly improve the durability of soil under freeze‒thaw conditions. However, with an increase in the number of freeze‒thaw cycles, the resistance of root reinforcement to freeze‒thaw action gradually decreases. The observed effect of freeze‒thaw cycles on soil degradation was divided into three stages: a significant decrease in strength, a slight decrease in strength and strength stability. Freeze‒thaw cycles and VC mainly affect the cohesion of the soil and have little effect on the internal friction angle. Compared with that of a bare soil slope, the safety factor of a slope covered with plants is larger, the maximum displacement of a landslide is smaller, and it is less affected by freezing and thawing. These findings can provide a reference for research on ecological slope protection technology.</div

Strength attenuation mechanism of the root–soil composite under freeze‒thaw action.

Strength attenuation mechanism of the root–soil composite under freeze‒thaw action.</p

Slope displacement and safety factor statistics of different VCs.

Slope displacement and safety factor statistics of different VCs.</p

Deformation calculation parameters of the Baijiabao landslide.

Deformation calculation parameters of the Baijiabao landslide.</p