Two-Dimensional Hydrodynamic Modelling of Flood Inundation for a Part of the Mekong River with TELEMAC-2D.

Aims: This paper presents a study on the development of a 2-dimensional (2D) hydrodynamic model based on TELEMAC-2D for the flood simulation of the river from Kratie to Kampong Cham in Cambodia, a part of the Mekong River. The motivation behind the research was to study the feasibility of TELEMAC-2D in flood forecasting, and specifically to determine its adequacy in flood simulations with a focus on the reduction in model run-time through parallelization. Place and Duration of Study: DHI-NTU Centre, Nanyang Environment and Water Research Institute (NEWRI), Nanyang Technological University, between November 2013 and March 2014. Methodology: We simulated an actual flood event which occurred between June to November in 2001 for the stretch of the Mekong River from Kratie to Kampong Cham and compared the model simulations with MODIS satellite Images for specific days in the pre-, peak- and post-flood period. Results: It was found that during the peak-flood period, there was high percentage (> 90%) match between the simulation results and observation obtained from satellite images while the match was below 50% for the pre- and post- flood periods. Conclusion: The 2D simulation results were consistent with observations from satellite imaging. The discrepancy at pre- and post-flood may be due to the fact that (i) the model takes into account only hydrodynamic processes of flows in the river and flood plain, it does not consider other hydrological processes such as infiltration or evaporation which may be important during the preand post- flood periods, and (ii) the resolution of MODIS satellite image at 500m x 500m may be too coarse and therefore not sufficient to identify flooded areas when the area is small or water depth low. Finally, it was found that the computing time can be reduced significantly with parallelization using multi-core processors, albeit with lesser advantage in speedup when the number of cores increased beyond 4.

Contrast sensitivity of pattern transient VEP components: contribution from M and P pathways

The purpose of this study was to compare contrast sensitivity estimated from transient visual evoked potentials (VEPs) elicited by achromatic pattern-reversal and pattern-onset/offset modes. The stimuli were 2-cpd, achromatic horizontal gratings presented either as a 1 Hz pattern reversal or a 300 ms onset/700 ms offset stimulus. Contrast thresholds were estimated by linear regression to amplitudes of VEP components vs. the logarithm of the stimulus contrasts, and these regressions were extrapolated to the zero amplitude level. Contrast sensitivity was defined as the inverse of contrast threshold. For pattern reversal, the relation between the P100 amplitude and log of the stimulus contrast was best described by two separate linear regressions. For the N135 component, a single straight line was sufficient. In the case of pattern onset/offset for both the C1 and C2 components, single straight lines described their amplitude vs. log contrast relations in the medium-to-low contrast range. Some saturation was observed for C2 components. The contrast sensitivity estimated from the low-contrast limb of the P100, from the N135, and from the C2 were all similar but higher than those obtained from the high-contrast limb of the P100 and C1 data, which were also similar to each other. With 2 cpd stimuli, a mechanism possibly driven by the M pathway appeared to contribute to the P100 component at medium-to-low contrasts and to the N135 and C2 components at all contrast levels, whereas another mechanism, possibly driven by the P and M pathways, appeared to contribute to the P100 component at high contrast and C1 component at all contrast levels...

Division of labor between M and P visual pathways: different visual pathways minimize joint entropy differently

Visual perception and action are strongly linked with parallel processing channels connecting the retina, the lateral geniculate nucleus, and the input layers of the primary visual cortex. Achromatic vision is provided by at least two of such channels formed by the M and P neurons. These cell pathways are similarly organized in primates having different lifestyles, including species that are diurnal, nocturnal, and which exhibit a variety of color vision phenotypes. We describe the M and P cell properties by 3D Gábor functions and their 3D Fourier transform. The M and P cells occupy different loci in the Gábor information diagram or Fourier Space. This separation allows the M and P pathways to transmit visual signals with distinct 6D joint entropy for space, spatial frequency, time, and temporal frequency. By combining the M and P impacts on the cortical neurons beyond V1 input layers, the cortical pathways are able to process aspects of visual stimuli with a better precision than it would be possible using the M or P pathway alone. This performance fulfils the requirements of different behavioral tasks.