Entorhinal grid cells have periodic, hexagonally patterned firing locations that scale up progressively along the dorsal-ventral axis of medial entorhinal cortex. This topographic expansion corresponds with parallel changes in cellular properties dependent on the hyperpolarization-activated cation current (Ih), which is conducted by hyperpolarization-activated cyclic nucleotide-gated (HCN) channels. To test the hypothesis that grid scale is determined by Ih, we recorded grid cells in mice with forebrain-specific knockout of HCN1. We find that, although the dorsal-ventral gradient of the grid pattern was preserved in HCN1 knockout mice, the size and spacing of the grid fields, as well as the period of the accompanying theta modulation, was expanded at all dorsal-ventral levels. There was no change in theta modulation of simultaneously recorded entorhinal interneurons. These observations raise the possibility that, during self-motion-based navigation, Ih contributes to the gain of the transformation from movement signals to spatial firing fields.

In this paper, a guided wave tomography method based on Full Waveform Inversion (FWI) is developed for accurate and high resolu- tion reconstruction of the remaining wall thickness in isotropic plates. The forward model is computed in the frequency domain by solving a full-wave equation in a two-dimensional acoustic model, accounting for higher order eects such as diractions and multiple scattering. Both numerical simulations and experiments were carried out to obtain the signals of a dispersive guided mode propagating through defects. The inversion was based on local optimization of a waveform mist func- tion between modeled and measured data, and was applied iteratively to discrete frequency components from low to high frequencies. The resulting wave velocity maps were then converted to thickness maps by the dispersion characteristics of selected guided modes. The results suggest that the FWI method is capable to reconstruct the thickness map of a irregularly shaped defect accurately on a 10 mm thick plate with the thickness error within 0.5 mm.

The inspection of large areas of complex structures is a growing interest for industry. An experimental observation on a large welded plate found that the weld can concentrate and guide the energy of a guided wave travelling along the direction of the weld. This is attractive for non-destructive evaluation (NDE) since it offers the potential to quickly inspect for defects such as cracking or corrosion along long lengths of welds. In this paper, a twodimensional semi-analytical finite-element (SAFE) method is applied to provide a modal study of the elastic waves that are guided by the welded joint in a plate. This brings understanding to the compression wave that was previously observed in the experiment. However, during the study, a shear weld-guided mode, which is non-leaky and almost non-dispersive, has also been discovered. Its characteristics are particularly attractive for NDE, so this is a significant new finding. The properties for both the compression and the shear mode are discussed and compared, and the physical reason for the energy trapping phenomena is then explained. Experiments have been undertaken to validate the existence of the shear weld-guided mode and the accuracy of the FE model, showing very good results.

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Space-coiling acoustic metasurfaces have been largely exploited and shown their outstanding wave manipulation capacity. However, they are complex in realization and cannot directly manipulate acoustic near-fields by controlling the effective path length. Here, we propose a comprehensive paradigm for acoustic metasurfaces to extend the wave manipulations to both far- and near-fields and markedly reduce the implementation complexity with a simple structure, which consists of an array of deep-subwavelength-spaced slits perforated in a thin plate. A semi-analytical approach for such a design is established using a microscopic coupled-wave model, which reveals that the acoustic diffractive pattern at every slit exit is the sum of the initial transmission and the secondary scatterings of the coupled fields from other slits. For proof-of-concept, we examine two metasurface lenses for sound focusing within and beyond the diffraction limit. This work provides a feasible strategy for creating ultra-compact acoustic components with versatile potentials.