Cognitive decline is a virtually universal aspect of the aging process. However, its neurophysiological basis remains poorly understood. We describe here more than 20 age-related cortical processing deficits in the primary auditory cortex of aging versus young rats that appear to be strongly contributed to by altered cortical inhibition. Consistent with these changes, we recorded in old rats a decrease in parvalbuminlabeled inhibitory cortical neurons. Furthermore, old rats were slower to master a simple behavior, with learning progressions marked by more false-positive responses. We then examined the effect of intensive auditory training on the primary auditory cortex in these aged rats by using an oddball discrimination task. Following training, we found a nearly complete reversal of the majority of previously observed functional and structural cortical impairments. These findings suggest that age-related cognitive decline is a tightly regulated plastic process, and demonstrate that most of these age-related changes are, by their fundamental nature, reversible. aging | cognitive decline | plasticity | inhibition | parvalbumin P erceptual and cognitive decline are near-universal aspects of normal aging (1, 2). Such deficits cannot be explained solely by a dysfunction of peripheral sensory organs and frequently translate to slowed perceptual processing and difficulty in accurately identifying stimuli under challenging (e.g., noisy, time-limited, attentionally demanding) conditions (3, 4). In the human auditory system, psychophysical and electroencephalography experiments have examined aspects of cognitive decline by using oddball detection paradigms, successive-signal masking studies, speech-in-noise studies, and compressed speech (5-7), among other strategies. These studies have shown that degraded signal salience, defective sensory adaptation and a slowing of sensory processing contribute to the deterioration of a wide range of perceptual and cognitive processes recorded in aged populations (3,(8)(9)(10). Animal models have been instrumental in defining the cellular and molecular basis of agerelated perceptual impairments. In rats, significant alterations in inhibitory function in various subcortical nuclei and the auditory cortex have been linked to abnormal temporal and spectral processing (11)(12)(13)(14). Interestingly, although these changes are often described as progressive plastic compensations secondary to a combination of slow peripheral deafferentation and chemical or molecular insults (11,15), the possibility that age-related changes might be by their plastic nature largely reversible has seldom been explored or proposed (16). Compelling evidence that these agerelated functional alterations can be prevented to some extent by sensory enrichment (9, 17) or even dietary improvements (15) certainly supports this concept. During adulthood, after the closure of developmental plasticity windows, attention-demanding intensive training strategies remain one of the most powerful means of directing plastic re...

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In transformation optics, the space transformation is viewed as the deformation of a material. The permittivity and permeability tensors in the transformed space are found to correlate with the deformation field of the material. By solving the Laplace's equation, which describes how the material will deform during a transformation, we can design electromagnetic cloaks with arbitrary shapes if the boundary conditions of the cloak are considered. As examples, the material parameters of the spherical and elliptical cylindrical cloaks are derived based on the analytical solutions of the Laplace's equation. For cloaks with irregular shapes, the material parameters of the transformation medium are determined numerically by solving the Laplace's equation. Full-wave simulations based on the Maxwell's equations validate the designed cloaks. The proposed method can be easily extended to design other transformation materials for electromagnetic and acoustic wave phenomena.

A unified analytic model for effective mass density, effective bulk modulus, and effective shear modulus is presented for elastic metamaterials composed of coated spheres embedded in a host matrix. The effective material properties are derived directly from the averages of local momentum, stress, and strain defined in a single doubly coated sphere. It is shown that the effective material parameters predicted by the proposed model are in excellent agreements with the coherent-potential approximation results at low filling fractions where the anisotropy of periodic structures can be neglected for elastic waves. The advantage of the proposed method is that it can reveal clearly the physical mechanism for negative effective material parameters induced by the resonant effect. It is found that negative effective mass density is induced by negative total momentum of the composite for a positive momentum excitation. Negative effective bulk modulus appears for composites with an increasing ͑decreasing͒ total volume under a compressive ͑tensile͒ stress. Negative effective shear modulus describes composites with axisymmetric deformation under an opposite axisymmetric loading. Numerical examples are also given to illustrate these mechanisms. These findings may be useful in design of elastic metamaterials.

Recently, there are emerging demands for isotropic material parameters, arising from the broadband requirement of the functional devices. Since inverse Laplace's equation with sliding boundary condition will determine a quasi-conformal mapping, and a quasi-conformal mapping will minimize the transformation material anisotropy, so in this work, the inverse Laplace's equation with sliding boundary condition is proposed for quasi-isotropic transformation material design. Examples of quasi-isotropic arbitrary carpet cloak and waveguide with arbitrary cross sections are provided to validate the proposed method. The proposed method is very simple compared with other quasi-conformal methods based on grid generation tools.