Objective: To investigate whether longitudinal declines in cognition are associated with higher fibrillar amyloid-beta (A␤) deposition in vivo in individuals without dementia. Method: [11 C]PiB images were obtained to measure fibrillar A␤ burden in 57 participants without dementia from the Baltimore Longitudinal Study of Aging. Participants (33 men, 24 women) had a mean (SD) age of 78.7 (6.2) years. Six participants (4 men, 2 women) had mild cognitive impairment defined as Clinical Dementia Rating ϭ 0.5. To measure [11 C]PiB retention, distribution volume ratios (DVR) for 15 regions of interest were estimated by fitting a simplified reference tissue model to the measured time activity curves. Mixed effects regression was used to predict cognitive trajectories over time using data before and including time of PiB (mean follow-up 10.8 years), with mean cortical DVR, age at baseline, sex, and education as independent predictors. Voxel-based analysis identified local associations. Results: [11 C]PiB retention was higher in older individuals. Greater declines over time in mental status and verbal learning and memory, but not visual memory, were associated significantly with higher PiB retention. Voxel-based analysis showed significant associations in frontal and lateral temporal regions. Conclusions:Higher A␤ deposition is associated with greater longitudinal decline in mental status and verbal memory in the preceding years. The differential association for verbal but not visual memory may reflect the greater reliance of verbal word list learning on prefrontal regions, which show early A␤ deposition. Prospective imaging may help distinguish between individuals with evolving neuropathology who develop accelerated cognitive decline vs those with normal aging. Postmortem studies of associations between amyloid-beta (A␤) burden and antemortem cognition yield conflicting findings. 1,2 We found that longitudinal cognitive trajectories were similar between cognitively normal individuals with and without Alzheimer disease (AD) neuropathology and that both groups differed from individuals with mild cognitive impairment (MCI) or AD, who showed marked cognitive decline. 3Radiotracers for in vivo imaging of fibrillar A␤ burden allow prospective investigation of relationships between cognitive performance and A␤. Consistent with postmortem studies, 20% to 30% of clinically normal individuals show A␤ deposition on imaging.4,5 Across the spectrum of cognitive function, lower memory correlates cross-sectionally with higher [ 11 C]PiB. 6,7 The only study investigating longitudinal changes in cognition in relation to A␤ e-Pub ahead of print on February 10, 2010, at www.neurology.org.

As in other countries, suburbanization in China occurred after the cities had experienced a period of sustained industrial and population growth. This study examines suburbanization in Beijing, Shanghai, Shenyang, and Dalian. As a result of economic restructuring, the urban core registered net population loss from 1982 to 1990 because of decentralization while the inner suburbs gained population. Among the forces driving suburbanization were marketization of urban land, the shift of industrial land to tertiary use, transportation improvement, the availability of foreign and domestic capital, housing rehabilitation in the city, and new housing construction in the suburbs. There were certain similarities but major differences between American and Chinese suburbanization. Unlike the current metropolitan landscape in the United States where suburban growth has given rise to a polycentric spatial structure, suburbanization in China is still at the incipient stage of development with suburbs dominated by central cities. The role of the state in China has been more direct and powerful in setting the suburbanization process in motion. [

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The existing empirical research of urban population density is rich on developed countries but much less so on developing countries. On cities in a planned economy such as that of China, very little is reported. Part of the reason is the lack of reliable data sources. This research models the population densities in Beijing 1982-90. Data are from the third and fourth national population censuses in 1982 and 1990 respectively, and are aggregated at the level of sub-district (jie-dao). Spatial boundaries of these sub-districts are obtained from various sources of local governments and numerous field trips. The research shows that the negative exponential function also fits the density distributions in Beijing, the density gradient becomes flatter, and the city-centre intercept drops over time. The results are consistent with the findings on Western cities, implying that even a socialist city cannot escape the universal forces shaping urban structure. In addition, GIS surface modelling is used to analyse the spatial patterns. While central-city sub-districts have experienced significant loss of population, suburban sub-districts have gained growth at various scales. This signals the beginning of suburbanisation in Beijing. However, some of the causes are different from those in Western countries.

This paper contributes to the demonstration that the self-similar city hierarchies with cascade structure can be modeled with a pair of scaling laws reflecting the recursive process of urban systems. First we transform the Beckmann's model on city hierarchies and generalize Davis's 2n -rule to an rn-rule on the size – number relationship of cities (r > 1), and then reduce both Beckmann's and Davis's models to a pair of scaling laws taking the form of exponentials. Then we derive an exact three-parameter Zipf-type model from the scaling laws to revise the commonly used two-parameter Zipf model. By doing so, we reveal the fractal essence of central place hierarchies and link the rank-size rule to central place model logically. The new mathematical frameworks are applied to the class counts of the 1950–70 world city hierarchy presented by Davis in 1978, and several alternative approaches are illustrated to estimate the fractal dimension.