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4AbstrackThis study investigated the cortical sources of the early (5&250 ms) components of the pattern-onset visual evoked potential (VEP). VEPs were recorded in response to a small circular checkerboard stimulus that was flashed over a range of visual field positions. Temporally and spatially overlapping VEP components were distinguished by differences in retinotopic sensitivity and scalp topography, and by inverse dipole modeling. The C1 component (5Cr80 ms) was found to change its polarity and topography systematically as a function of stimulus position in a manner consistent with the retinotopic organization of the striate cortex. The P1 component (comprised of the P75 and PlO0 subcomponents) had a time course that overlapped the C1 but could be distinguished from the C1 by its differing topography and reduced sensitivity to stimulus position. The P1 generators were localized to the lateral extrastriate cortex. Inverse dipole models were consistent with these striate and extrastriate source locations for the C1 and P1, respectively. The N1 component (120-180 ms) was found to originate from several spatially distinct generators that differed in their retinotopic organization.

Atmospheric carbon dioxide increased at a rate of 2.8 petagrams of carbon per year (Pg C year-1) during 1988 to 1992 (1 Pg = 10(15) grams). Given estimates of fossil carbon dioxide emissions, and net oceanic uptake, this implies a global terrestrial uptake of 1.0 to 2. 2 Pg C year-1. The spatial distribution of the terrestrial carbon dioxide uptake is estimated by means of the observed spatial patterns of the greatly increased atmospheric carbon dioxide data set available from 1988 onward, together with two atmospheric transport models, two estimates of the sea-air flux, and an estimate of the spatial distribution of fossil carbon dioxide emissions. North America is the best constrained continent, with a mean uptake of 1.7 +/- 0.5 Pg C year-1, mostly south of 51 degrees north. Eurasia-North Africa is relatively weakly constrained, with a mean uptake of 0.1 +/- 0.6 Pg C year-1. The rest of the world's land surface is poorly constrained, with a mean source of 0.2 +/- 0.9 Pg C year-1.