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<jats:p> <jats:bold>SAN FRANCISCO</jats:bold> —A new analysis of sediment from the floor of the North Pacific has strengthened an earlier claim ( <jats:italic>Science</jats:italic> , 18 June 1993, p. 1725) that a series of volcanic eruptions may have triggered the world's precipitous descent into the ice ages 2.6 million years ago. Ash from the volcanoes, which would have blocked the sun and cooled the Earth, appears in sediment from the same 1000-year period when glaciers formed on nearby continents. </jats:p>

<jats:p>Solids are usually shown in textbooks as perfect periodic structures, but real solids often have substantial disorder. This disorder leads to completely new and often complex behavior. In their Perspective, Aeppli and Chandra describe the search for systems that are simple enough to understand theoretically, but that still exhibit interesting complexity. Magnetic systems may fulfill this need and might lead to new systems for information storage and computation.</jats:p>

<jats:p> Octopus-like neurons, the cells of the brain, typically have many input “arms”—the dendrites, which receive signals from other neurons—and one output arm, the axon. Two new reports in this week's <jats:italic>Science</jats:italic> show that the dendrites process information in a manner even more complex than expected. In his Perspective, Sejnowski explains how the observations of Magee and Johnston ( <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="209" related-article-type="in-this-issue" vol="275" xlink:href="10.1126/science.275.5297.209" xlink:type="simple">p. 209</jats:related-article> ) and Markram <jats:italic>et al.</jats:italic> ( <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="213" related-article-type="in-this-issue" vol="275" xlink:href="10.1126/science.275.5297.213" xlink:type="simple">p. 213</jats:related-article> ) on backpropagating action potentials illuminate the mechanism of Hebbian learning, the synaptic strengthening that occurs when a receiving neuron is active at the same time that a signal comes in. </jats:p>

<jats:p> Connection from neuron to neuron in the cortex of the brain becomes less effective with each successive action potential but eventually recovers after the end of the action potential burst. Two new reports, one in this issue ( <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="221" related-article-type="in-this-issue" vol="275" xlink:href="10.1126/science.275.5297.221" xlink:type="simple">p. 220</jats:related-article> ), present many-neuron models with these properties and show that these neurons respond to large changes in the frequency of individual inputs (they act as frequency detectors) or to simultaneous changes in many inputs (they act as coincidence detectors). In her Perspective, Thomson explains the origin of the modeled properties and discusses the accuracy with which these models mimic the behavior of real cortical cells. Such modeling of many neurons with real properties reveals computational characteristics of the circuit not apparent from analyses of individual cells. </jats:p>

<jats:p>Atmospheric weather systems become unpredictable beyond a few weeks, but climate variations can be predictable over much longer periods because of the coupling of the ocean and atmosphere. With the use of a global coupled ocean-atmosphere model, it is shown that the North Atlantic may have climatic predictability on the order of a decade or longer. These results suggest that variations of the dominant multidecadal sea surface temperature patterns in the North Atlantic, which have been associated with changes in climate over Eurasia, can be predicted if an adequate and sustainable system for monitoring the Atlantic Ocean exists.</jats:p>