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See Inside Scientific American Volume 307, Issue 1

Readers Respond to "A Man-Made Contagion" and Other Articles

Letters to the editor from the March 2012 issue of Scientific American



Scientific American

MIND'S WHY

In describing their conclusions that DNA segments, or jumping genes, that can copy themselves into different parts of the genome may be the cause of the uniqueness of individual brains in “What Makes Each Brain Unique,” Fred H. Gage and Alysson R. Muotri misrepresent the degree of similarity expected between the brains of identical twins. Their work does reveal an intriguing source of genetic variation between such brains, the significance of which remains to be elucidated.

Yet the authors gloss over the fact that all brains, genetically identical or otherwise, are almost certainly quite distinct as a result of inherent variation in neural stem cell divisions, cell migration events and neural circuit formation. Such variation is created by the unique, probably often random, cellular and experience-dependent interactions that occur during the development of any given brain.

Nicholas Gaiano
Johns Hopkins University
School of Medicine

Having worked with start-ups and early-stage businesses my entire career, I have participated in many debates about whether entrepreneurs are born or made. What entices otherwise employable people to work for practically—maybe explicitly—nothing beyond the lottery system known as stock options (the tool Silicon Valley uses to lure young engineers into working crazy hours with no guarantees)? Now I learn that with jumping genes, natural selection is “rolling the dice” to build brains to meet the challenges of ever changing conditions. Yep, that sounds like an entrepreneur.

Michael J. Connelly
President Emeritus
New York Venture Capital Forum

STAR BRIGHT

In “The Far, Far Future of Stars,” Donald Goldsmith states that the luminosity of a star richer in heavy elements declines because of opacity of its outer layers. Then, in the next sentence, he goes on to say that “the lower luminosity means that the star consumes its nuclear fuel at a lower rate.” So what is it, opacity or lower rate?

Also, one expects that the trapping of radiation by the outer layers would cause the temperature to increase and, consequently, the fusion rate to increase as well. So what is going on?

Dov Elyada
Haifa, Israel

GOLDSMITH REPLIES: Stars produce energy in their cores, which passes to their surfaces and radiates into space. The lower luminosity results from a higher opacity, which hinders the passage of radiation. A lower luminosity also implies a lower rate of energy production and thus a slower consumption of nuclear fuel. The higher opacity that leads to a greater trapping of radiation produces complex effects in a star's atmosphere, including a possible increase in its size and temperature, but it is the temperature in the core, not in the atmosphere, that determines the star's rate of energy production.

DEPRESSION RELIEF

“Lifting the Black Cloud,” by Robin Marantz Henig, succinctly and correctly sums up the state of evidence regarding the efficacy of antidepressant medications in saying that they “leave a lot to be desired”.

There are other alternatives. High-quality randomized controlled trials have shown that aerobic exercise reduces depressive symptoms in cancer survivors and in other populations. In contrast to the common side effects of antidepressants—such as sexual dysfunction, headaches, insomnia and nausea—those of exercise are a pure delight—enhanced libido, better sleep, decreased body fat, longer life, increased strength and endurance, and more. Exercise has the added benefit of low cost and may be modified to match individual needs. Must solutions to the black cloud of depression always be medicinal?

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