# chickens and egg or egg and chickens



## trbell (Nov 1, 2000)

Suzana Herculano-HouzelUniversidade Federal do Rio de JaneiroAnswer quickly: are your legs strong because you run often, or do yourun often because your legs are strong? Do you like maths because youare good with numbers, or are you good with numbers because you likemaths? In people with post-traumatic stress disorder (PTSD), is thehippocampus small because of the disease, or is PTSD present becausethe hippocampus is small?The difference between cause and effect is a constant pain in theneck of neuroscientists. Demonstrations over the last few years thatthe brain can change significantly over time because of what itexperiences have only complicated matters. Braille readers haveenlarged cortical representations of their fingertips; London taxidrivers, forced to know by heart their way around town, have anenlarged anterior hippocampus; women with chronic depression have asmaller hippocampus. But which comes first, the altered size of abrain area or the condition?The usual way out is to look for a relationship between changes inthe brain and the duration of practice or exposure. For example, thelonger the time as a London cabbie, the larger the anterior part ofthe hippocampus; the longer the duration of chronic depression, thesmaller the female hippocampus. Put these facts together with thediscovery of continuous production of new neurons in the humanhippocampus, and it seems perfectly plausible that this brainstructure undergoes remodelling as it sees fit.And this seemed to be the case with PTSD. In the 1990s, PTSDtriggered by intense trauma and stress - two factors that causeneuronal atrophy and even death - was found to correlate with reducedhippocampal size in veterans of the Vietnam War. The severity ofsymptoms even correlated with the degree of hippocampal shrinkagerelative to controls, which seemed to fit perfectly with thehypothesis of neuronal atrophy caused by stress.Enter Mark Gilbertson and his data from 40 pairs of identical twins,published in the November issue of Nature Neuroscience. According tomagnetic resonance imaging scans, the worse the symptoms of thedisorder in combat veterans, the smaller the volume of thehippocampus in the twins who stayed at home.Reduced hippocampal volume in the healthy twins correlated, ofcourse, with a similar reduction in the veteran brother. The latter,however, was not related to combat severity - although those thatdeveloped PTSD had taken part in more severe combat than veteranswhom had not. The most parsimonious explanation is that, in the caseof PTSD, reduced hippocampal size results neither from trauma norfrom the disorder, but is instead a pre-existing condition thatpredisposes subjects to the development of PTSD once the brain isexposed to trauma. In this case, the reduced size of the hippocampusis probably conducive to an exaggerated hormonal and behaviouralresponse to stress. This scenario would also explain why only a fewpeople develop the disorder, despite the fact that many are exposedto the same trauma.In the end, the relationship between stress and hippocampal size isprobably a two-way street. In other words, the egg (stress) oftencomes first, but sometimes the chicken (a reduced hippocampus) hasprecedence. This should be taken as a sign that it is time to droprhetorical conundrums about chickens and eggs in the face of evidencethat nature and environment, brain and society, can often not bedissociated. After all, chicken and egg are one and the same animal.References and linksORIGINAL RESEARCH PAPER Gilbertson, M. W. et al. Smaller hippocampalvolume predicts pathologic vulnerability to psychological trauma.Nature Neurosci. 5, 1242-1247 (2002)FURTHER READING Sapolsky, R. M. Chickens, eggs and hippocampalatrophy. Nature Neurosci. 5, 1111-1113 (2002)--tom


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