Those who know even a tiny bit of toxicology, are well aware about difference between acute and chronic effects of xenobiotics. Some chemicals (or other factors, for that matter) seem to be harmless at the first touch (acute) but tend to develop terrible effects if taken continuously over long time (chronic). The common sense explanation for that is accumulation of small damage over time in our inner organs (liver, for instance), although it could be a misconception.
But the nervous system is a different beast. As Otchy et al found in the recent paper, acute stimulation of brain areas produces much more prominent effects than chronic stimulation over a long time (the latter can have no effect at all!). They verified the observation in two models, rats and zebra finches, and found the same patterns: lesions in some brain areas tended to have no effect on performed task after certain time (lever pressing in rats or singing in birds). At the same time, acute optogenetic stimulations drastically impaired the performance. The authors suggested that it is the adaptability of the brain that accounts for the difference. According to them, acute manipulations produce overestimated output due to off-target effects, and this may lead to wrong conclusions about functional roles of studied brain areas. These effects are attenuated under the ‘steady-state’ chronic stimulation conditions. So the authors urge other neuroscientists to revise their experiment design and findings, while still adhering to reductionism.
Despite the sensation-like headlines made by the paper, it is unlikely that optogenetics will repeat the fate of fMRI. At least not yet. For sure, neuroscientists have enough plasticity to cope with the shock. At most they’ll be a little more cautious with the conclusions.