<qa>  What is Gastrodia elatas main active constituent?Gastrodia elata, a famous Chinese medical herb, has effects on preventing ischemic brain injury, neuronal cell damage or apoptosis, suppressing inflammatory response, inhibiting glutamine receptors and nNOS, and scavenging free radicals. Gastrodin 4-hydroxymethylphenyl-β-D-glucopyranoside, the primary active constituent of gastrodia elata, has been widely used for the treatment of paralysis, hemiplegia, headache, vertigo, and Alzheimers disease. Based on its pharmacological effects, we postulated that gastrodin would attenuate the causative parameters of cognitive dysfunction related to CPB and would be an effective drug to prevent the decline as a result. 