This word is acceptable for play in the US & UK dictionaries that are being used in the following games:

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adj. Of or pertaining to the subcortex, the portion of the brain located below the cerebral cortex

Situated beneath the cortex.

adj. of or relating to or being or involving nerve centers below the cerebral cortex

"Most of them are located below the cerebral cortex and are known as subcortical."

subcortically   

limbic    arteriosclerotic    thalamic    cerebellar    neoplastic