I presume you want to avoid a negative connotation word because you have been told/informed that it is better to use a positive word to express something than a negative. However, unimpaired is widely used in the medical literature, because it stands so directly for the positive opposite to impaired, regardless of grammar. Unimpaired carried a positive connotation, and I would most definitely use it in an academic paper on neuroscience (though my areas are medicine and molecular biology).
From this chapter on fluent aphasias (referring here to Wernicke's):
Although the form of language may be relatively unimpaired, speech may be essentially meaningless in the most severe cases, and sound a little like "Jabberwocky."
Or this paper on how the cross-linguistic prevalence of SOV and SVO word
orders reflects the sequential and hierarchical representation of activity in Broca’s Area:
c) Lesion sites significantly linked with impaired vs. unimpaired performances on tasks probing action concepts. Colors indicate areas associated with impairment on 6/6 tasks...