EXCLUSIVE

'Star Trek' icon Nichelle Nichols has been diagnosed with dementia and the disease has progressed.

According to docs filed in Nichelle's conservatorship, Dr. Meena Makhijani -- who specializes in osteopathic medicine -- says Nichols has been a patient of hers for the last 2-3 years ... and she has "moderate progressive dementia."

Makhijani gives details of the severity of the disease by rating various cognitive functions. Most alarming ... she says Nichols has major impairment of her short-term memory and moderate impairment of understanding abstract concepts, sense of time, place and immediate recall.

On the bright side, the doctor says there's no apparent impairment for things like long-term memory, orientation of her body, comprehension, verbal communication, concentration, recognition of familiar people, as well as ability to reason logically and plan actions.