A neurocognitive deficit is a reduction or impairment of cognitive function in one of these areas, but particularly when physical changes can be seen to have occurred in the brain, such as after neurological illness, mental illness, drug use or brain injury.
A clinical neuropsychologist may specialise in using neuropsychological tests to detect and understand such deficits, and may be involved in the rehabilitation of an affected person. The discipline that studies neurocognitive deficits to infer normal psychological function is called cognitive neuropsychology.