Research in how people with language or hearing impairment perceive speech is not only intended to discover possible treatments. It can provide insight into what principles underlie non-impaired speech perception. Two areas of research can serve as an example:
- Listeners with aphasia. Aphasia affects both the expression and reception of language. Both two most common types, Broca's and Wernike's aphasia, affect speech perception to some extent. Broca’s aphasia causes moderate difficulties for language understanding. The effect of Wernike’s aphasia on understanding is much more severe. It is agreed upon, that aphasics suffer from perceptual deficits. They are usually unable to fully distinguish place of articulation and voicing. As for other features, the difficulties vary. It has not yet been proven whether low-level speech-perception skills are affected in aphasia sufferers or whether their difficulties are caused by higher-level impairment alone.
- Listeners with cochlear implants. Cochlear implantation allows partial restoration of hearing in deaf people. The acoustic information conveyed by an implant is usually sufficient for implant users to properly recognize speech of people they know even without visual clues. For cochlear implant users, it is more difficult to understand unknown speakers and sounds. The perceptual abilities of children that received an implant after the age of two are significantly better than of those who were implanted in adulthood. A number of factors have been shown to influence perceptual performance. These are especially duration of deafness prior to implantation, age of onset of deafness, age at implantation (such age affects may be related to the Critical period hypothesis) and the duration of using an implant. There are differences between children with congenital and acquired deafness. Postlingually deaf children have better results than the prelingually deaf and adapt to a cochlear implant faster.