Speech perception
Agnosia is "the loss or diminution of the ability to recognize familiar objects or stimuli usually as a result of brain damage". There are several different kinds of agnosia that affect every one of our senses, but the two most common related to speech are speech agnosia and phonagnosia.
Apperceptive agnosia
Visual agnosia (both apperceptive and associative) is prevalent in Alzheimer’s Disease (AD) patients. Visual agnosia may be present in early stages of AD and can often act as an indicator of AD. Apperceptive agnosia results from diffuse cortical pathology of AD. There is early involvement in the hippocampus and the entorhinal cortex followed by a spread to adjacent areas with neurofibrillary tangles (NFT). Gradual extension of NFT throughout the occipital, parietal, and temporal regions devoted to vision occur resulting in visual agnosia.