Agnosia

Agnosia refers to a heterogeneous collection of neurological disorders characterized by the inability to recognize or interpret sensory stimuli despite intact primary sensory systems and general intellect. It results from injury or dysfunction in higher-level cortical areas responsible for perception, most often in the occipitotemporal and temporoparietal regions.

Visual agnosia is the most common form. In apperceptive visual agnosia, the person cannot form a coherent percept of an object, so recognition fails even with good basic vision. In associative visual agnosia, percepts are formed, but linking them to meaning fails, so a person can copy a drawing yet cannot name the object. Prosopagnosia, a specific type, is the difficulty recognizing familiar faces, sometimes with preserved object recognition. Color agnosia is the inability to recognize colors despite normal color perception. Tactile agnosia (astereognosis) is the failure to identify objects by touch despite intact touch and proprioception. Auditory agnosia impairs recognition of sounds; pure word deafness is a form where spoken words are not understood despite hearing.

Agnosia can affect other senses, such as tactile or auditory, and may be unilateral or bilateral depending on the lesion. It is distinct from neglect (inattention to stimuli on one side) and from aphasia (language impairment). It often follows stroke, traumatic brain injury, tumors, infections, or neurodegenerative disease, and can be congenital in rare cases.

Diagnosis relies on clinical assessment and neuropsychological testing, supported by neuroimaging. There is no cure; management emphasizes compensatory strategies, rehabilitation, and using intact modalities to aid communication and daily function. Outcomes vary with the underlying cause and extent of brain damage.