Agnosias

Agnosia refers to a family of perceptual disorders in which there is a failure to recognize sensory stimuli despite intact primary sensory function. The deficits arise from damage or dysfunction in higher-level brain areas responsible for processing and interpreting sensory information, rather than from problems with the senses themselves. Agnosias can affect one or more modalities, and may be acquired after injury or disease or present developmentally.

Visual agnosias involve difficulties with recognizing visual information. Object agnosia impairs identification of objects, while apperceptive visual agnosia disrupts the ability to form a coherent percept of a visual scene, and associative visual agnosia impairs linking percepts to meaning. Prosopagnosia is a well-known form that specifically impairs face recognition. Other visual conditions include color agnosia, where colors cannot be named or matched despite normal color perception.

Tactile agnosia, or astereognosis, is the inability to recognize objects by touch despite intact tactile sensation. Auditory agnosias affect interpretation of sounds or speech; for example, pure word deafness impairs recognition of spoken language while preserving the ability to hear nonverbal sounds.

Causes and localization: most agnosias are acquired due to brain injury or disease, frequently involving posterior cortical regions in the temporal and parietal lobes. Stroke, traumatic brain injury, tumors, infections, and neurodegenerative diseases can produce various forms. Developmental agnosias exist from early life and may co-occur with learning disorders.

Diagnosis and management: diagnosis relies on targeted neuropsychological testing and imaging to identify lesions. Treatment is typically multidisciplinary, emphasizing compensatory strategies, training to use intact senses, environmental adaptations, and rehabilitation. Prognosis varies by type and extent of brain injury, with some individuals achieving partial recovery through therapy.