Agnosia

Definition and Core Concepts
Apperceptive vs. Associative Agnosia
Sensory Modalities and Anatomical Correlates
References

Definition and Core Concepts

Agnosia is a deficit of higher sensory processing that results in an impaired ability to recognize objects, persons, sounds, shapes, or smells. The term, coined by Sigmund Freud in 1891, literally means "absence of knowledge." For a diagnosis of agnosia to be made, the deficit should not be explicable by a concurrent intellectual impairment, a disorder of attention, a primary sensory deficit (like blindness or deafness), or a language disorder (aphasia) that would prevent the patient from naming the stimulus.

Conceptual representation of visual agnosia

Agnosia is often described as a "perception without knowledge," where a person can see an object's features but cannot recognize what the object is.

Apperceptive vs. Associative Agnosia

The classic model, proposed by Lissauer in 1890, divides agnosias into two main categories:

Apperceptive Agnosia: A defect in higher-order perceptual processing. The patient has difficulty perceiving the structure of an object and may be unable to copy a drawing of it, despite having normal basic visual functions.
Associative Agnosia: Perception is thought to be intact (the patient can copy a drawing), but there is a defect in giving meaning to the percept. It has been described as "a normal percept that has somehow been stripped of its meaning."

This distinction remains debated. The idea that perception is entirely normal in associative agnosia is likely not true, and some argue that all agnosias represent a form of higher-order perceptual deficit. This definitional difficulty reflects the ongoing challenge of understanding perception at a physiological level.

Sensory Modalities and Anatomical Correlates

Theoretically, agnosias can occur in any sensory modality, but the most unequivocal examples are in the visual (e.g., prosopagnosia) and auditory (e.g., pure word deafness) domains. Many other forms, including tactile agnosias like astereognosis, have been described.

Anatomically, agnosias generally reflect dysfunction at the level of the association cortex, though they can occasionally result from thalamic pathology. Over time, persistent agnosic defects may merge into anterograde amnesia, representing a failure to learn new sensory information.