Agnosia is a rare disorder whereby a patient is unable to recognize and identify objects, persons, or sounds using one or more of their senses despite otherwise normally functioning senses. The deficit cannot be explained by memory, attention, language problems, or unfamiliarity to the stimuli. Usually, one of the sensory modalities is affected. For example, a patient with agnosia may not be able to identify a cup by sight, although they may be able to tell its color and identify it by touch by its shape and texture. It is not same as anomia. Anomia is a naming disorder in which patients cannot name an object despite using their other sensory modalities like touch and smell. 
Classically, there are 2 forms of agnosia: apperceptive and associative.
- Apperceptive agnosia is a failure in recognition due to deficits in the early stages of perceptual processing.
- Associative agnosia is a failure in recognition despite no deficit in perception. Associative agnosia patients can typically draw, match or copy objects while apperceptive agnosia patients cannot.
Types of Agnosia
There are 3 main types of agnosia, based on the type of sensation involved.
- Visual (vision)
- Auditory (hearing)
- Tactile (touch)
Visual agnosia refers to an impairment in recognizing visually presented objects, despite otherwise normal visual field, acuity, color vision, brightness discrimination, language, and memory. Patients can recognize objects using other sensory modalities. Sometimes impairment is worse for certain types of objects, so a variety of objects should be tested to diagnose precisely. Visual agnosia is the most common and better-understood agnosia.
Agnosia is further divided in 2 subtypes: apperceptive visual agnosia and associative visual agnosia.
- Apperceptive visual agnosia refers to an abnormality in visual perception and discriminative process, despite the absence of elementary visual deficits. These people are unable to recognize objects, draw, or copy a figure. They cannot perceive correct forms of the object, although knowledge of the object is intact. Apperceptive visual agnosia is typically associated with lesions to the parietal, occipital cortex
- Associative visual agnosia refers to difficulty with understanding the meaning of what they are seeing. They can draw or copy but do not know what they have drawn. They correctly perceive the form and know the object when tested with verbal or tactile information, but cannot identify the object. They are unable to link the fully perceived visual stimulus to prior experience to help them recognize the stimulus. Associative visual agnosia is usually associated with damage to the bilateral inferior occipitotemporal cortex.
Types of Visual Agnosia
- Prosopagnosia is the inability to recognize familiar faces. Patients can often identify other aspects like gender, hair, emotions. Prosopagnosia results from damage to fusiform face area (located in the inferior temporal cortex in fusiform gyrus). People with apperceptive prosopagnosia cannot perceive facial expression and cues but can recognize non-facial clues like hair and clothing. Associative prosopagnosia patients can derive some facial information like gender and age. Mr. P in Oliver Sack’s book The Man Who Mistook His Wife for a Hat presents a classic case of prosopagnosia.
- Simultanagnosia is the inability to recognize and sort out objects when they appear together, but they can recognize them when they appear alone. Patients are unable to perceive the overall meaning of a picture or multiple things together, although they can describe isolated elements. Two forms of simultagnosia have been described.
- Dorsal simultagnosia: Patients cannot see more than one object at a time. For example, when presented with a picture with a table, chair and flower vase, they may report only one thing at a time. When their attention is diverted to the other thing, they can then identify only that thing; other things disappear to them. They often have difficulty reading as it involves viewing more than one word at a time. They often bump into objects that are close together. Dorsal simultagnosia is typically associated with lesions in the bilateral occipitotemporal cortex.
- Ventral simultagnosia: These individuals also cannot identify more than one object or complex objects at one time, although they can see more than one object at a time. They are unable to perceive the whole picture as one and derive a meaning out of it. For example, in a night sky picture with stars and full moon, they may identify the moon as a ball, unable to derive the meaning of the whole picture. Ventral simultagnosia is associated with lesions in the left inferior occipital area.
- Color agnosia is the inability to identify and distinguish colors, despite intact basic color vision and brightness discrimination mechanisms. It is very difficult to diagnose this type of agnosia as colors can only be appreciated visually. Usually, these patients have a lesion in the left occipitotemporal region of the brain.
- Topographical agnosia is the inability to orient to surroundings because of the inability to interpret the spatial information. These patients have a good memory of the layout and specifics of the places well known to them, but they are unable to navigate their way through. They are unable to use visual cues to guide them in the right direction. Associated with a lesion in the right posterior cingulate area of the brain.
- Finger agnosia is a difficulty in naming and differentiating among the fingers of either hand as well the hand of others. It does not refer to the inability to identify a finger as a finger, as the name may suggest. It is a part of the constellation of symptoms often referred to as Gerstmann syndrome which includes acalculia, agraphia, finger agnosia, and left-right disorientation.
- Akinetopsia refers to the inability to perceive motion.
- Agnostic alexia refers to the inability to recognize words visually. They can still write and talk without difficulty.
- Auditory agnosia is an inability to recognize sounds despite intact hearing. Auditory agnosias are typically associated with right side temporal lesions.
Types of Auditory Agnosia
- Phonagnosia is the inability to recognize familiar voices. They can recognize words spoken by others. It is caused by damage to certain parts of the sound association region.
- Verbal auditory agnosia or pure word deafness is the inability to comprehend spoken words but can read, write, and speak in a relatively normal manner.
- Nonverbal auditory agnosia is the inability to comprehend nonverbal sounds and noises, with sparing of speech comprehension.
- Amusia is the inability to recognize the music. They are unable to comprehend that certain types of sounds represent music and therefore cannot distinguish music from other sounds.
Tactile agnosia refers to the inability to recognize objects by touch. They can name objects by sight.
- Amorphognosia is the inability to identify the size and shape of objects by touch, for example, a triangle or square.
- Anosognosia is the inability to identify distinctive qualities like texture and weight, for example, a piece of wood, cotton or metal.
- Tactile asymbolia is impaired recognition by touch in the absence of amorphognosia and ahylognosia.