Agnosia is loss of the ability to discriminate one sensation from another. People with agnosia may have difficulty recognizing the geometric features of an object or face or may be able to perceive the geometric features but not know what the object is used for or whether a face is familiar or not. Agnosia can be limited to one sensory modality such as vision or hearing. For example, a person may have difficulty in recognizing an object as a cup or identifying a sound as a cough. Agnosia can result from strokes, dementia, or other neurological disorders. It typically results from damage to specific brain areas in the occipital or parietal lobes of the brain. People with agnosia may retain their cognitive abilities in other areas.
Agnosia refers to the inability to recognize people or objects even when basic sensory modalities, such as vision, are intact. As an example, patients with visual agnosia may not recognize an object upon visual presentation even though they can describe its shape and other visual characteristics or the person cannot recognize their wife's face and in some cases even their own. It is therefore apparent that sensory perception of the object / face is disconnected from memories associated with the object / face. The patient can perceive the object but has no meaningful associations to it. In agnosia, while perception itself, feeling an objects shape, normal, recognition of objects is not.
The occipital lobe is the very back part of your brain. In the picture below it is colored orange. As for the parietal lobe there are on the side of the brain. In the picture it is colored pink. People with agnosia may have difficulty recognizing the geometric features of an object or face (I know they are my keys, but I cannot describe their shape) or may be able to perceive the geometric features but not know what the object is used for or whether a face is familiar or not. Agnosia can be limited to one sense such as vision or hearing. For example, a person may have difficulty in recognizing an object as a cup or identifying a sound as a cough. People with agnosia may retain their cognitive abilities in other areas, like memory, ability to learn and conversation skills.
There are basically three different forms of agnosia; visual agnosia where the person has difficultly recognizing objects, faces and words. The second is Auditory Agnosia which involves the inability to recognize sounds and thirdly, Somatosensory Agnosia where the person has difficulty perceiving objects through tactile stimulation.
Visual agnosia is the inability to understand the meaning of visual stimuli despite apparently normal vision. There is a loss of recognition of visual perception. Visual agnosia is associated with lesion of the left occipital lobe and temporal lobes. Many patients who have visual agnosia also have a severe problems with their visual field. Apperceptive agnosia is more severe and represents an inability to categorise or manipulate the visual percept in any way. The patient may be unable to distinguish shape or say if they have seen an object before. Associative agnosia is less severe because a limited understanding of the relevance of a visual object is retained. The patient may realise they are looking at a face but be unable to recognise who's face it is (prosopagnosia)
Visual agnosia is associated with lesions of the left occipital and temporal lobes. Many patients have a severe visual field defect on the right side because of the injury to the left occipital lobe. It is common for patients to have visual agnosia but intact recognition in the tactile and auditory modalities. In prosopagnosia bilateral lesions of the gray and white matter of the occipitotemporal gyrus cause prosopagnosia. In particular, the inferior longitudinal fasciculus, a pathway that interconnects the occipital and temporal lobes, has been suggested as the lesioned area in prosopagnosia.
Auditory Agnosia refers to the inability to recognize specific sounds in the context of intact hearing. There is a distinction in this syndrome between pure word deafness, that is considered an agnosia for auditory/verbal information, and auditory agnosia, that involves an agnosia for environmental, nonverbal sounds. Cortical deafness is also a term applied to patients who essentially do not respond to any auditory information even when hearing is intact. Finally, receptive amusia is a term that refers to agnosia for music.
Patients with somatosensory Agnosia (Astereognosis) have difficulty perceiving objects through tactile stimulation although basic tactile sensation is intact. There is a distinction between the inability to recognize basic features of an object, such as size, weight, and texture, and the inability to name or recognize the object. Patients who cannot recognize an object by touch may still be able to draw the object and recognize the object pictured in the drawing. Sometimes they can describe the physical features of the object but cannot recognize it. Pure astereognosis is thought to be caused by lesions of the somatosensory cortex (post central gyrus) in the area subsumed by the hand.