Agnosia is a loss of the ability to recognize objects we see (visual agnosia) or noises we hear (auditory agnosia). At the same time, sensory functions (vision, hearing, touch, etc.) are standard. 

Clinical studies show that people suffering from neurodegenerative diseases have gnostic disorders – the inability to recognize objects, shapes, and faces. Face recognition is particularly impaired in frontotemporal and Alzheimer’s -type dementia, as well as in Parkinson’s disease. These gnostic disorders would explain the appearance of behavioral and psychological disorders.

Agnosia and Alzheimer’s disease

Alzheimer’s disease is primarily characterized by progressive degeneration of the hippocampus and entorhinal cortex, resulting in impaired episodic memory and learning, making it challenging to learn new faces.

A loss of semantic memory is subsequently observed as the disease progresses: the patient has more difficulty retaining information related to people from their face. He hardly perceives the relative as familiar and no longer recognizes him.

Patients have difficulty interpreting negative facial expressions expressing fear and sadness. Delusional-type behavioral disorders appear as the disease worsens, resulting in the belief that the loved one has been replaced by a look-alike (Capgras syndrome). These disorders hinder communication, generate inappropriate behaviors and affect the relational life of patients.

In the severe stage of the disease, the patient no longer recognizes himself in a mirror. Faced with his image, the patient presents different attitudes: hesitation, indifference, or avoidance.

Neuroimaging studies have reported lesions of neurons located in the temporal and prefrontal cortex’s limbic regions (e.g., the amygdala), undoubtedly at the origin of these gnostic disorders.

Agnosia and frontotemporal dementia

Frontotemporal dementias (FTD) are a group of neurodegenerative diseases characterized by behavioral and language disorders associated with intellectual deterioration.

Patients are often anosognosic (anosognosia: neuropsychological disorder defined by the individual’s lack of knowledge of his illness). They can recognize familiar faces and extract features unrelated to emotions (i.e., age, sex) but have more difficulty identifying negative facial emotions, such as fear, anger, and disgust. Recognition of positive emotions may also be affected.

These deficits would explain the appearance of specific behavioral and psychological disorders (inappropriate behavior in society, empathy). They would be linked to progressive damage to the amygdala, the frontal lobe, then the temporal lobe.

When the temporal lobe is affected, the patient has difficulty identifying famous people, which is more akin to semantic dementia or primary progressive aphasia, which are two subtypes of FTD where language disorders predominate.

Semantic dementia is characterized by early behavioral disturbances, impaired word comprehension, altered social functioning, and a lack of empathy. The patient loses the notion of concepts in connection with temporal lobe degeneration.

The patient can no longer put a name to a famous or familiar face. The inability to recognize faces can be associative (the patient identifies the person from their nature and not their face) or semantic (the patient cannot remember the person).

In addition, patients have difficulty identifying facial expressions evoking fear, anger, or sadness. This deficit is correlated with atrophy of the temporal lobe, the amygdala, and the frontal cortex.

Parkinson’s disease

Although motor symptoms predominate in the disease, the existence of cognitive, psychological, and behavioral disorders (anxiety, depression, apathy) would be associated with degeneration of the cerebral regions located in the basal ganglia.

Early impairment of emotion processing (recognition of facial expressions of disgust and fear) has been observed in Parkinson’s patients. In contrast, the ability to recognize positive expressions (expressions of joy) is preserved. The involvement of the basal ganglia and the amygdala would be at the origin of this deficit in emotion processing.