Introduction
Agnosia and neurodegenerative diseases are both results of damage to the brain. Certain neurodegenerative diseases have been shown to cause specific types of agnosia. Agnosia arises from lesions in particular parts of the brain, which can be caused by damage to the brain from neurodegenerative diseases over time.
Definition of agnosia
Agnosia is an umbrella term for neurological disorders that result in difficulty recognising objects, people, sounds, or textures despite intact senses. Importantly, agnosia is not related to memory.
Overview of neurodegenerative disorders
Neurodegenerative disorders are characterised by neurodegeneration (i.e., the loss of nerve cell function or structure over time). Neurodegeneration is the key feature of many brain disorders, including but not limited to:
- Alzheimer's disease
- Parkinson's disease
- Frontotemporal dementia
- Huntington's disease
- Amyotrophic lateral sclerosis (ALS)
- Motor neurone disease (MND)
Types of agnosia
Categorised by the different senses, agnosia can come under three sub-groups: visual, auditory, and tactile.
Visual agnosia
Visual agnosia is the inability to recognise "things" (items, faces, etc.) on sight alone. Put simply, someone with visual agnosia cannot process and comprehend what they see before them.
There are two types of visual agnosia - apperceptive and associative.1
Apperceptive visual agnosia
Apperceptive visual agnosia is associated with an inability to recognise the form of something.
It is when a visual stimulus can be identified and understood, but cannot be copied or drawn.
For example, if you place a black cat in front of a person with apperceptive visual agnosia, they should be able to tell you that it is a black cat and that it purrs. However, they may not be able to generate an accurate drawing of the cat on paper. They may also struggle to read and write.
Associative visual agnosia
This is when a visual stimulus can be accurately copied or drawn, but an association between what has been copied and its meaning cannot be made. With this type of visual agnosia, there would be little difficulty drawing a picture of the cat. However, once drawn, any knowledge about the black cat – for example that it is a black cat and it purrs – will be lost and unable to be expressed.
Specific types of visual agnosia
Additionally, there are specific types of visual agnosia. Two examples of this are detailed below:
Prosopagnosia
Prosopagnosia, also known as face blindness, is an inability to recognise faces. It is not an inability to recall faces; rather, it is where the memory of a face cannot be made in the first place. As with all agnosias, someone with prosopagnosia will have the corresponding sense – in this case, vision – intact. However, they will be unable to recognise familiar faces (associative prosopagnosia) or facial expressions (apperceptive prosopagnosia) using sight alone.
People with prosopagnosia can suffer from the disorder for a large portion of their life not knowing that what they have is a diagnosable illness, but rather believing that they are intellectually stunted. It can be a source of shame or anxiety for these people, especially since widespread awareness of the disorder is low and the number of diagnoses is even lower.
Approximately 2.5% of the world is believed to suffer from some level of prosopagnosia. So why is diagnosis rare? The issue lies mainly in the fact that people with prosopagnosia are exceptionally good at "out of the box" thinking. To avoid embarrassment, they consistently use clues to aid them in their recognition. Knowledge of hairstyles, clothing, smells, and key features like bushy eyebrows or beards are frequently used to identify a familiar person.
This is particularly the case for people with congenital prosopagnosia – that is, people who were born face-blind. Because they can ultimately "recognise faces" with this puzzle-piece approach, people may not even know that the difficulty they face in facial recognition is not universal.
In 2008, a study on the social consequences of prosopagnosia concluded that the psychological issues that manifested in people with prosopagnosia were equally as debilitating as those resulting from dyslexia or stuttering, where awareness and both professional and psychological support are more prominent.2
Simultanagnosia
Like prosopagnosia, simultanagnosia also comes under two specific types:
- Dorsal simultanagnosia is the inability to see more than one object at a time
- Ventral simultanagnosia is the inability to recognise more than one object at a time
For example, take a simple child-like painting of the sun in a meadow. Someone with dorsal simultanagnosia may only be able to see the sun initially, then the field, then a cow – though never simultaneously. People with this type of simultanagnosia may find themselves frequently bumping into large, typically "obvious" objects if something smaller like a mug catches their eye.
Applying the same example, a person with ventral simultanagnosia would be able to see the sun, field, and cow together, but could only apply meaning to one at a time. In this case, the cow may be correctly recognised as a cow whilst the sun is a yellow ball.
Auditory agnosia
Auditory agnosia is a difficulty in perceiving sounds despite having no hearing impairments. Some examples of auditory agnosias (and the specific deficits of recognition) are:
- Phonagnosia (familiar voices)
- Verbal auditory agnosia/word deafness (spoken words)
- Nonverbal auditory agnosia (nonverbal sounds and noises)
- Amusia (music)
Tactile agnosia
Tactile agnosia is an inability to recognise objects by touch. Some examples of tactile agnosias (and the specific deficits of recognition) are:
- Amorphognosia (size and shape)
- Ahylognosia (distinctive qualities like texture/weight e.g., cotton, wood, metal)
- Asymbolia (identity of objects where neither amorphognosia nor ahylognosia is present)
Other agnosias
People may also suffer from agnosias related to the other senses, smell and taste. These are known as olfactory and gustatory agnosia, respectively.
Sometimes, a person may not be able to recognise that they have a medical condition (such as Parkinson's disease). Alternatively, they may be able to recognise that the medical condition exists but are unable to recognise its importance. These agnosias are known as:
- Anosognosia (existence of medical condition)
- Anosodiaphoria (importance of medical condition)
Causes of agnosia
Agnosia can be a resulting condition of:
- Strokes
- Brain tumours
- Brain infections
- Hypoxia
- Carbon monoxide (CO) poisoning
- Head injury
- Developmental disorders
- Neurological disorders
- Dementia
Key brain regions
The brain is split into three main parts:
- The cerebrum – comprising the frontal, parietal, occipital, and temporal lobes
- The cerebellum
- The brainstem
Agnosia is caused by lesions in the brain. The location of these lesions is specific to a type of agnosia. The table below shows some of the aforementioned agnosias, and where the lesions are located in the brain.
Figure 1: Images of the cerebrum showing all 4 lobes (left) and other important brain structures (right). Image taken from hopkinsmedicine.org.
Type of Agnosia | Location of lesion |
Apperceptive visual agnosia | Parietal/occipital lobes |
Associative visual agnosia | Occipital/temporal lobes |
Prosopagnosia | Occipital/temporal lobes (fusiform face area FFA in temporal cortex) |
Simultanagnosia | Parietal/occipital lobes |
Auditory agnosia | Temporal lobes |
Tactile agnosia | Parietal lobes |
Olfactory agnosia | Temporal lobes |
Anosognosia | Typically to the parietal lobes (but also: thalamus, basal ganglia, and frontal/temporal lobes) |
Common neurodegenerative disorders associated with agnosia
Alzheimer's disease
Alzheimer's disease (AD) is a common neurodegenerative disorder associated with agnosia. AD is suggested to develop due to an abnormal accumulation of proteins that form "plaques" or "tangles" in and around brain cells. This leads to blockages at neuron-neuron connections, disrupting communication between the brain cells and resulting in cell death.
Typically, the hippocampus (the memory centre) shrinks first. This is why a lot of the early-stage symptoms of AD are memory-related, such as forgetting recent events, misplacing items, or repeatedly asking the same questions.
Over time, other parts of the brain experience similar reductions in size. Eventually, AD will progress from struggling to think of the right word and remember conversations/faces, to having severe short-term and long-term memory issues. This includes having extreme difficulty eating, moving, or speaking.
Alzheimer's disease and agnosia case studies
It is believed that agnosia can occur as a symptom of AD. Specifically, poor facial recognition in people with AD has been linked to prosopagnosia. This was suggested following a study in 2016 that showed this deficit was due to issues with visual processing as well as memory. Another investigation into this was carried out in 2020.3
This study used three electrochemical markers associated with facial processing in a single person with Alzheimer's. Although the main conclusion to be drawn was regarding the importance of these markers for future AD diagnosis, in one patient the face recognition deficits were found to be likely related to prosopagnosia.
AD has also been linked to mirror-image agnosia. In a 2014 study, the results suggested that specific damage to the right parietal lobe, which may occur in the early stages of AD, was the cause.4
Parkinson's disease dementia
Parkinson’s disease (PD) dementia is a neurodegenerative disorder associated with the progressive loss of dopaminergic neurons in the substantia nigra (a part of the basal ganglia). PD has also been linked to agnosia – in particular, anosognosia. Anosognosia is also a common symptom of Alzheimer’s disease.5
A research study from the NHS noted that reports of anosognosia of motor symptoms in later-stage PD were being made.
Current treatments
There is no cure for agnosia. Instead, treating the underlying cause – if possible – is regarded as the best course of action. Unfortunately, where the underlying cause is an incurable neurodegenerative disorder, agnosia is often a life-long condition. This is understandably very frustrating and difficult to come to terms with, so there is an increasing emphasis on rehabilitation to help those with the condition live a good quality of life.
Some of these methods include:
- Support groups
- Speech and occupational therapy (to teach patients how to use intact senses)
- Counselling
Summary
- Agnosia is a term for a group of conditions where a person has a specific recognition deficit
- Agnosia is not related to memory
- One cause of agnosia is neurological disorders, such as Alzheimer's disease (AD)
- Despite previously being attributed solely to memory problems, poor facial recognition in AD has been linked with prosopagnosia (face blindness)
- Both Parkinson’s disease dementia and Alzheimer’s have been linked to anosognosia, (the inability to recognise their symptoms and medical conditions)
- Although agnosia with neurodegenerative diseases is incurable, there is an effort being put into increasing awareness and support so that patients can lead a good quality of life
References
- Kumar A, Wroten M. Agnosia. StatPearls [Internet]. 2023. [cited 10 June 2024]. Available from: https://www.ncbi.nlm.nih.gov/books/NBK493156/
- Yardley L, McDermott L, Pisarski S, Duchaine B, Nakayama K. Psychosocial consequences of developmental prosopagnosia: A problem of recognition. Journal of Psychosomatic Research. 2008 [cited 10 June 2024]; 65(5): 445–51. Available from: https://www.sciencedirect.com/science/article/abs/pii/S0022399908001578?via%3Dihub
- Mazzi C, Massironi G, Sanchez-Lopez J, De Togni L, Savazzi S. Face recognition deficits in a patient with alzheimer’s disease: amnesia or agnosia? The importance of electrophysiological markers for differential diagnosis. Front Aging Neurosci. 2020 [cited 10 June 2024]; 12: 580-609. Available from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7779478/
- Chandra S, Issac T. Neurodegeneration and mirror image agnosia. North Am J Med Sci. 2014 [cited 10 June 2024]; 6(9): 472. Available from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4193155/
- Vannini P, Hanseeuw B, Munro CE, Amariglio RE, Marshall GA, Rentz DM, et al. Anosognosia for memory deficits in mild cognitive impairment: Insight into the neural mechanism using functional and molecular imaging. NeuroImage: Clinical. 2017 [cited 10 June 2024]; 15: 408–14. Available from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5458095/