Introduction
Agnosia is a condition characterized by the brain's inability to recognize stimuli through a specific sense, despite intact sensory function, cognitive ability, familiarity, attention, and memory.1 Individuals with agnosia can still identify objects, faces, or people using their other senses. For instance, a person with visual agnosia might struggle to recognize a cat by sight but can identify it through touch (feeling its fur) or sound (hearing its purr).2
This article aims to increase awareness of agnosia, explore its various types, and explain the roles of the occipital and temporal lobes in the condition.
What is agnosia?
Agnosia is not due to a deficit in the senses themselves; for instance, in visual agnosia, there is no impairment in visual fields, colour, or depth perception. The problem lies in the brain's ability to process stimuli. Rapid onset of agnosia can occur from brain damage due to strokes or head trauma, while a slower onset may result from neurodegenerative conditions like Alzheimer’s disease, other forms of dementia, or brain tumours. Additionally, agnosia can also be caused by factors such as carbon monoxide poisoning and various developmental and neurological conditions.2
Types of agnosia
Agnosia can be divided into two main subtypes:
- Apperceptive Agnosia: This type involves a failure to recognize stimuli due to impairments in the early stages of perceptual processing. While the senses themselves remain intact, the brain struggles to accurately process the information. For example, individuals with visual apperceptive agnosia can see objects but have difficulty copying or matching them, reflecting a problem with integrating visual information.
- Associative Visual Agnosia: This subtype is characterized by a failure to recognize stimuli despite intact sensory processing. In this case, the brain can process the visual information but cannot make sense of it. For instance, a person with visual associative agnosia may be able to draw or copy an object but would struggle to identify it.
Agnosia can impact any of the senses, though some forms are more prevalent than others. The types of agnosia based on the affected sense include:
- Visual Agnosia (seeing)
- Auditory Agnosia (hearing)
- Tactile Agnosia (touching)
- Gustatory Agnosia (tasting)
- Olfactory Agnosia (smelling)
Diagnosis
Diagnosing agnosia involves specific steps that vary depending on the affected modality, but some common procedures apply across all cases:
- A patient history
- Ruling out alternative explanation
- Assessing the patient’s brain function and memory
- Checking if there are sensory deficits in more than one modality (unlikely in the case of agnosia)
- Conducting detailed interviews with the patient, family members, and caregivers to gather clues about the diagnosis and underlying cause
- Ask the patient to identify common objects through sight, touch, and smell
- Asking how this is affecting their day-to-day functioning
Brain regions involved in Agnosia
Visual and auditory agnosias are particularly associated with lesions in the occipital and temporal regions of the brain. These forms will be the primary focus of this article.
Temporal lobe
The temporal lobe is named for its proximity to the temples, it is located behind the parietal lobe and in front of the occipital lobe. The lobe is present on both the left and right hemispheres of the brain.
Its main functions include:3
- Emotional processing
- Producing and understanding language
- Learning and memory
- Helps the body maintain homeostasis
- Object recognition
- Formation of visual memories
Occipital lobe
The smallest of the brain lobes is the occipital lobe, known as the visual processing hub of the brain.
Functions associated with the occipital lobe include:4
- Visual processing
- Spatial processing
- Depth perception
- Colour determination
Types of agnosia and corresponding brain regions
Visual agnosia
Visual agnosia is the most common and well-understood type of agnosia. It refers to impairment in visually recognising a stimulus despite the visual system functioning normally (e.g. colour vision and visual field).1
Brain regions involved in visual agnosia
There are pathways between brain regions that allow us to make meaningful associations between the stimuli we see and how we perceive them. The two distinct visual pathways that allow us to do so are:
The dorsal pathway (the ‘what’ pathway)
- The dorsal stream leads from the primary visual cortex in the occipital lobe to the parietal lobe
- It is involved in processing where visual stimuli are, relative to the viewer5
The ventral pathway (the where/how pathway)
- The ventral stream is associated with object recognition
- Information from the primary visual system in the occipital cortex is connected to regions in the temporal lobe
- The area of the temporal lobe it connects to is associated with long-term memories and helps us derive meaning from what we are seeing5
Damage to the ventral visual stream is specifically associated with visual agnosia, impairing the ability to recognise specific categories of stimuli depending on the lesion location. For example, prosopagnosia also known as ‘facial blindness’ is a subtype of visual agnosia, where individuals cannot recognise familiar faces. It is often associated with lesions in the inferior temporal cortex.2
Auditory agnosia
Auditory agnosia refers to the inability to recognise and distinguish between sounds, even when an individual’s hearing is not impaired. In auditory agnosia, the abilities to read, write, and speak remain unaffected.
Brain regions involved in auditory agnosia
The primary auditory cortex, located in the temporal lobe, is where the detection of sound and discrimination between frequencies occur. Auditory stimuli are processed through two pathways, similar to that of the visual pathway streams:
Ventral auditory stream (the auditory ‘what’ pathway)
- Originating in the primary auditory cortex, the pathway then joins the visual ventral stream at the middle temporal gyrus and is involved in sound recognition.6
Dorsal auditory stream
- The dorsal stream is known as the ‘spatial’ pathway and allows us to determine the direction of sound and recognise when multiple different sounds are occurring.6
An example of a type of auditory agnosia is phonagnosia, associated with damage to the right temporal lobe.7 For example, a person with phonagnosia would not recognise the voice of a loved one on the phone, despite being able to hear the voice clearly and comprehend what they are saying.
Summary
Understanding the roles of the occipital and temporal lobes is crucial for accurately diagnosing and effectively treating different types of agnosia. The occipital lobe, located at the back of the brain, is primarily responsible for processing visual information. In cases of visual agnosia, individuals have difficulty recognizing objects, faces, or other visual stimuli despite having normal vision. This impairment arises because the occipital lobe, although functioning to some extent, fails to integrate and interpret visual data correctly.
On the other hand, the temporal lobe, situated on the sides of the brain, plays a vital role in object recognition, language comprehension, and memory formation. This lobe is essential in both visual and auditory agnosia. In visual agnosia, the temporal lobe's involvement is crucial for interpreting and making sense of visual stimuli, while in auditory agnosia, it affects how sounds and spoken words are processed and understood. By understanding these brain regions' specific functions and their impact on sensory processing, healthcare providers can better diagnose and tailor treatments for individuals with agnosia.
References
- Coslett HB, Gottfried JA. Sensory Agnosias. Neurobiology of Sensation and Reward. 2011 [cited 13 June 2024]. Available from: http://www.ncbi.nlm.nih.gov/books/NBK92800/
- Kumar A, Wroten M. Agnosia. StatPearls [Internet]. 2023 [cited 12 June 2024]. Available from: http://www.ncbi.nlm.nih.gov/books/NBK493156/
- Patel A, Biso GMNR, Fowler JB. Neuroanatomy, Temporal Lobe. StatPearls [Internet]. 2023 [cited 14 June 2024]. Available from: http://www.ncbi.nlm.nih.gov/books/NBK519512/
- Rehman A, Al Khalili Y. Neuroanatomy, Occipital Lobe. StatPearls [Internet]. 2023 [cited 13 June 2024]. Available from: http://www.ncbi.nlm.nih.gov/books/NBK544320/
- Rolls ET. Two what, two where, visual cortical streams in humans. Neuroscience & Biobehavioral Reviews. 2024 [cited 13 June 2024]; 160:105650. Available from: https://www.sciencedirect.com/science/article/pii/S0149763424001192
- Cohen YE, Bennur S, Christison-Lagay K, Gifford A, Tsunada J. Functional Organization of the Ventral Auditory Pathway. Adv Exp Med Biol. 2016 [cited 14 June 2024]; 894:381–8. Available from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5444378/
- Van Lancker DR, Cummings JL, Kreiman J, Dobkin BH. Phonagnosia: A Dissociation Between Familiar and Unfamiliar Voices. Cortex. 1988 [cited 14 June 2024]; 24(2):195–209. Available from: https://www.sciencedirect.com/science/article/pii/S0010945288800297