Overview

The term ‘agnosia’ is a Greek word which means ‘not knowing’ or ‘without knowledge’. Agnosia is a broad term for a rare condition where the brain cannot recognise objects, people, or sounds in their environment, even though the senses can perceive them. This condition can affect your vision, smell, hearing, taste and touch. For example, you may not be able to identify a cup just by looking at it, but you can identify it by its shape and texture through touch.¹

This is because your brain finds it difficult to gather information and understand this information from the senses, even though you were able to recognise the object in the past. The most common types of agnosia are visual (sight) agnosia, auditory (hearing) agnosia, tactile (touch) agnosia, and prosopagnosia (face blindness), which are covered in this article.

Agnosia usually occurs upon damage to the brain areas responsible for primary sensory processing, which is why the five senses are affected in this condition.¹ Several neurological conditions like head injury, stroke, dementia, infections, and others can cause damage to the brain, leading to agnosia.¹ 

Traumatic brain injuries (TBI) are caused by damage to the brain due to external excessive physical force applied to the head. Damage to the brain can either be focal (specific to one area) or diffuse (more than one area of the brain). Out of 200,000 people who are admitted to hospitals due to head injuries in the UK each year, 40,000 of them are cases of traumatic brain injury, and 2,000 people succumb to their injury.^2,3 TBIs are also one of the leading causes of death and disability that affect both children and adults worldwide and cause a major socioeconomic burden for both patients and their families, public health systems and society.⁴ 

Due to the prevalence and massive impact that TBIs have in our society, it is important to understand the symptoms, causes, and dire consequences to provide appropriate treatment and management upon injury. This article highlights the effect of traumatic brain injuries on recognition abilities that can lead to different types of agnosia.

Types of agnosia 

Agnosia can be broadly divided into apperceptive and associative agnosias. 

• Apperceptive agnosia is the inability to recognise due to incomplete processing in the early stages of perception¹ 
• Associative agnosia is the inability to recognise due to a lack of association between visual information and preexisting knowledge about the visible object or person¹

Visual agnosia (sight)

Visual agnosia affects the perception of sight in patients. Although you can see things properly, your brain cannot process this visual information and recognise what your eyes see. Visual field, acuity, colour vision, brightness, discrimination, motion perception, language, and memory are unaffected, but recognition of an object based only on visual representation is affected. Damage to the lateral area of the occipital lobes and/or the ventral section of the temporal lobes has been identified as the primary cause of visual agnosia.⁵

Apperceptive visual agnosia 

In this form of visual agnosia, patients can access pre-existing knowledge about the object but cannot identify, copy, or match the object to a drawing. So, identifying the elementary characteristics of the object and forming a structural description of the object is incomplete.⁶

Associative visual agnosia 

In associative visual agnosia, patients can visually perceive the object. Still, they find it difficult to associate any meaning or knowledge regarding the object, even though they may have pre-existing knowledge or experiences with the object. So this disorder is commonly known to be ‘perception stripped of meaning’.⁷ 

Prosopagnosia (face blindness)

Prosopagnosia or ‘face blindness’ (a type of visual agnosia) is the inability to recognise individual faces even if they are familiar.^1,6 In severe cases, the person may be unable to identify their faces in photographs or faces of family members and close friends. They can identify people based on other features like their clothes, gender, or voices. In apperceptive prosopagnosia, patients use hair colour or clothing to identify a person because they cannot perceive facial expressions. In associative prosopagnosia, patients use gender and age along with some facial information to identify a person.¹ 

Auditory agnosia (sound)

Auditory agnosia is the inability to recognise sounds even though you can hear them.¹ 

Verbal auditory agnosia

Also called pure word deafness, verbal auditory agnosia is a condition where patients cannot comprehend spoken words aloud, but they can read, write and speak normally.¹ 

Non-verbal auditory agnosia

In non-verbal auditory agnosia, patients cannot recognise nonverbal sounds and noises, but they can understand words when people speak to them.¹

Tactile agnosia (touch) 

Tactile agnosia is the impairment of tactile perception. In other words, patients cannot differentiate between objects through touch, but they can recognise them through their other senses.¹

Astereognosis

Astereognosis is a type of tactile agnosia where you cannot identify an object by feeling it, but you can recognise it through sight.⁹

Mechanisms underlying normal recognition

Visual pathways 

The visual processing pathways in the brain are broadly divided into two streams, ventral for object recognition and dorsal for action recognition and control.⁹ The ventral stream is commonly called the ‘what’ pathway, and the dorsal stream is called the ‘where’ pathway.¹⁰ 

The ventral stream connects to the lower part of the brain, where the temporal lobe is located. Visual information captured by the retina is passed on to the primary visual cortex and then projected to the inferior temporal cortex of the brain, where processing for complex visual information like complex shapes, depth perception, form and colour vision, contrast sensitivity, face perception, and route-finding takes place.¹¹

The dorsal stream connects to the upper part of the brain, where the parietal lobe is located. This pathway is responsible for processing spatial locations of objects, visual guidance for actions like reaching or grasping, spatial awareness, spatial perception and interpretation, and coordination of movements that take place.¹²

Auditory pathways 

The central auditory nervous system is responsible for identifying and processing sound frequency, intensity, spatial location, attenuations, and combinations of these three abilities that allow people to understand and accurately interpret sound.¹³ The structures involved in the central auditory system are auditory nerves from the left and right ear, cochlear nucleus and superior olivary nucleus, inferior colliculus, medial geniculate nucleus, and the primary auditory cortex.¹³

The primary auditory cortex is the brain region where all auditory information is received and is located bilaterally in the superior temporal gyrus of the temporal lobe.¹⁴ Like the visual pathway, auditory pathways can be categorised into ‘what’ and ‘where’ for auditory information processing. 

The ventral stream (‘what’) is responsible for identifying sounds and is processed in the temporal lobe and prefrontal cortex of the brain, whereas the dorsal stream (‘where’) is responsible for locating sounds and is processed in the parietal lobe and the prefrontal cortex of the brain.¹⁵ The endpoints of the ventral stream (temporal and prefrontal cortex) are involved in complex sound processing, such as understanding the meaning behind language. In contrast, the endpoints of the dorsal stream (parietal and prefrontal cortex) are involved in understanding separate segments of speech, learning vocabulary, and comprehending the articulation of words.¹⁶

Somatosensory pathways

The somatosensory system is responsible for processing sensory modalities like touch, pain, proprioception (body positioning), and temperature. Receptors in the skin responsible for each sensory modality carry afferent information (information for the sensation and perception of touch) to the primary somatosensory cortex in the parietal lobe (via the ventral posterolateral nucleus in the thalamus) of the brain, where somatosensory information is processed.¹⁷ 

Traumatic brain injuries (TBIs)

TBIs can be categorised into mild, moderate, and severe brain injuries based on the severity of symptoms and are also categorised into closed brain injury and penetrating brain injury based on how the injury occurs. Closed brain injuries are internal damages to the brain within the skull, whereas penetrating brain injuries are caused by external objects that penetrate the skull and severely damage the brain.

Types and causes of TBIs 

Concussions

Concussions are mild traumatic brain injuries where short-term effects on brain function like headaches, difficulties in memory, balance, mood, sleep or concentration, are observed without any visible changes in the brain structure upon damage to the head. Concussions are commonly caused when people fall or after sports-related injuries, and not everyone loses consciousness during a concussion. Most people recover fully after a concussion, which lasts less than 6 hours. 

Contusions 

Contusions or intracerebral hematoma occur when damage to the head inflicts a bruise on the brain and causes bleeding or swelling in that brain region. A forceful blow to the head can cause it, or when sudden movement comes into contact with an immobile object (e.g. a steering wheel during motor vehicle accidents). Contusions can range from moderate to severe TBIs depending on the extent of bleeding and swelling of the brain. They may occur hours or days after injury and cause serious deterioration in brain functions. If there is severe bleeding detected through a CT scan, doctors will treat contusions as severe TBIs. 

Diffuse axonal injury (DAI)

DAIs are classified as severe TBIs because axons responsible for nerve cell communications in the brain are damaged in more than one area when shearing forces act on the brain as it moves inside the bony skull.¹⁸ The most common causes of DAIs are high-speed motor vehicle crashes, falls, and shaken baby syndrome. 

Penetrating injuries

Penetrating injuries are also called open head injuries when there is a break in the skull and protective barriers of the brain, usually caused by gunshot wounds, falls, stabs, or accidents with sharp objects that are responsible for the head trauma.¹⁹ 70-90% of patients die before reaching the hospital, and 50% of those who do make it to the hospital unfortunately succumb to their injuries during resuscitation.¹⁹ 

Impact of TBIs on recognition abilities

Pure forms of agnosia are very rare, so most cases of diagnosed agnosias are acquired forms or appear as symptoms of traumatic brain injuries, stroke, dementia, brain tumours, or infections.¹

Visual agnosia and TBIs

Visual agnosia caused by traumatic brain injuries can have long-lasting effects and are a major cause of visual agnosia. Damage to the bilateral or unilateral inferior occipitotemporal cortex causes the inability to access stored information about an object’s structural description in associative visual agnosia.⁶ In apperceptive visual agnosia, lesions or damage in the parietal or occipital cortex (due to diffuse brain damage) cause a breakdown in the early stages of visual information processing in the brain.⁶

Auditory agnosia and TBIs

Although it is less frequent, traumatic brain injuries have been identified as a cause of auditory agnosias.²¹ Damage or lesions in the bilateral superior temporal lobe and unilateral damage to the primary auditory cortex can result in auditory agnosia.⁸ Damage to the pathways responsible for the processing of acoustic input (verbal and non-verbal sounds or noises) was found to cause generalised auditory agnosia.²¹ 

Tactile agnosia and TBIs

Head injuries have been found to cause tactile agnosia like astereognosis upon trauma to the parietal lobes of the brain, which is a common site for depressed skull fracture, and has been discussed as an important part of the brain responsible for the processing of somatosensory information like touch.⁹

Prosopagnosia and TBIs

The brain regions responsible for facial recognition abilities include the prefrontal cortex, anterior temporal lobe, inferior and middle temporal cortex, hippocampus, and amygdala, which work together to encode, store, and retrieve facial memories which are affected in acquired cases of prosopagnosia caused by traumatic brain injuries.²⁰ Damage to the fusiform face area in the inferior temporal cortex affects facial perception, memory and recognition.¹

Summary

Agnosia is a rare set of conditions where people cannot recognise objects, sounds, or people in their environment, even though their other senses and cognitive abilities are functioning properly. Traumatic brain injuries have been identified as a common cause for different types of agnosia like visual, auditory, tactile and prosopagnosia.

Damage to the brain through TBIs disrupts the normal processing of the sensory information sent to the brain and the functioning of the senses, which may lead to agnosia. TBIs can range from mild to severe and differ in how the damage is inflicted upon the brain, which ultimately determines the appropriate diagnosis, management, and treatment for the injury.