Categories of Agnosia
As mentioned earlier, agnosias affect a single sensory modality and because of this they can be described in terms of the modality affected. There are several categories of agnosias but there are three most common categories presented in the literature: visual, auditory and tactile.
Visual Agnosia
Visual agnosia is the most common agnosia and refers to the inability to recognize familiar objects and faces in the context of preserved visional functions (i.e. visual acuity, visual field). Visual agnosia can be classified into two broad categories: appreceptive and associative. In 1890 Heinrich Lissauer came up with these categories to explain object recognition deficits following brain damage. Lissauer theorized that processing what we see occurs in a two-stage process: 1) the brain assembles incoming information into an image; and 2) the brain then associates that image to past memories to understand its meaning (Coslett, 2007).
Appreceptive agnosia occurs due to impairment at the first stage and associative agnosia occurs at the second stage. Appreceptive visual agnosia is characterized by the inability to reliable name, match, or discriminate visually presented objects, despite adequate function (Baugh, Desanghere, & Marotta, 2010). For instance, an individual with appreceptive agnosia is unable to copy and match a drawing of an object but able to verbally identify the object (see diagram below).
Appreceptive agnosia occurs due to impairment at the first stage and associative agnosia occurs at the second stage. Appreceptive visual agnosia is characterized by the inability to reliable name, match, or discriminate visually presented objects, despite adequate function (Baugh, Desanghere, & Marotta, 2010). For instance, an individual with appreceptive agnosia is unable to copy and match a drawing of an object but able to verbally identify the object (see diagram below).
Associative agnosia is characterized by the inability to use derived perceptual representation to access stored information of the object's functions and associations (Baugh, Desanghere, & Marotta, 2010). Contrary to appreceptive agnosia, an individual with associative visual agnosia is able to copy and match a drawing of an object but unable to identify it (see diagram above). Farah (2004) emphasized, however, that although those with associative agnosia may be able to copy a drawing with detail, they do so without being informed by stored knowledge of the stimulus. In other words, if given a copy of a deliberately distorted drawing, an individual with associative agnosia may include the distortion in their drawing without being unaware of the error. Some individuals get diagnosed with a form of visual agnosia called integrative agnosia, whereby symptoms of both aperceptive and associative agnosias are present.
Lissauer's distinction has continue to be clinically useful but it has been clearly shown that all forms of agnosia have some degree of apperceptive ("perceptual") disturbances (Bauer, 2006).
Since Lissauer's classification of agnosia, several other forms and syndromes of visual agnosia have been recognized in the literature. It is important to note that "pure" forms of agnosias are quite rare, whereas subtypes are more common clinically and in the literature. Some subtypes
of visual agnosia include: Colour Agnosia: Inability to recognize and discriminate colours. Object Agnosia: Inability to recognize and name objects. Simultanagnosia: Inability to perceive overall meaning of a whole image or shape, despite ability to recognize individual details or elements. Prosopagnosia: Inability to recognize familiar faces. Also referred to as facial agnosia (play video to right to learn more). |
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Auditory Agnosia
Auditory agnosia is characterized by an inability to recognize spoken words or environmental sounds despite preserved hearing. Individuals with auditory agnosia can still have the ability to detect and make simple judgments about sounds but they are unable to identify the sources of sounds (Coslett, 2007).
For example, upon hearing a car horn, an individual with auditory agnosia may describe the sound as loud and of short duration but would not be able to recognize that it is a car horn. |
Compared to visual agnosia, auditory agnosia have not been extensively studied. However, similar to visual agnosia, auditory agnosia can be described in terms of apperception and association terms. Individuals with apperceptive auditory agnosia will not be able to repeat a word or imitate a sound (auditory). In contrast, individuals with associative auditory agnosias will be able to perform the above tasks normally but will not be able to match different examples of sounds. For example, an individual would not be able to match two different doorbell sounds, baby cries, or a man and woman saying the same word. The following are several forms and syndromes of auditory agnosia identified in the literature. These forms and syndrome may have apperceptive and associative subtypes or forms (Mesulam, 2000).
Auditory Sound Agnosia: Inability to identify non-verbal sounds.
Pure Word Deafness: Inability to comprehend spoken language.
Nonverbal Auditory Agnosia: Inability to recognize environmental sounds.
Cortical Deafness: Loss of awareness of sound; In the most severe form, an individual may appear to be deaf.
Amusia: Inability to express and perceive music. In addition, there is also a loss in the ability to sing, hum or whistle and provide recognition or emotional response to music (Ghadiali, 2004).
Auditory Sound Agnosia: Inability to identify non-verbal sounds.
Pure Word Deafness: Inability to comprehend spoken language.
Nonverbal Auditory Agnosia: Inability to recognize environmental sounds.
Cortical Deafness: Loss of awareness of sound; In the most severe form, an individual may appear to be deaf.
Amusia: Inability to express and perceive music. In addition, there is also a loss in the ability to sing, hum or whistle and provide recognition or emotional response to music (Ghadiali, 2004).
tactile Agnosia
Similar to visual and auditory agnosia, tactile or somatosensory agnosia refers to the inability to recognize an object through touch despite the physical ability to manipulate the object. Individuals with tactile agnosia can still perceive an object's tactile characteristics such as its weight and texture, but unable to name or comprehend the significance of the object. For example, an individual with auditory agnosia would feel a quarter and be able to apprehend its size, temperature, and shape, but not be able to recognize that it is a quarter. Individuals with tactile agnosia often describe their hands as being "numb" or "stiff" and that the feeling is not distinct or strong enough to identify objects by touch (Reed, 2010).
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Interestingly this category of agnosia causes unilateral lesions in the brain. As a result, the hand that is contralateral to the lesion is the hand that is "agnostic". For example, an individual with a right hemisphere stroke causing agnosia would have difficulty with recognizing objects with his or her left hand.
Several classifications of tactile agnosia have been identified in the literature. However, the understanding of these classifications is not well understood, especially when compared against other conditions. However, the literature covers apperceptive and associative tactile agnosias. Apperceptive tactile agnosia refers to the ability to distinguish tactile characteristics (i.e. temperature, texture, size) but difficulty with making an association with the tactile characteristics and a perceptual representation of an object. For example, an individual with apperceptive agnosia, when feeling a shoe, would be able to describe its hard sole and skinny laces but would not be able to put the descriptions together and recognize that the object is a shoe.
Associative tactile agnosia, on the other hand, refers to deficits that occur when individuals cannot use the integrated tactile percepts to access their knowledge or an object (Reed, 2010). In other words, an individual with associative agnosia cannot verbally describe the characteristics of an object such as its texture or shape. However, the ability to draw objects that are tactually presented is in tact but despite this capability, individuals are unable to recognize the objects, determine if two objects felt are from the same category, or whether both objects perform the same function (Reed, 2010). Like the other categories of agnosias, this lack of recognition is due to disrupted pathways between incoming perceptual information and stored knowledge of the object.
Several classifications of tactile agnosia have been identified in the literature. However, the understanding of these classifications is not well understood, especially when compared against other conditions. However, the literature covers apperceptive and associative tactile agnosias. Apperceptive tactile agnosia refers to the ability to distinguish tactile characteristics (i.e. temperature, texture, size) but difficulty with making an association with the tactile characteristics and a perceptual representation of an object. For example, an individual with apperceptive agnosia, when feeling a shoe, would be able to describe its hard sole and skinny laces but would not be able to put the descriptions together and recognize that the object is a shoe.
Associative tactile agnosia, on the other hand, refers to deficits that occur when individuals cannot use the integrated tactile percepts to access their knowledge or an object (Reed, 2010). In other words, an individual with associative agnosia cannot verbally describe the characteristics of an object such as its texture or shape. However, the ability to draw objects that are tactually presented is in tact but despite this capability, individuals are unable to recognize the objects, determine if two objects felt are from the same category, or whether both objects perform the same function (Reed, 2010). Like the other categories of agnosias, this lack of recognition is due to disrupted pathways between incoming perceptual information and stored knowledge of the object.
literature on agnosia
As mentioned above, agnosia tends to only affect one sensory modality. Individuals with tactile agnosia can still identify objects with their eyes and ears. Hence, tactile agnosia and auditory agnosias are less often identified and discussed in the literature than visual agnosia. In particular, tactile agnosia has received the least attention and is overall poorly understood compared to visual and auditory agnosias. A possible reason why is because audition and touch is often not needed in the context of normal vision for object identification (Coslett, 2011). This is true if we consider the fact that we rarely identify an object on the basis of sound or touch alone. This was evident when I was searching for literature and multimedia on tactile and auditory agnosias. The amount of material on visual agnosia significantly overweighs the other two categories of agnosias. Nevertheless, another possible reason why tactile and auditory agnosias are less prevalent could be because they are underreported in practice. To read more on the different categories and types of agnosias, please refer to the Resources page.