Definition and basic characteristics
The term agnosia comes from the Greek word gnosis, meaning «knowledge». Accordingly, the prefix «a-» denotes the loss or absence of this knowledge. In normal physiology and neurology, this condition refers to a specific impairment of higher nervous activity.
The main clinical feature of agnosia is that the patient's basic analyzers are undamaged. Elementary sensation remains completely intact. The person is fully conscious; they see, hear, and feel touch, yet the brain loses the ability to synthesize these signals into a unified percept and compare them with past experience. In other words, there is a disruption in the very process of recognizing previously familiar objects or phenomena.
Main types of agnosia and their localization
Depending on which area of the cerebral cortex is damaged, several main types of pathology are distinguished. Each form has its strict topical localization and characteristic clinical picture.
- Visual agnosia
- Localization: Occipital cortex.
- Clinical picture: The patient physically sees surrounding objects and can navigate around obstacles (as elementary vision is unimpaired), but completely fails to recognize them or identify what is in front of them.
- Auditory agnosia
- Localization: Temporal cortex.
- Clinical picture: The person hears various sounds but loses the ability to associate them with a specific source (or the object making the sound). This form also involves a loss of the ability to perceive and decode the meaning of speech from an interlocutor, leading to sensory aphasia.
- Tactile agnosia
- Localization: Superior parietal cortex.
- Clinical picture: The patient retains completely basic touch sensation. However, if given an object and asked to identify it solely by touch (with eyes closed), they are unable to do so, as the ability to recognize objects through tactile exploration is lost.
Systemic mechanism of the pathology
From the perspective of normal physiology, the development of agnosia is explained through the theory of functional systems. When specific cortical areas are damaged (visual in the occipital lobe, auditory in the temporal lobe, or tactile in the parietal lobe), the complex physiological information processing algorithm breaks down.
Normally, the brain constantly predicts results and evaluates incoming signals. In agnosia, the previously established mechanism of evaluating action results is severely impaired. This happens because a crucial link in the functional system—the process of feedback (reverse afferentation)—suffers. Information coming from sensory organs can no longer correctly undergo the stage of comparison with the action result acceptor, making image synthesis and object recognition impossible.
Briefly: answers to topical questions
What are the specific types of visual agnosia?
Specific types of visual agnosia include impairments in recognizing various visual stimuli with preserved visual acuity.
Depending on the nature of the impairment, the following types are distinguished:
- Object visual agnosia — inability to recognize well-known objects or their images.
- Facial agnosia (prosopagnosia, from Greek prosopon — face) — impairment in recognizing the faces of familiar people in person or in photographs.
- Alexia — loss of the ability to read (recognize text).
- Opticospatial agnosia — impairment in perceiving spatial characteristics, including topographical agnosia (disorientation in familiar places).
- Left-sided spatial visual agnosia — neglect of the left half of space.
What medical term denotes the inability to recognize objects by touch with eyes closed?
The inability to recognize objects by touch with eyes closed is denoted by the medical terms astereognosis (astereognostia) or tactile agnosia.
In this condition, the patient retains all types of elementary sensation and the feeling of touch itself, but completely loses the ability to identify familiar objects by feeling them with the hand. For clinical evaluation (testing stereognosis), a familiar object is placed in the patient's hand with eyes closed, and they are asked to recognize it by touch.
The V-A-T localization rule: Vision — Occipital lobe (Visual), Auditory — Temporal lobe, Tactile — Parietal lobe (upper part).
Class and exam questions
Is a person blind in visual agnosia?
No, vision as an elementary sensation is preserved. The person sees objects and can navigate around obstacles, but the brain is unable to identify and recognize what is seen.
Why is speech comprehension impaired in auditory agnosia?
When temporal cortical areas are damaged, the ability to associate sounds with their semantic meaning is lost. The person hears a voice but does not comprehend the meaning of spoken speech, which is termed sensory aphasia.
What is impaired in the functional system in agnosias?
The mechanism of evaluating action results is disrupted. A malfunction occurs in reverse afferentation and in the process of comparing the received information with the action result acceptor.
What else to study on this topic
- Mechanism of sensory aphasia development in temporal lobe lesions
- The role of the action result acceptor in the theory of functional systems
- The significance of reverse afferentation in higher nervous activity processes
- Physiology of the superior parietal cortex and processing of tactile information
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