Barognosia: What Is It And How Does This Ability Work?
The human body is a complex machine. Often performs seemingly simple tasks; but they require the coordination of a very precise mechanism of organs, muscles, nerves and ligaments.
What has been outlined is particularly relevant in the process that mediates from the perception of a stimulus (in any sensory modality) until its arrival at the port of the brain where it must be processed.
In this article we will address one of these phenomena: barognosia. The truth is that we all use it in our daily activities, but… did you know it?
The term barognosia can be dissected for a more precise etymological analysis, which offers information about its origin and meaning. The elements that make it up, “baro” and “gnosia”, come from classical Greek. The first of them refers to weight or pressure (hence the word barometer for the measurement of this atmospheric condition) and the second refers to the ability to recognize an aspect of reality after receiving it through the senses.
So that, Barognosia is the ability that people have to recognize the weight of the objects with which we interact on a daily basis, with total independence of other visual or acoustic parameters that could suggest such a property of these. This capacity, included within the functions of sensory recognition, would be used when an object is held with one hand and its weight is calculated in contrast to a different one.
In fact, this is one of the essential mechanisms through which the preservation of barognosia is explored in any neurological examination, something very common, since it is a function that can be affected as a consequence of certain pathologies of the central nervous system. or the peripheral. More specifically, objects of similar morphology (balls, for example) are placed in each of the two hands, in order to estimate which of them has greater or lesser relative weight (varying the magnitude in this variable). to improve the precision of the measurement).
It is a capacity included within the general category of stereognosias, a modality of perceptual recognition that is related to the sense of touch (haptics), and from which the physical properties of objects are extracted without having to resort to the rest of the senses. Although aspects such as texture or temperature can be included here, with which a more precise knowledge of the stimulus with which one interacts is obtained, for barognosia alludes to deep sensitivity, ignoring receptors of another nature (superficial).
Barognosia is possible thanks to a group of mechanoreceptors located in the skin, periosteum, joint capsule, ligaments and menisci; called Pacinian corpuscles.
At an anatomical level they can be described as nerve endings that arise from the cell axon and whose end takes on the appearance of an encapsulated cone, which perimeters the central space. This cone is formed by the succession of about 40-80 sheets, made up of connective tissue cells and separated by collagen. These are receptors sensitive to vibration and tissue deformation.
It is precisely this same capsule, which deforms when pressure is exerted on the tissues, that informs the nervous system of the magnitude of such external force so that it can be interpreted in terms of weight. Since its distribution is concentrated in the deep layers of the skin (dermis), as well as in the hypodermis of the fingers and palms of the hands (glabrous areas), these are usually the parts of the body most frequently used for value the preservation of barognosia.
The detection of alterations in barognosia has an essential clinical component, since these are discrimination tasks that only the subject can report and for which objective tests are lacking. However, once this symptom is detected, attention should focus on a thorough examination of the skin tissues and the central and peripheral nervous system, with the aim of identifying an organic cause that is susceptible to intervention.
How is barognosia evaluated?
Tests to evaluate barognosia are included in the neurological examination protocol for tactile discrimination, along with a variety of procedures aimed at assessing deep sensitivity. In all cases the person is asked to close their eyes, so that the interference of any adjuvant perception is minimized. After this, you are asked to recognize the object that has been left on your hand, the intensity of its vibration or the location of different points when they are stimulated by pressure.
In the particular case of the evaluation of barognosia, the evaluee is urged to remain in a seated posture while placing the palms of his hands on the thighs (without contact) and positioned upward. Two pieces with different weights, whose size and shape are comparable, are placed on them and they are asked to discriminate which of them has a greater weight. The examination begins with a significant dissonance, and the parameter is progressively adjusted until a differential threshold is detected.
The differential threshold refers to the minimum weight, expressed in grams, from which one is able to recognize a difference between two objects. Thus, it is not always a function whose commitment is absolute, but sometimes there are different degrees within a spectrum that ranges from normality to complete impairment. It is informative to explore the lower and upper limits, so the totality of possible alternatives at both extremities must be considered.
It may be useful to additionally explore sensitivity, stereognosia (recognition of objects such as coins, balls or dice), tactile pressure (not painful), graphesthesia (recognition of signs drawn on the palm of the hand), textures, kinesthesia (movement), proprioception (location of the body with respect to the space that surrounds it) and vibration; since all of them depend (like weight discrimination) on the dorsal lemniscal-medial pathway.
If the subject is able to respond appropriately, his barognosia is considered perfectly preserved. Otherwise, the presence of a problem can be inferred at any of the points through which the sensitive information passes from the receptors to the brain areas in which it is processed. Detection of the cause is essential to develop the treatment and differential diagnosis of any underlying pathology.
In the event that the examination detects an alteration limited to deep sensitivity and with preservation of the superficial, tabes dorsalis may be suspected. It would be a degeneration of the dorsal cords of the spinal cord, which would disturb the sensory discriminative ability while maintaining the action of the nociceptors (widely distributed in the skin) and thermoreceptors (Ruffini corpuscles).
How is barognosia expressed when it is compromised?
Barognosia is a specific form of recognition that requires the participation of the Pacinian corpuscles and many other structures located in the nervous system.
When an object is taken by hand, The aforementioned cells send information to the spinal cord through sensory nerves, in afferent descent until reaching the medulla oblongata and finally to the thalamus (from which a large part of the sensory information is managed). It is from this point that the signal can be processed and interpreted, all mediated by the parietal cortex (which integrates the sensation into a perception).
Any point along the path can be altered by different unexpected circumstances, which would produce abarognosia. This is expressed as a severe difficulty in estimating, recognizing and discriminating the weight of objects located in areas of the body susceptible to assessment. It very frequently occurs as a result of a lesion in the parietal cortex, whose symptoms manifest at a contralateral level (if it is in the right hemisphere, the left hand would be affected, for example).
There is evidence that lesions in the postcentral gyrus are those most frequently detected in people with abarognosia which also present with tactile/haptic agnosia (inability to recognize textures and shapes), agraphesthesia (inability to interpret numbers or letters that are drawn on the affected surface) and atopognosia (difficulty in recognizing contact with one or more stimuli on the skin).
When abarognosia is present without apparent damage to the tissues, joints or bones (burn, fracture, etc.), it is necessary to have diagnostic procedures among which neuroimaging techniques play an essential role (especially structural). Through these, the initial examination carried out by the neurology doctor can be complemented, determining the state of the spinal cord and the brain regions potentially involved.
