Barognosia: what is it and how does it work?

This brain capacity allows us to know approximately the weight of objects.

The human body is a complex machine. It often performs seemingly simple tasks, but they require the coordination of a very precise interplay of organs, muscles, nerves and ligaments.

This is particularly relevant in the process that mediates from the perception of a stimulus (in any sensory modality) to its arrival at the port of the brain where it is to be processed.

In the present article we will deal with one of these phenomena: barognosia.. The truth is that we all make use of it in our daily activities, but... did you know about it?

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What is barognosia?

The term barognosia can be dissected for a more precise etymological analysis to provide information on its origin and meaning. The elements that form it, "baro" and "gnosia", come from classical Greek. The former refers to weight or pressure (hence the word barometer for the measurement of this atmospheric condition) and the latter alludes to the ability to recognize an aspect of reality after its reception through the senses.

Thus, barognosia is the ability of people to recognize the weight of the objects we interact with on a daily basis, completely independent of other visual parameters.This ability is independent of any other visual or acoustic parameters that might suggest such a property of these objects. This ability, included within the functions of sensitive recognition, would be used when holding an object with one hand and calculating its weight 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 recognition, something very common, since it is a function that can be affected as a consequence of certain pathologies of the central or peripheral nervous system. 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 accuracy of the measurement).

It is a capacity included within the general category of sterognosias, 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 the need 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 can be obtained, barognosia refers to deep sensitivity, ignoring receptors of a different nature (superficial).

How does barognosia occur?

Barognosia is made possible by a group of mechanoreceptors located in the skin, periosteum, joint capsule, ligaments and menisci; called Pacini's corpuscles.

At the anatomical level they can be described as nerve endings arising from the cellular 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 lamellae, formed by 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 this 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 to assess the preservation of barognosia.

The detection of alterations in barognosia has an essential clinical component, as these are discrimination tasks that can only be reported by the subject and for which there is a lack of objective evidence. However, once this symptom has been detected, attention should be focused 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 amenable to intervention.

How is barognosia evaluated?

Tests to assess 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 his or her eyes, so that interference from any adjuvant perception is minimized. After that, he/she is asked to recognize which object has been left on his/her 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 examinee is urged to remain in a seated position while placing the palms of his hands on his thighs (without contact) and positioned upwards. Two pieces with different weights, whose size and shape are comparable, are placed on them and the examinee is asked to discriminate which of them has a higher weight. The scan starts 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, at which a difference between two objects can be recognized.. Thus, it is not always a function whose compromise is absolute, but sometimes there are different degrees within a spectrum ranging from normal to complete impairment. It is informative to explore the lower and upper limits, so the totality of possible alternatives in both extremities should be considered.

It may be useful to additionally explore sensitivity, stereognosia (recognition of objects such as coins, balls or dice), tactile pressure (non-painful), graphesthesia (recognition of signs traced on the palm of the hand), textures, kinesthesia (movement), proprioception (location of the body in relation to the surrounding space) and vibration, all of which depend (like weight discrimination) on the dorsal lemniscal-medial pathway.

If the subject is able to respond adequately, it is considered that his barognosia is perfectly preserved. Otherwise, the presence of a problem can be inferred in any of the points through which the sensitive information passes from the receptors to the brain areas where it is processed. The detection of the cause is essential to elaborate the treatment and the differential diagnosis of any underlying pathology.

In the case that the examination detects an alteration limited to deep sensitivity and with preservation of superficial sensitivity, tabes dorsalis may be suspected. This would be a degeneration of the dorsal cords of the spinal cord, which would disturb the sensory discriminative ability while maintaining the action of nociceptors (widely distributed in the skin) and thermoreceptors (Ruffini corpuscles).

How is barognosia expressed when it is compromised?

Barognosia is a concrete form of recognition that requires the participation of Pacini's corpuscles and many other structures located in the nervous system.

When an object is grasped by hand, the aforementioned cells send the information to the spinal cord through sensory nerves, in afferent ascension until it reaches 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 pathway can be altered by different circumstances, resulting in 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 valuation. It most frequently occurs as a result of a lesion in the parietal cortex, whose clinical manifestation is manifested 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 the most frequently detected lesions in people with abarognosia, and that they also present with a contralateral level.These also include tactile/haptic agnosia (inability to recognize textures and shapes), agraphesthesia (inability to interpret numbers or letters traced 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 tissues, joints or bones (burn, fracture, etc.), diagnostic procedures are necessary, among which neuroimaging techniques play an essential role (especially structural). Through these, it will be possible to complement the initial exploration carried out by the neurologist, determining the state of the spinal cord and the potentially involved brain regions.

Bibliographic references:

• Biber, U.Visual Stability and the Motion Aftereffect: A Psychophysical Study Revealing Spatial Updating. PLoS One, 6(1), e16265.
• Cássia, M., Meireles, V., Lalone, E., Claro, N., Barbosa, R., Marcolino, A.M. ... Pessoni, F. (2018). Functional, motor, and sensory assessment instruments upon nerve repair in adult hands: systematic review of psychometric properties. Systematic Reviews, 7, 175-186.