SENSIBILITY. - The concept of sensibility is not univocally established. It is commonly said that a subject has sensibility for certain problems or aspects of life, meaning by this a special receptivity and fineness of inner elaboration, or an innate predisposition to understand, in mediated or immediate form, certain forms of the real. The expression is thus used in analogy to the much more precise, psychophysiological meaning of "capacity to perceive determined sensory stimuli"; in this latter sense one speaks of visual, auditory, tactile sensibility, etc., to indicate the capacity of a sensory stimulus, quantitatively variable, to reach the threshold of consciousness, a capacity that is exactly measurable.
One will say, for example, that the maximum limit of man's auditory sensibility, that is, of the capacity to perceive sound vibrations as an acoustic sensation, is lower than that of the dog, because man no longer perceives as an acoustic sensation vibrations with a frequency higher than 20,000 per second, whereas the dog is still able to perceive acoustically vibrations that reach frequencies of 38,000.
On this concept of sensibility one of the most interesting and fertile chapters of psychophysiology has been built. It is well to remember that sensibility, thus understood, must be kept distinct from "excitability," proper to living forms in general, unicellular ones included, which constitutes a vast and complex chapter of general biology.
One can speak of sensibility only where there exist differentiated nervous structures that specifically underlie this function; the more integrated and differentiated the anatomical structures, the more varied and complex the forms of sensibility.
It is generally customary to distinguish, from a neurobiological and clinical point of view, protopathic sensibility (Head) from epicritic sensibility, according to the greater or lesser intellectual content of the various forms of sensibility. These concepts are biologically very important, being linked to the meaning of "consciousness" as "vigilance" (v. SENSAZIONE).
The disturbances of sensibility, very important from the standpoint of neurological semeiotics, concern the subjective and objective forms of sensibility. Subjectively: paresthesias (tingling, numbness, tickling, sensation of "dead flesh," etc.), causalgias (pains of a burning type, produced by irritation of the sympathetic nerve network that surrounds the nerve and the satellite blood vessels), pains (e.g., "neuralgias"). Objectively one will distinguish: disturbances of tactile, pain, thermal sensibility, of tactile localization, of tactile discrimination, of sensibility to pressure or weight (baresthesia), of pain sensibility from exaggerated pressure, of vibratory sensibility (pallesthesia), of that of pressure localization, of the recognition of the position of the different segments of a limb, of a limb in toto, and of passive movements (bathyesthesia, arthrokinetic sensibility). Generally considered separately are the disturbances of stereognosis, that is, of the capacity to recognize the shape, consistency, and volume of an object; this is in fact a synthetic process of the various forms of elementary sensibility.
In clinical practice one observes - as a consequence of the organization into anatomo-topographic structures of the various pathways of sensibility - the so-called dissociations of sensibility: of peripheral type (deep sensibilities are preserved and superficial ones abolished); of tabetic type (thermo-algesic, baresthesia and superficial tactile sensibility intact, pallesthesia, bathyesthesia, tactile discrimination abolished); of syringomyelic type (roughly, the opposite picture of the preceding form occurs).
The pathways of sensibility arrive in the optic thalamus (posterior part of the external nucleus), from which the sensory tracts that go to the cerebral cortex depart. The function of the thalamus is very important: it seems in fact that to it belongs the elaboration of the emotional, affective aspect of every form of sensibility, leaving to the cortex the more delicate task of discriminating, identifying, and evaluating the various stimuli. It is evident that in this regard there must not exist an equal behavior for all forms of sensibility; pain impulses, which are those most charged with emotional content, are for the most part elaborated intrathalamically; for touch, on the other hand, the emotional content is far scantier and therefore the thalamo-cortical current is much richer. It seems that this anatomo-functional organization is the true basis of protopathic and epicritic sensibility, and on this basis the protopathic aspect of sensibility presents intimate traits of union with the so-called cenesthesia (v.). Thalamic lesions can determine very intense, paroxysmal, persistent, intolerable pains, uninfluenced by any curative means; they are to be considered, perhaps, as symptoms of thalamic "release" from the inhibitory influence of the cortex.
From what has been said, one can understand how there may exist profound alterations of the various sensibilities, without the psychic state having to suffer thereby in its integrity, and, vice versa, how there may exist profound psychic alterations, without any, or very slight, impairment of the sensibilities.
Bibliography3 works
Bruno Callieri