SENSIBILITÀ. — The concept of s. is not univocally established. It is commonly said that a subject has s. for certain problems or aspects of life, by which is meant a special receptivity and subtlety of inner elaboration, or an innate predisposition to understand, in mediated or immediate form, certain forms of reality. The expression is thus used by analogy with the much more precise psychophysiological meaning of “capacity to perceive certain sensory stimuli”; in the latter sense one speaks of visual, auditory, tactile s., etc., to indicate the capacity of a quantitatively variable sensory stimulus to reach the threshold of consciousness, a capacity that can be exactly measured.
One will say, for example, that the upper limit of man’s auditory s., that is, his capacity to perceive sound vibrations as an auditory sensation, is lower than that of the dog, because man no longer perceives as an auditory sensation vibrations with a frequency higher than 20,000 per second, whereas the dog can still perceive acoustically vibrations reaching frequencies of 38,000.
One of the most interesting and fruitful chapters of psychophysiology has been constructed upon this concept of s. It should be remembered that s., understood in this way, must be distinguished from “excitability,” which is proper to living forms in general, including unicellular organisms, and constitutes a broad and complex chapter of general biology.
One may speak of s. only where differentiated nervous structures exist that specifically underlie this function; the more highly integrated and differentiated the anatomical structures, the more varied and complex the forms of s.
From a neurobiological and clinical point of view, it is customary to distinguish generically between protopathic s. (Head) and epicritic s., according to the greater or lesser intellectual content of the various forms of s. These concepts are biologically very important, being connected with the meaning of “consciousness” as “vigilance” (v. SENSAZIONE).
Disorders of s., of the utmost importance from the standpoint of neurological semiotics, concern subjective and objective forms of s. Subjectively: paresthesias (tingling, numbness, tickling, the sensation of “dead flesh,” etc.), causalgias (burning-type pains produced by irritation of the sympathetic nervous network surrounding the nerve and its satellite blood vessels), and pains (e.g., “neuralgias”). Objectively, one distinguishes disorders of tactile, painful, and thermal s.; of tactile localization; of tactile discrimination; of s. to pressure or weight (baresthesia); of painful s. due to excessive pressure; of vibratory s. (pallesthesia); of the localization of pressure; of recognition of the position of the various segments of a limb, of a limb as a whole, and of passive movements (batiesthesia, arthrokinetic s.). Disorders of stereognosis, that is, the capacity to recognize the shape, consistency, and volume of an object, are generally considered separately; this is in fact a synthetic process involving the various forms of elementary s.
In clinical practice one observes—in consequence of the organization of the various pathways of s. into anatomotopographical structures—the so-called dissociations of s.: of the peripheral type (deep s. are preserved and superficial s. abolished); of the tabetic type (thermoalgesic, baresthetic, and superficial tactile s. intact, pallesthesia, batiesthesia, and tactile discrimination abolished); and of the syringomyelic type (the picture roughly opposite to that of the preceding form occurs).
The pathways of s. reach the optic thalamus (posterior part of the external nucleus), from which arise the sensory tracts proceeding to the cerebral cortex. The function of the thalamus is of the greatest importance: it appears in fact to be responsible for elaborating the emotional and affective aspect of every form of s., leaving to the cortex the more delicate task of discriminating, identifying, and evaluating the various stimuli. It is evident that there cannot be an identical pattern in this regard for all forms of s.; painful impulses, which are those most heavily charged with emotional content, are for the most part elaborated intrathalamically; for touch, on the other hand, the emotional content is much slighter and therefore the thalamocortical current is much richer. This anatomofunctional organization appears to be the true basis of protopathic and epicritic s., and on this basis the protopathic aspect of s. presents intimate points of connection with so-called cenesthesis (v.). Thalamic lesions can cause extremely intense, paroxysmal, persistent, and intolerable pains, unaffected by any therapeutic measure; these are perhaps to be regarded as symptoms of thalamic “release” from the inhibitory influence of the cortex.
From what has been said, it can be understood how profound alterations of the various s. may exist without the psychic state thereby suffering any impairment of its integrity, and, conversely, how profound psychic alterations may exist without any, or only very slight, impairment of s.