An actual or apparent exaltation of the perceptive faculties, or superacuity of the normal senses, characteristic of the hypnotic state. It has been observed frequently in hysterics. They may feel a piece of wire on their hands as heavy as a bar of iron. The smallest suggestion—whether given by word, look, gesture, or even breathing or unconscious movement—is instantly seized upon and interpreted by the entranced subject, who for this reason is often called "sensitive."
The phenomenon of hyperesthesia, observed but wrongly interpreted by the early magnetists and mesmerists, was largely responsible for the so-called clairvoyance, thought reading, community of sensation, and other kindred phenomena. In its manifestation, hyperesthesia is often difficult to distinguish from telepathy or clairvoyance. Theoretically the dividing line is that hyperesthesia is a peripheral perception. Telepathy or clairvoyance is a central perception that does not reach us through the sensory organs. In practice it is difficult to decide whether the perception takes place through the sensory organs or not.
The realization of a relationship between suggestion and hyperesthesia by Alexandre Bertrand and James Braid brought hypnotism into the domain of scientific fact. The significance of hyperesthesia in connection with every form of psychic phenomena can hardly be overestimated. Nor is it found only in the trance state. It enters into the normal existence to an extent that is but imperfectly understood. Dreams, for instance, frequently reproduce impressions that have been recorded in some obscure stratum of consciousness, while much that we call intuition is made up of inferences subconsciously drawn from indications too subtle to reach the normal consciousness.
Hyperesthesia has been defined as "an actual or apparent exaltation of the preceptive faculties," modern scientists being unsure whether the senses are actually sharpened or not. Most probably the hyperesthetic perception is merely a normal perception that, through cerebral dissociation, operates in a free field. Very slight sense impressions may be recorded in the brain during normal consciousness but other impressions may inhibit them from reaching the conscious mind.
Gilbert Murray conducted telepathic experiments by placing himself in a different room from the sensitive and having a sentence spoken to him in a very low voice. The sensitive in the other room reproduced the sentence. The British Society for Psychical Research considered this a case of telepathy. Charles Richet considered it exceptional auditory hyperesthesia. Similarly, the sudden movements that save people from falling objects in the street may be attributed to subconscious hearing of an almost inaudible sound that generates and sends an urgent impulse to the motor centers.
Emile Boirac recorded interesting cases of tactile and visual hyperesthesia. His subject read with his fingertips in complete darkness. Being bandaged, his back turned to Boirac, but holding his elbow, he could also read if Boirac passed his own fingertips along the lines of a newspaper. It did not make the least difference if Boirac closed his eyes. (See also Eyeless Sight. ) Another subject could tell the time from a watch wrapped up in a handkerchief. A Mme. M., before the Medical Society of Tamboff, told the colors of 30 flasks wrapped in paper and placed under a thick cloth. Further complicating the issue, Mme. M. could also taste by the sense of touch.
James Braid found the olfactory sense so acute in some hypnotic patients that by the smell of a glove they could unhesitatingly and unerringly detect its owner in a large company. It is questionable whether auditory hyperesthesia could explain the astounding phonic imitations he observed, such as patients repeating accurately what was spoken in any language, or singing correctly in a language they had never heard before. Braid noted,
"A patient of mine who, when awake, knew not the grammar even of her own language, and who had very little knowledge of music, was enabled to follow Mlle. Jenny Lind correctly in songs in different languages, giving both words and music so correctly and simultaneously with Jenny Lind, that two parties in the room could not for some time imagine that there were two voices, so perfectly did they accord, both in musical tone and vocal pronunciation of Swiss, German, and Italian songs."