Table 1.

Sonoception: rationale and technology.

Inner body sensory system	Body site	Technology	Proposed approach
Interoception	Stomach	Ultrasound	Ultrasound waves (>20 KHz)—frequencies higher than the upper audible limit of human hearing—are often used in medicine (i.e., sonography of fetus) as totally free from side effects for human health. The ultrasonic technological devices developed for medical applications are basically used for imaging visceral anatomy. However, in recent research (Marzo et al., 2015), usage of ultrasonic transducers has been suggested as a new methodology that “can exert radiation forces and form acoustic traps at points where these forces converge permitting the levitation of particles of a wide range of materials and sizes through air, water or biological tissues” (p. 2). In this vein, holographic acoustic elements could be employed to translate the particles of food eaten with consequent motion of the stomach walls (Kang and Yeh, 2010; Hong et al., 2011).
Interoception	Heart	Low bass frequency	Bass sounds (50–120 Hz) are also prevalent in living and working environments and, despite its low audibility, low frequency noise often causes a person to experience a vibratory sensation. One of the most prominent effects of high-level low frequency sound is the so-called “chest slam”, i.e., the sensation that the chest is resonating. Studies report that pure tones with sound pressure levels of 100 dB enable the perception of chest vibration (Schust, 2004; Takahashi, 2011).
Proprioception	Muscles	Vibrotactile transducers	Cutaneous receptors in the skin around fingers, elbows, ankles and knee joints provide exteroceptive and proprioceptive information. Similar to muscle spindles, these receptors encode both movement kinematics and show directional sensitivity (Lee et al., 2013). When a vibration of approximately 70–100 Hz is applied to a tendon of the biceps or triceps muscle of a physically immobile limb obstructed from view, a sensation of arm displacement is generated (Naito et al., 1999). Notably, increasing the vibration frequency increases the velocity of the perceived illusory movement (Roll and Vedel, 1982). When the vibratory stimulation is interrupted, the spindle discharge decreases, inducing the perception that the limb is returning towards its original position.
Vestibular input	Otolith organs	Vibrotactile trasnducers	The otoliths (the utricular and saccular maculae) are the gravity sensing organs of the inner ears. Air-conducted sounds and bone-conducted vibration have been proposed as two effective methods to evoke vestibular myogenic potentials originating from selective activation of the otolithic end organs (Manzari et al., 2010). Bone-conduced vibration at frequency of 500 Hz produces consistent craniocentric whole-body responses in standing subjects (Welgampola and Day, 2006; Curthoys and Grant, 2015). The characteristics of the response are compatible with mediation by vestibular input, although the sway direction is different from that evoked by galvanic vestibular stimulation. This suggests that different patterns of input are produced by the two types of stimulation, possibly involving different proportions of afferents from the otoliths and semicircular canals. If so, bone-conducted sound, used either in isolation or combination with galvanic vestibular stimulation, may enable investigation of hitherto unexplored aspects of vestibular function in intact freely behaving human subjects.