For Interoception we will employ contactless acoustic transducers to stimulate mechanoreceptors from the skin,chest and abdomen and induce the perception of movements from the heart and the stomach, respectively. A different strategy will be employed for the different organs; whilst ultrasounds will be used for the stomach, we plan to use the low bass frequency for the heart and skin pressure for the insular cortex

Stomach: Ultrasounds represent sound waves with frequencies (> 20kHz) higher than the upper audible limit of human hearing. They are commonly used in medicine (i.e., sonography of foetus) being totally free from side effects to human health. However, the devices based on ultrasonic technology developed for medical applications are basically used for imaging of visceral anatomy. Recently, Marzo and coworkers (2015, Nature Communication) have suggested the usage of ultrasonic transducers 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…the device could be applied directly onto the skin with the manipulation taking place inside the body; similar to an ultrasound scanner but for manipulating particles (that is, drug capsules, kidney stones or micro-surgical instruments)” (see also Kang et al., 2010; Hong et al. 201Oa1). Based on these findings, our first aim is to optimize an ultrasonic transducer array (working from 40 to 60 kHz) and show that acoustic levitation can be employed to translate the particles of the eaten food with a consequent motion of the stomach walls to ultimately perceive movements from the stomach. 

Heart: Our second aim is to adopt the low bass frequency (from 50 to 120 Hz) to stimulate mechanoreceptors from the chest to ultimately experience the own heart beat sensation. Low bass sounds (20-200Hz) are prevalent in living and working environments, and in spite of their low audibility, low-frequency noise often causes a person to experience a vibratory sensation (Takahashi et al., 1999). 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 reported that pure tones with sound pressure levels of 100 dB enable the perception of chest vibration (Takahashi, 2011; Schust, 2003). Whilst the exposure to ultrasonic waves does not present side effects for internal human organs, bass sounds (although present in our environments) may have side effects for health. In particular, the exposure to sound in excess of 120 dB may lead to hearing loss. A further side effect on health is represented by possible body’s resonant frequencies; indeed, the exposure of an organ to its own resonant frequency may cause irreversible damages. It is noteworthy however that the body’s resonant frequencies are in the range of the infrasounds (1-20Hz) while the frequencies (from 50 to 120Hz) used for induce sensation of “chest slam”, well above the body’s resonance. 

Insular Cortex: Our final aim is to target directly the anterior insular cortex, which receives information through a vast network of small un-myelinated fibers connected to the Lamina I spinothalamocortical pathway. These specific fibers, called C-fibers, compose a poly-modal afferent system that innervates the entire organism and report a wide range of inputs such as: hunger, thirst, pain, itch, temperature, muscle contraction, hormonal and immune activity, and cardiorespiratory function, along with a specific type of tactile perception called C-tactile (CT). Parasympathetic interoceptive inputs have been recently discovered as a promising research field, and among these kinds of stimuli, CT (or affective touch) is arguably the most interesting one. CT afferent fibers constitute a secondary touch system with a deep involvement in different psycho-physiological pathways.