Data-Driven Playback of Natural Tactile Texture Via Broadband Friction Modulation

Tasty vibes: Uncovering crossmodal correspondences between tactile vibrations and basic tastes

The interest in crossmodal correspondences individually involving the senses of touch and taste has grown rapidly in the last few decades. Several correspondences involving different tactile dimensions (e.g., hardness/softness, roughness/smoothness) have been uncovered, such as those between sweetness and softness and between roughness and sourness. However, a dimension that has been long overlooked, despite its pervasiveness and importance in everyday experiences, relates to tactile vibrations. The present study aimed to fill this gap and investigate crossmodal correspondences between basic tastes and vibrations. In the present study (N = 72), we uncovered these associations by having participants sample basic taste (i.e., sweet, salty, sour, bitter, umami) aqueous solutions and chose the frequency of vibrations, delivered via a consumer-grade subwoofer wristband on their dominant hand, that they most strongly associated with each taste. We found that sourness was most strongly associated with frequencies around 98 Hz, and that sweetness and umami were associated with frequencies around 77 Hz. These correspondences may, to different extents, be based on affective and semantic mechanisms. The findings have relevant implications for theoretical research on multisensory integration and perception and the potential future applications of these associations, through wearable technologies, to enhance eating experiences and promote healthier eating habits.

The neural basis of tactile texture perception

Running our fingers across a textured surface gives rise to two types of skin deformations, each transduced by different tactile nerve fibers. Coarse features produce large-scale skin deformations whose spatial configuration is reflected in the spatial pattern of activation of some tactile fibers. Scanning a finely textured surface elicits vibrations in the skin, which in turn evoked temporally patterned responses in other fibers. These two neural codes-spatial and temporal-drive a spectrum of neural response properties in somatosensory cortex: At one extreme, neurons are sensitive to spatial patterns and encode coarse features; at the other extreme, neurons are sensitive to vibrations and encode fine features. While the texture responses of nerve fibers are dependent on scanning speed, those of cortical neurons are less so, giving rise to a speed invariant texture percept. Neurons in high-level somatosensory cortices combine information about texture with information about task variables.

The spatial profile of skin indentation shapes tactile perception across stimulus frequencies

Multiple human sensory systems exhibit sensitivity to spatial and temporal variations of physical stimuli. Vision has evolved to offer high spatial acuity with limited temporal sensitivity, while audition has developed complementary characteristics. Neural coding in touch has been believed to transition from a spatial to a temporal domain in relation to surface scale, such that coarse features (e.g., a braille cell or corduroy texture) are coded as spatially distributed signals, while fine textures (e.g., fine-grit sandpaper) are encoded by temporal variation. However, the interplay between the two domains is not well understood. We studied tactile encoding with a custom-designed pin array apparatus capable of deforming the fingerpad at 5 to 80 Hz in each of 14 individual locations spaced 2.5 mm apart. Spatial variation of skin indentation was controlled by moving each of the pins at the same frequency and amplitude, but with phase delays distributed across the array. Results indicate that such stimuli enable rendering of shape features at actuation frequencies up to 20 Hz. Even at frequencies > 20 Hz, however, spatial variation of skin indentation continues to play a vital role. In particular, perceived roughness is affected by spatial variation within the fingerpad even at 80 Hz. We provide evidence that perceived roughness is encoded via a summary measure of skin displacement. Relative displacements in neighboring pins of less than 10 µm generate skin stretch, which regulates the roughness percept.