Our sixth sense: Humans locate the source of ground vibrations with their feet.

Our sixth sense: Humans locate the source of ground vibrations with their feet. Nicolas Grimault1,4, Eli Kabriti1, Nicolas Mathevon2,3,5 and Fabien Perrin1,5 1Equipe Cognition Auditive et Psychoacoustique, CRNL, University Lyon 1, CNRS, Inserm, Lyon, France. 2ENES Bioacoustics Research Lab, CRNL, University of Saint-Etienne, CNRS, Inserm, Saint-Etienne, France. 3Ecole Pratique des Hautes Etudes, CHArt lab, EPHE - PSL University, Paris, France. 4Lead contact. 5Co-last authors *E-mail: [email protected] *** eTOC blurb Grimault et al.’s study reveals that humans possess a vibrotactile “sixth sense”. Standing individuals can locate the source of ground vibrations using their feet, relying on timing and/or intensity differences between them. This ability, analogous to binaural hearing, represents a previously unknown channel for human spatial perception. *** 1 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 Vision, hearing and olfaction are considered the primary sensory channels for locating distant objects or living beings beyond our immediate reach1. While some animals also use ground-borne vibrations to sense their environment, whether humans could exploit this channel to locate distant vibration sources remained unexplored. Here, we show that humans standing barefoot can pinpoint the source of mechanical waves propagating through the ground. Our results suggest this ability relies on comparing vibration intensity and arrival time between the two feet, a process strikingly analogous to binaural sound localization by the auditory system. Acting as a directional antenna, the feet provide a hitherto overlooked sensory channel for accessing potentially vital spatial information. This discovery not only reveals a hidden facet of human perception but also raises important questions about the impact of pervasive anthropogenic ground vibrations on how we navigate our sensory world. Locating the sources of sensory stimuli is a fundamental prerequisite for animals, including humans, to interpret their environment and respond appropriately. A variety of species are sensitive to vibrational signals and may use them to communicate, escape predators or find their prey 2,3 (Figure 1A). Some species of invertebrates and snakes have been shown to locate the source of ground vibration with great accuracy (e.g., sand scorpion, Saharan sand vipers, sandfish lizard 2,4 ). In mammals however, this localizing ability has been experimentally demonstrated only in mole rats 4,5 . While Humans readily perceive foot vibrations 6 , this sensory channel is often considered functionally limited, and it is unknown whether Humans can localize a distant source of vibration. Yet, this ability could have been of primary importance in the context of our ancestral hunter-gatherer’s way of life, where oriented displacements including anti-predatory behaviors and foraging decisions depended on a mixture of sensory cues provided by the environment. 2 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 In the present study, we tested human adults (N= 6 men and 6 women, aged 23-50 years) on their ability to report the direction of a ground-borne vibration source under conditions that eliminated all visual and auditory cues. During each trial (n = 80 / participant), participants stood barefoot on a metal plate inside a semi-anechoic booth, blindfolded and wearing a noise-cancelling headphone playing low-pass filtered noise to mask any airborne sound (Figure 1B). A vibratory device placed one meter away generated sinusoidal vibrations at either 30 Hz or 125 Hz. These frequencies were chosen to differentially activate cutaneous mechanoreceptors: 30 Hz stimulates both fast-adapting (FA) and slow adapting (SA) receptors, whereas 125 Hz primarily activates FA receptors 7 . Vibrations were delivered from one of four directions relative to the participant (0° = front, 180° = back, + 90° = right, - 90° = left). To test our core hypothesis that localization depends on comparing inputs between the feet, participants performed the task while standing on either a single foot or both feet. Stimulus order and conditions were counterbalanced (Supplemental Information, Supplemental Table S2). Our results provide the first direct evidence that humans can determine the direction of ground-borne vibrations. As illustrated in Figure 1C, performance was significantly above the 25% chance level when participants used both feet to detect the 125 Hz stimulus, reaching an average of 50% correct decisions (Bayesian Factor > 100, indicating very strong evidence; Supplemental Table S1). Under this optimal condition, accuracy was robust for sources located to the front and sides, though we observed a notable increase in confusion for sources originating from the back of participants (Figure 1D). This pattern is a classic signature of spatial hearing that relies on binaural cues. The dramatic improvement in performance when using two feet compared to one strongly supports the hypothesis that the feet function as a “vibrotactile antenna”, with the brain integrating information from both to compute direction. 3 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 The computational mechanisms appear to be analogous to that of auditory localization, which relies mainly on Interaural Time Difference (delay of sound arrival between both ears) and Interaural Level Difference (difference in sound amplitude). To investigate whether analogous cues operate for vibrotactile localization via the feet, we queried participants about their strategy. Eleven out of twelve reported actively comparing sensations between their feet. This subjective report aligns with the physical reality of the stimulus. Given the average inter-feet distance of 26.4 ± 3.3 cm, we calculated the estimated Inter-Feet Time Difference (IFTD) for a lateral source (± 90°) to be approximately 9 ms (See Supplemental Information). This temporal delay is well within the discriminative capacity of the human somatosensory system, which can resolve temporal differences on the order of milliseconds 7,8 . Furthermore, we measured Inter-Feet Level Differences (IFLD, amplitude difference in terms of displacement) of 7 mm for the 30 Hz stimulus and 0.15 mm for the 125 Hz stimulus (Supplemental Information). Considering the known sensitivity of cutaneous mechanoreceptors 9 , these subtle amplitude differences should also be perceptible. Since both IFTD and IFLD vary predictably with source direction, they represent plausible and sufficient cues for localization. The superior performance at 125 Hz likely stems from the properties of FA receptors, whose rapid response characteristics are ideally suited for encoding the precise onset timing and transient amplitude information critical for resolving these subtle inter-feet differences 8 . In summary, this study demonstrates that humans possess a latent “sixth sense”, a capacity to utilize vibrotactile perception through their feet to gain spatial information about distant vibration sources. The underlying sensory processing appears to share fundamental principles with binaural hearing, suggesting that the comparison of signals from paired sensors is a conserved, cross-modal strategy for spatial computation in the brain. This discovery opens several exciting avenues for future research. Neuroimaging studies could explore 4 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 whether vibrotactile localization recruits cortical areas that overlap with those involved in auditory spatial processing. Further work could also investigate whether this ability can be enhanced through training, potentially offering new sensory substitution pathways for individuals with visual impairments. Finally, our findings have profound implications for understanding our interaction with modern environments. We are constantly exposed to a cacophony of lowfrequency, ground-borne vibrations from traffic, machinery, and infrastructure. This “vibrational noise pollution” may not only mask our ability to perceive meaningful environmental signals but could also have unsuspected, subliminal effects on our navigation, stress, and cognitive load. Uncovering this dormant sensory skill is not just a matter of scientific curiosity; it prompts a re-evaluation of our relationship with the very ground beneath our feet. Supplemental information Supplemental information including experimental procedures, 2 Supplemental Tables, and supplemental references can be found with this article online at XXX. Acknowledgements We thank Etienne Parizet and Etienne Gaudrain for helpful advice on the design of the experimental and the statistical analyses. Funding was provided by CeLyA (ANR 10-LABX-0060), the University Lyon 1 (UCBL, France), the University of Saint-Etienne and the Centre National de la Recherche Scientifique (CNRS, France). References 1. Wu Y., Chen K., Ye Y., Zhang T., and Zhou W. (2020) Humans navigate with stereo olfaction, PNAS 117 (27) 16065-16071. DOIS:10.1073/pnas.2004642117 5 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 2. Cocroft, R. B., Gogala, M., Hill, P. S. and Wessel, A. (eds) Studying Vibrational Communication Vol. 3 of Animal Signals and Communication (Springer Berlin Heidelberg, Berlin, Heidelberg, 2014). DOIS: 10.1007/978-3-662-43607-3. 3. Hill, P. S. and Wessel, A. Biotremology. Current Biology 26 (5), R187–R191 (2016). DOIS: /10.1016/j.cub.2016.01.054 . 4. Narins, P. M. in Vibration Communication in Vertebrates (eds Barth, F. G. & Schmid, A.) Ecology of Sensing 127–148 (Springer Berlin Heidelberg, Berlin, Heidelberg, 2001). DOIS: 10.1007/978-3-662-22644-5 7. 5. Mason M.J., and Wenger L.M.D. (2019) Mechanisms of vibration detection in mammals in BiotremologyS: Studying Vibrational Behavior, Animal Signals and Communication 6, PSM Hill et al (eds) Springer Nature Switzerland AG 2019, DOIS: 10.1007/987-3-030-22293-2_10 6. Forta, N.G., Griffin, M. J., and Morioka, M. Difference thresholds for vibration of the foot: Dependence on frequency and magnitude of vibration. Journal of Sound and Vibration 330 (4), 805–815 (2011). DOIS: 10.1016/j.jsv.2010.08.039 7. Siedentopf, C., Heubach, K., Ischebeck, A., Gallasch, E., Fend, M., Mottaghy, F., Koppelstaetter, F., Haala, I..A., Krause, B., Felber, SS. et al (2008). Variability of BOLD response evoked by foot vibrotactile stimulation: Influence of vibration amplitude and stimulus waveform. NeuroImage. 41. 504-10. 10.1016/j.neuroimage.2008.02.049. 8. Strzalkowski, N.D.J., Ali, R.A., and Bent, L.R. The firing characteristics of foot sole cutaneous mechanoreceptor afferents in response to vibration stimuli. Journal of Neurophysiology 118 (4), 1931–1942 (2017). DOI: 10.1152/jn.00647.2016 9. Kekoni, J., Hmlinen, H., Rautio, J. & Tukeva, T. Mechanical sensibility of the sole of the foot determined with vibratory stimuli of varying frequency. Experimental Brain Research 78 (2) (1989). DOIS: 10.1007/BF00228915 Declaration of Interests The authors declare no competing interests. 6 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 Figure 1. Humans can pinpoint the direction of a ground vibration source. (A) Sensibility to substrate vibrations is known in various animals (blue and red), but the ability to localize vibration source has been experimentally confirmed in only a few (red). Human localization ability was previously unexplored. (B) Experimental set-up. Blindfolded participants, wearing noise-cancelling headphones, stood barefoot (one or two feet) on a plate and reported the perceived direction (Front, 0°; Back, 180°; Left, -90°; Right, +90°) of a ground vibration source (1 m away; 30 Hz or 125 Hz sine waves; ~10 dB above detection threshold). (C) Percentage of correct localization responses across conditions. Performance significantly surpassed chance (25%, dashed red line), with highest accuracy for two feet and 125 Hz stimulus. (D) Distribution of reported directions under optimal conditions (two feet, 125 Hz) for each source location (purple arrows). Dots indicate the mean number of trials reported per direction (max 5); grey areas represent SD across participants. Responses generally aligned with the true source direction, except for back condition. 7 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 SUPPLEMENTAL INFORMATION Supplemental Table S1. Effect of experimental conditions on localization accuracy. Results of Bayesian one sample t-test assessing whether accuracy in each condition exceeded the 25% chance level (H1). Bayesian factors and associated errors are shown. BFs > 10 = strong support for H1S; > 3 = moderate support for H1S; < 0.3 = moderate support for H0S; < 0.1 = strong support for H0. 8 203 204 205 206 207 208 209 210 211 212 Supplemental Table S2. Detection thresholds and intensity levels used for the experiments. 9 213 214 215 216 217 218