Sensitivity to Haptic Sound-Localization Cues at Different Body Locations.

Cochlear implants (CIs) recover hearing in severely to profoundly hearing-impaired people by electrically stimulating the cochlea. While they are extremely effective, spatial hearing is typically severely limited. Recent studies have shown that haptic stimulation can supplement the electrical CI signal (electro-haptic stimulation) and substantially improve sound localization. In haptic sound-localization studies, the signal is extracted from the audio received by behind-the-ear devices and delivered to each wrist. Localization is achieved using tactile intensity differences (TIDs) across the wrists, which match sound intensity differences across the ears (a key sound localization cue). The current study established sensitivity to across-limb TIDs at three candidate locations for a wearable haptic device, namely: the lower tricep and the palmar and dorsal wrist. At all locations, TID sensitivity was similar to the sensitivity to across-ear intensity differences for normal-hearing listeners. This suggests that greater haptic sound-localization accuracy than previously shown can be achieved. The dynamic range was also measured and far exceeded that available through electrical CI stimulation for all of the locations, suggesting that haptic stimulation could provide additional sound-intensity information. These results indicate that an effective haptic aid could be deployed for any of the candidate locations, and could offer a low-cost, non-invasive means of improving outcomes for hearing-impaired listeners.

Sensitivity to haptic sound-localisation cues.

Hearing aid and cochlear implant (CI) users often struggle to locate and segregate sounds. The dominant sound-localisation cues are time and intensity differences across the ears. A recent study showed that CI users locate sounds substantially better when these cues are provided through haptic stimulation on each wrist. However, the sensitivity of the wrists to these cues and the robustness of this sensitivity to aging is unknown. The current study showed that time difference sensitivity is much poorer across the wrists than across the ears and declines with age. In contrast, high sensitivity to across-wrist intensity differences was found that was robust to aging. This high sensitivity was observed across a range of stimulation intensities for both amplitude modulated and unmodulated sinusoids and matched across-ear intensity difference sensitivity for normal-hearing individuals. Furthermore, the usable dynamic range for haptic stimulation on the wrists was found to be around four times larger than for CIs. These findings suggest that high-precision haptic sound-localisation can be achieved, which could aid many hearing-impaired listeners. Furthermore, the finding that high-fidelity across-wrist intensity information can be transferred could be exploited in human-machine interfaces to enhance virtual reality and improve remote control of military, medical, or research robots.

Haptic sound-localisation for use in cochlear implant and hearing-aid users.

Users of hearing-assistive devices often struggle to locate and segregate sounds, which can make listening in schools, cafes, and busy workplaces extremely challenging. A recent study in unilaterally implanted CI users showed that sound-localisation was improved when the audio received by behind-the-ear devices was converted to haptic stimulation on each wrist. We built on this work, using a new signal-processing approach to improve localisation accuracy and increase generalisability to a wide range of stimuli. We aimed to: (1) improve haptic sound-localisation accuracy using a varied stimulus set and (2) assess whether accuracy improved with prolonged training. Thirty-two adults with normal touch perception were randomly assigned to an experimental or control group. The experimental group completed a 5-h training regime and the control group were not trained. Without training, haptic sound-localisation was substantially better than in previous work on haptic sound-localisation. It was also markedly better than sound-localisation by either unilaterally or bilaterally implanted CI users. After training, accuracy improved, becoming better than for sound-localisation by bilateral hearing-aid users. These findings suggest that a wrist-worn haptic device could be effective for improving spatial hearing for a range of hearing-impaired listeners.