Sensitivity to Haptic Sound-Localization Cues at Different Body Locations

Abstract

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.

Keywords: cochlear implant; cross-modal; electro-haptic stimulation; haptic sound-localization; hearing aid; hearing impaired; neuroprosthetic; somatosensory; tactile; vibrotactile.

Figure 1

Illustration of the tactile stimulation probe placements that were used in the current study: lower tricep (panel (A)) and dorsal and palmar wrist (panel (B)).

Figure 2

A speech sample and its amplitude envelope, which was used to modulate the amplitude of the tactile stimulus in the current study. The original audio waveform is shown in blue and the extracted amplitude-envelope is highlighted with a thick orange line. The spoken text is marked above the stimulus.

Figure 3

Schematic representation of the experimental set up for the testing phase, with the participant sitting in front of two tactile vibrometers and a computer monitor.

Figure 4

Panel (A): across-wrist tactile intensity difference thresholds for each body location. For reference, the dashed light blue line shows across-ear intensity difference discrimination thresholds in young normal-hearing adults (based on data from [20]). Panel (B): detection thresholds for the unmodulated stimuli at each location (averaged across left and right limbs). Panel (C): the usable dynamic range at each location (averaged across left and right limbs). The dotted dark blue line shows the dynamic range available through electrical CI stimulation (based on data from [25,26]). In all of the panels, the error bars show the standard error of the mean.