Cortical Processing Related To Sound Source Localization-A Functional Near-Infrared Spectroscopy Study

BackgroundSound localization is an important life skill for human beings,and most patients with clinical hearing loss have sound localization impairment,which seriously affects daily life,learning,and work.Interaural level differences(ILD)is known to be an important acoustic cue for horizontal sound source localization,and those hearing impaired patients with cochlear implant and hearing aid rely mainly on ILD cues,however,the responsible cortex and response patterns associated with horizontal sound source localization remain unclear.Compared with fMRI which has severe limitations on experimental conditions and populations,functional near-infrared spectroscopy(fNIRS)is less affected by environmental noise highly applicable to populations with electronic device implant,and has obvious advantages in the field of brain function studies of spatial hearing,especially sound source localization,but there is still a lack of relevant studies in this field.Part ⅠObjective:To establish an experimental paradigm and data analysis model for detecting the cortical response to horizontal sound source localization via fNIRS.Materials and methods:25 subjects with normal hearing(age 26±2.7 years)were included.The sound localization test was performed in an anechoic chamber arranged with a 180° fan-shaped sound field formed by 7 loudspeakers.During the experiment,the subjects wore a 20-channel fNIRS to record the cortical changes during acoustic stimulation in different directions.MATLAB was used for dichotomous analysis of the data.Feature sets with≥70%accuracy were extracted as the final results.Results:A paradigm of simultaneous measurement of acoustic localization behavior and cortical response analysis via fNIRS was successfully established.In the dichotomous results,there were significant differences in the changes of relative values of oxyhemoglobin in bilateral posterior superior temporal gyrus and part of dorsolateral prefrontal regions(analysis time window of 5-8s,classification accuracy≥70%).Conclusions:fNIRS has higher accuracy in detecting cortical responses to sound source localization in normal subjects,and ±90° elicits more cortical responses in the bilateral posterior superior temporal gyrus than 0° sound sources.Part IIObjective:To explore the changing pattern of cortical responses related to horizontal sound localization during binaural loudness imbalance based on the study model established in Part I.Materials and methods:By Physically attenuating monaural sound wave afferents causing bilateral loudness imbalance model,twenty-five normal hearing individuals were tested for horizontal localization in a 180°sector sound field and recorded cortical responses simultaneously using fNIRS with 20 channels.Homer3 and SPSS 26.0 were used for data processing and statistical analysis.Results:The error in sound localization was about 10°-30° after the physical attenuation method caused binaural loudness imbalance(mean interaural hearing threshold difference of about 20-40 dB HL).When the left ear loudness was attenuated,the mean oxyhemoglobin value in the right temporal region was lower than 0° and +90°for-90° sound source stimulation(P<0.05);when the right ear loudness was attenuated,the mean oxyhemoglobin waveform value in the right temporal region was higher than 0° and-90° for+90° sound source stimulation(P<0.5).In contrast,the90° sound source caused stronger activation in the premotor area when the binaural loudness was balanced.Conclusion:Binaural loudness imbalance leads to poor sound source localization ability,and different cortical response patterns are seen in the bilateral temporal regions.Part IIIObjective:To explore the cortical response to acoustic source localization activity captured using fNIRS in bimodal cochlear implant(CI)recipients and to explore the response pattern for the optimization of clinical rehabilitation strategies.Materials and methods:Four bimodal CI subjects(ages 7,12,17,and 24 years).The cortical responses were recorded simultaneously under three listening conditions(hearing aid(HA)+CI,CI only,HA only).Results:The degree of benefit of sound source localization in dual-mode listening varied considerably between individuals.Differences in activation to different source orientations were seen in the right superior temporal gyrus in subjects with better sound source localization performance under bimodal listening;no regular cortical response pattern was seen under CI and HA listening alone.No statistical differences in cortical response were seen in the subjects with poor sound localization performance in all three listening states.Conclusion:Despite the large inter-individual differences in the degree of benefit in sound source localization ability with bimodal compared to unilateral device listening,cortical response related to sound localization was seen to correlate with discrimination ability in this group of subjects.Future use of fNIRS assessment is expected to provide an objective basis for bimodal CI tuning and rehabilitation.