Sound Level Changes the Auditory Cortical Activation Detected with Functional Near-Infrared Spectroscopy

Abstract

Background: Functional near-infrared spectroscopy (fNIRS) is a viable non-invasive technique for functional neuroimaging in the cochlear implant (CI) population; however, the effects of acoustic stimulus features on the fNIRS signal have not been thoroughly examined. This study examined the effect of stimulus level on fNIRS responses in adults with normal hearing or bilateral CIs. We hypothesized that fNIRS responses would correlate with both stimulus level and subjective loudness ratings, but that the correlation would be weaker with CIs due to the compression of acoustic input to electric output.

Methods: Thirteen adults with bilateral CIs and 16 with normal hearing (NH) completed the study. Signal-correlated noise, a speech-shaped noise modulated by the temporal envelope of speech stimuli, was used to determine the effect of stimulus level in an unintelligible speech-like stimulus between the range of soft to loud speech. Cortical activity in the left hemisphere was recorded.

Results: Results indicated a positive correlation of cortical activation in the left superior temporal gyrus with stimulus level in both NH and CI listeners with an additional correlation between cortical activity and perceived loudness for the CI group. The results are consistent with the literature and our hypothesis.

Conclusions: These results support the potential of fNIRS to examine auditory stimulus level effects at a group level and the importance of controlling for stimulus level and loudness in speech recognition studies. Further research is needed to better understand cortical activation patterns for speech recognition as a function of both stimulus presentation level and perceived loudness.

Keywords: Cochlear implants; Functional near-infrared spectroscopy; Loudness ratings; Stimulus level.

Fig. 6

The LME model results (z-scores) for the sound vs. silence contrast for HbO data. After FDR correction, no channels were significantly activated to sound for the any group in the LME model

Fig. 7

LME model results (z-scores) for the linear correlation of activation with stimulus level for HbO data. After FDR correction, two channels were significantly correlated with level when combining groups and two channels for the CI group, but no significant channels were found for the NH group

Fig. 1

Representation of the location of each fNIRS channel displayed on a representative structural MRI. These locations are based on average 10–20 system locations being spatially registered to MNI coordinates. The location of participants’ CI processors disrupted recordings at four posterior channels (9, 13,18, and 22; blue circle) and were omitted from analysis on a case-by-case basis

Fig. 2

Mean perceived loudness ratings for each group on the LDL scale with a maximum of seven and a minimum of one. Error bars represent one standard error of the mean

Fig. 3

The LME model results (z-scores) for the sound vs. silence contrast. After FDR correction, two channels were significantly activated to sound for the NH group and one channel when combining the two groups together in the LME model. No channels reached significance in the separate LME model for the CI group

Fig. 4

LME model results (z-scores) for the linear correlation of activation with stimulus level. After FDR correction, three channels were significantly correlated with level for the NH group and five channels when combining groups, but no significant channels were found for the CI group

Fig. 5

Top row: the relative (for each subject, relative to their mean) loudness rating (y) and relative HbR activation (x) were significantly negatively correlated for the CI group (right) but not significantly correlated for the NH group (left). Bottom row: a similar analysis using sound presentation level (y) there is not a clear relationship with neural activity (x)