Abstract
Functional neuroimaging has identified that the temporal, frontal and parietal cortex support core aspects of speech processing. An objective measure of speech intelligibility based on cortical activation in these brain regions would be extremely useful to speech communication and hearing device applications. In the current study, we used noise-vocoded speech to examine cortical correlates of speech intelligibility in normally-hearing listeners using functional near-infrared spectroscopy (fNIRS), a non-invasive, neuroimaging technique that is fully-compatible with hearing devices, including cochlear implants. In twenty-three normally-hearing adults we measured (1) activation in superior temporal, inferior frontal and inferior parietal cortex bilaterally and (2) behavioural speech intelligibility. Listeners heard noise-vocoded sentences targeting five equally spaced levels of intelligibility between 0 and 100% correct. Activation in superior temporal regions increased linearly with intelligibility. This relationship appears to have been driven in part by changing acoustic properties across stimulation conditions, rather than solely by intelligibility per se. Superior temporal activation was also predictive of individual differences in intelligibility in a challenging listening condition. Beyond superior temporal cortex, we identified regions in which activation varied non-linearly with intelligibility. For example, in left inferior frontal cortex, activation peaked in response to heavily degraded, yet still somewhat intelligible, speech. Activation in this region was linearly related to response time on a simultaneous behavioural task, suggesting it may contribute to decision making. Our results indicate that fNIRS has the potential to provide an objective measure of speech intelligibility in normally-hearing listeners. Should these results be found to apply similarly in the case of individuals listening through a cochlear implant, fNIRS would demonstrate potential for a clinically useful measure not only of speech intelligibility, but also of listening effort.
Highlights
Functional neuroimaging has identified critical components of the functional organization of speech and language processing in the human brain (Hickok and Poeppel, 2000; Scott and Johnsrude, 2003)
Isolation of the functional haemodynamic response: we applied the haemodynamic modality separation (HMS) algorithm described by Yamada et al (2012) to isolate the functional component of the haemodynamic signal and suppress systemic physiological interference. This algorithm attempts to separate functional and systemic signals based on the assumption that the correlation between HbO and HbR will be different in each case. This approach does not accurately account for all statistical properties of the noise typically found in functional near-infrared spectroscopy (fNIRS) data (Huppert, 2016), in previous studies we have found application of this algorithm to be beneficial to the detection of auditory cortical activation (Wiggins et al, 2016; Wijayasiri et al, 2017); in particular, application of the HMS algorithm was shown to substantially improve the test-retest reliability of auditory fNIRS measurements (Wiggins et al, 2016)
We confirm that fNIRS-measured activation in superior temporal regions correlates with speech intelligibility (Pollonini et al, 2014; Olds et al, 2016), both within and, to a lesser extent, between individuals
Summary
Functional neuroimaging has identified critical components of the functional organization of speech and language processing in the human brain (Hickok and Poeppel, 2000; Scott and Johnsrude, 2003). Functional magnetic resonance imaging (fMRI) studies show that temporal cortex responses to speech stimuli of varying intelligibility (Evans et al, 2014; Zekfeld et al, 2006; Belin et al, 2000). Using positron emission tomography (PET) Scott and colleagues (2006) found a selective response to intelligible speech in the left anterior superior temporal sulcus by varying the intelligibility of speech parametrically using vocoded stimuli. The risks associated with cumulative effects of radionuclide exposure limits the clinical usefulness of PET as a repeatable clinical tool to measure speech understanding in clinical populations
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