Speech Processing in CI-Users

Background

Equipping a patient with a cochlear implant(CI) affects speech signal processing in their brain. This area of study is important in terms of assessing neuroplasticity extent in sensory deprivation/rehabilitation. Only few PET studies have focused on that subject(Song 2015).

Objective

To further elucidate neuronal correlates of speech processing in patients with CIs.

Methods

A [¹⁵O]H₂O PET study was performed in 6 adult CI-users with postlingual high-frequency SNHL (partial deafness) and 6 normal hearing individuals(NH). Both groups listened binaurally to monosyllabic words. NH also listened to vocoded words, i.e. CI-simulations. For each PET emission scan of 10min (x5 in CI, x8 in NH) a bolus of 550 MBq[¹⁵O]H₂O was administered.

Results

When listening to words, both patients and NH revealed increased blood flow in bilateral posterior superior temporal gyri (STG;auditory cortex). Patients also showed statistically significant more involvement of the dorsal auditory stream, including bilateral frontal poles (action monitoring), SMA/premotor cortex (cognitive control of high task demands/motor planning), bilateral superior marginal gyri(SMG)/angular gyri(AG) (multimodal integration) (Song et al 2015, Hickock 2007, Suh 2017, Verger 2017). NH, when listening to vocoded words compared to words, revealed hypermetabolism in the left STG and aSMG which underlay phonological decoding (Kelly et al.2010; Eggermont 2014).

Conclusion

Since the signal delivered via a CI is new and degraded, patients use additional brain structures when processing speech and in order to combine it with their natural hearing. The revealed brain network does not fully overlap with the one used by NH when trying to understand CI-simulations.

[The study was facilitated by a grant from the Polish National Science Centre, 2012/05/N/NZ4/02202 and 2011/03/D/NZ4/02431].