Cerebral lateralization of language in deaf children and adults with cochlear implant (CI): a neurofunctional study with transcranial doppler ultrasound (fTCD)

Severe to profound sensorineural hearing loss (SNHL) is a pathological condition, that affects about 1-3/1000 newborns, but as shown by several studies on humans and animals, can be considered a scientific opportunity for understanding the role of auditory stimuli in neuroplasticity. In the late decades the advent of Cochlear Implantation (CI) has also permitted further investigation on how stimuli restoration can affect neuroplasticity in previously deprived subjects. This issue has been studied in terms of timing of restoration (age of implantation) and side of stimulation (afferented ear), and effects on auditory perception and language development in the case of humans. Within this theoretical frame, the present study focuses on language lateralization, measured through functional Transcranial Doppler ultrasonography (fTCD), a non invasive technique, that quantifies a Lateralization Index (LI) detecting blood flow during language performance. At this aim, 3 different groups of subjects were evaluated by fTCD: (i) 36 children with prelingual profound bilateral SNHL with monolateral CI (and 24 control subjects with normal hearing); they were also evaluated in language achievement (PPVT, TCGB, Inpe high and low frequencies, GASS and language composite score); (ii) 11 children with profound congenital unilateral hearing loss (UHL) (and 11 control subjects with normal hearing bilaterally); they were evaluated on verbal and non verbal development (PPVT, TROG2, IQ, PRI, WMI, PSI, VCI, VMI); (iii) 6 adults with preverbal profound bilateral SNHL, evaluated before and after CI. The results show that (i) left dominance is maintained, even if bilateral representation for language appears more frequent in children with monolateral CI; children with right ear implanted or left LI show better language performance. (ii) left activation was confirmed in children with right UHL while it was not confirmed in those with left UHL. Performance on verbal test were significantly better in children with right hearing. (iii) No significant changes LI were observed in adults, after implantation. On the basis of the present study neuroplasticity of auditory and language circuits appears to be a complex phenomenon in which some biological constraints for left dominance for language are confirmed, but other factors, such as age of reafferentation, and side of afference can play roles, that have still to be clearly understood. Furthermore the present study brings some support to the right ear advantage hypothesis and this should be taken into account while choosing the ear to be implanted, in the case of unilateral CI. From this point of view the right implant could be considered the first choice in monolateral or sequenced implantation. fTDC for LI evaluation can be considered in the case of late diagnosed deafness before implantation.