Subjects (Table I)

Five subjects received a HiRes90K® cochlear implant (Advanced Bionics Corporation, Sylmar, USA) at the Edouard Herriot University Hospital of Lyon (France). All subjects had full electrode insertion. They were full-time users of their cochlear implants during the one-month interval between both sessions. Their benefit from cochlear implant varied from subject to subject. This was assessed by their speech perception scores to Lafon lists (1964) (lists of three phoneme long words) presented in an open set format in quiet at 65 dB HL in the sound field. The possible prosthesis on the ear controlateral to the implanted side was switched off during this evaluation. The deafness and implantation characteristics, as well as the demographics of each subject are presented below and a summary is proposed in Table I.

S1 was a 16 years old female who was born profoundly deaf. The aetiology of her deafness was unknown. She had worn prosthesis on the two ears till she got implanted on the right side. She could understand 24 % of phonemes from Lafon test on the day her implant was switched on and 76 % one month later. She kept using her prosthesis on the left side after her implantation. S2 was 17 years old at the time of the study. He became profoundly deaf at birth because of perinatal asphyxia. He was implanted on the left side where he had never used prosthesis. His performance to Lafon test was 2 % at implant switch on and 46 % one month later. S3 was a 26 years old woman whose deafness appeared progressively probably because of a Turner syndrome. She became profoundly deaf five years before her implantation (on the left side) but continued to wear her prosthesis even if they would only give her almost no information (see Table 1). She could understand 6 % of phonemes from Lafon test on the first day her implant was switched on and 72 % one month later. S4, 32, was a woman whose deafness got settled progressively. She has an Usher syndrome. The right ear became totally deaf one year before it was implanted. She stopped using her prosthesis on the right ear once it became totally deaf but she kept using a prosthesis on the left ear. This provided her with sufficient information to enable her to telephone. Her scores to speech perception test was 18 % at implant switch on and 94 % one month later. S5 was a 65 years old woman who progressively became deaf because of a Meniere’s syndrome. Her deafness was total eight years before implantation. She got implanted on the right side where she was wearing a prosthesis (audiometric mean for frequencies ranging from 250 to 8000 Hz tones = - 100 dB HL). Her scores to speech perception test were 2 % at implant switch on and 32 % one month later.

Table I. Demographic information: aetiology and onset of deafness, age at test, duration of profound deafness on the implanted side prior to implantation, implanted side, presence of a prosthesis on the opposite side, audiometric mean (for tones of 500, 1000, 2000, and 4000 Hz) obtained before implantation from the future implanted side and from the contralateral side when there is a prosthesis, and speech perception scores at implant switch-on and 1 month later.

The fact that subjects are at least 16 years old should not keep from investigating plasticity. Indeed, significantly, cortical reorganization is not restricted to period of development (Kass, 1991) and functional organization was found to not be statically fixed but adjusted in response to alteration of behaviorally relevant input even in mature cortex (Buonomano & Merzenich,1998 ; Weinberger & Bakin, 1998 ; Elbert & Flor, 1999; Rauschecker, 1999). Subjects were fully informed about the experimental procedures in accordance with the decision of the local ethical committee. They signed a consent form prior to participation.