In this study, our results showed that the auditory performance and speech intelligibility of trained children in the rehabilitation centers were almost the same as those of untrained children with early implantation. After implantation, the CAP and SIR scores of both groups increased with increasing time of implant use during the follow-up period, and at each time point, the median scores of the two groups were comparable.
Speech therapy has been shown to be effective in the rehabilitation of patients with cochlear implantation in previous studies 
. It has been thought to be one of the methods that can accelerate the learning process because postlingually deafened cochlear implant patients must adapt to both spectrally reduced and spectrally shifted speech due to the limited number of electrodes and the limited length of the electrode array after implantation. Several studies have revealed better auditory resolution, speech perception, and music perception after receiving training 
, suggesting great promise as part of the aural rehabilitation of adult, postlingually deafened cochlear implant recipients.
Our results may seem inconsistent with previous studies. However, our negative result was not the only one demonstrating no efficacy in CI patients receiving speech training. Mixed and generally poor outcomes have been revealed from previous CI speech-training studies 
. For example, Dawson and Clark (1997) found that three of five patients showed minimal or no improvement in vowel perception after training 
. Most authors believe these results were due to training protocols, training materials, and training frequencies used. However, none of these factors has been confirmed, and the exact reason remains unknown.
Some studies have found that earlier implantation leads to better language outcomes, suggesting that there may be sensitive periods for central auditory and spoken language development at a younger age 
. There is considerable evidence for a developmentally sensitive period during which the auditory cortex is highly plastic, although it has not been defined clearly 
. If the auditory system is deprived of sensory input during this sensitive period, then the central auditory system is susceptible to large-scale reorganization. Very early implantation may be necessary to allow at least relatively normal organization of auditory pathways in congenitally deaf children. This might be a potential factor explaining why earlier implantation leads to good results in children even without speech training.
The length of the speech therapy for the children in this study might be insufficient to see results. A previous study reported that the generally poor outcomes of CI speech training patients might well be due to the amount and type of training used 
. We cannot exclude this possibility. However, many studies that reported marked benefits used training protocols of less than 6 months' duration. Since they focused on adult, postlingually deafened cochlear implant recipients, comparison of these studies and ours was not proper.
The CAP and SIR are non-linear, hierarchical scales, with poor accuracy and little detail 
, and therefore they may not reveal the real conditions in these children. This may be the main limitation of the present study. However, the very young children have not established spoken language skills or the level of behavior and cooperation necessary for a more formal assessment 
. Therefore, indirect measures may have to be used 
. Further investigations should be undertaken using more accurate and detailed parameters and should also be conducted over a longer term with more cases. On the other hand, the benefits of ST for the early implantation children were so subtle that they could not be distinguished using real-life situation-based measures, and so might be of little clinical significance.