One key factor in the success of the neuroprosthetic device is how the brain adapts to this device. The central auditory system has to learn to „understand“ the activity evoked by electrical stimulation. Does the congenitally deaf brain have sufficient plasticity and functionality to allow such learning?

Sensitive periods exist in the auditory system of congenitally deaf animals: early cochlear implantation resulted in functional maturation of the auditory cortex (Klinke et al., 1999) and the installation of feature-sensitivity after more than 2 months of hearing experience (Kral et al., 2006). The earlier the implantation took place, the more extensive adaptations in the auditory cortex were found in many different measures of cortical responses (Kral et al., 2001; 2002; 2006a; 2006b; 2013; Kral & Sharma, 2012; Kral, 2013). Also in single-sided deafness, a sensitive period could be demonstrated in brain reorganization toward the hearing ear (read more).

Sensitive periods result from a developmental decrease in synaptic plasticity with age, which is caused by a genetically-predetermined switch in synaptic properties. Plasticity driven by stimulus statistics is replaced by adult gated plasticity, controlled by the brain based on the needs of the individual. Critical periods (the closure of sensitive periods) is caused by excessive pruning of synapses in the cortex (Kral et al., 2005; review in Kral and Sharma, 2012) that prevent the installation of microcircuitry needed for the adult-like gated plasticity (Kral et al., 2019). The critical period for therapy of congenital deafness closes also by the naive circuitry of the auditory system that could not learn to control plastic reorganizations but has lost the high juvenile plasticity  observed in young (Kral, 2013; 2019).

Our sensory organs are exposed to many different stimuli at the same time; which of them will trigger reorganization in the brain and which not is determined by the brain itself. A trigger for plastic reorganization in hearing-competent subjects is a discrepancy between sensory inputs and the prediction on these inputs (Rescorla-Wagner learning theory and predictive coding, for details and evidence from brain imaging, see Kral et al., 2017). Such control of plastic adaptations requires top-down cortical interactions modulating the bottom-up processing. For this, functional cortical columns are essential. In congenital sensory deprivation, this function of cortical columns is compromised (Yusuf et al., 2022; Kral & Eggermont, 2007). Sensitive periods are thus also the consequence of disrupted timing between developmental changes in synaptic plasticity and the developmental changes of its top-down control (Yusuf et al., 2021; Kral & O‘Donoghue,2010). For details on bottom-up and top-down in deaf and the „decoupling hypothesis“, go here.

Experiments performed in the lab of Prof. Anu Sharma (Sharma et al., 2002, 2005) demonstrated a strong correlation of EEG-recordings in cochlear-implanted children with the neurophysiological data obtained from congenitally deaf animals in our lab. In our clinics the data from prelinually deaf children following cochlear implantation support both the critical period for monaural deprivation (Illg et al., 2013; 2019) as well as a continuous drop in brain plasticity within the critical period, documenting that the earlier the therapy is initiated, the better the outcomes (Illg et al., 2024).