Hearing with Cochlear Implants

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. 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).

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 (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 (Kral & Eggermont, 2007; 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.

cochlear implant and central plasticity
Cortical activation after long-term electrostimulation through cochlear implants. A. Implantation age of 3 mo. allows gradual expansion of active areas Science 1999; implantation age 6 months does not show the same process Cereb Cortex 2002. B: Active area increases gradually with stimulation duration (blue). Increasing implantation age deacreases the effect (red & green). C: Onset latency of the responses decrease in early implantation, but not in late implantation (Prog Brain Res 2006). Figure from Trends Neurosci 2012.Brain_plasticity_files/SCIENCE_1999_1.PDFBrain_plasticity_files/CerebCortex2002_1.pdfBrain_plasticity_files/CerebCortex2002_1.pdfBrain_plasticity_files/ProgBrainRes_Color_2006_2.pdfBrain_plasticity_files/TINS_2012.pdfshapeimage_3_link_0shapeimage_3_link_1shapeimage_3_link_2shapeimage_3_link_3shapeimage_3_link_4

Months-long hearing experience through a cochlear implant and a single-channel portable processor. The cortical area representing the stimulated cochlear region expands slowly but extensively (Kral et al., 2006a), provided the cochlear implantation takes place before the end of the sensitive period. Naive CDC = congenitally deaf cat without any hearing experience.

Sensitive period for aural preference change in single-sided deafness: age at implantation determines the effect of cochlear implant stimulation. In implantations after 4.2 months, the mutual relation of the activity evoked by the implanted (hearing) ear and the non-implanted ear are not much changed compared ti deaf animals. In earlier implantations or in congenital single-sided deafness (green) the relation is changed in favor of the hearing ear. Data from Brain 2013; review on critical periods: Neuroscience 2013

Changes in the minicolumn induced by chronic electrostimulation through a cochlear implant. A: Synaptic activity in a naive animal show circumscribed activity mainly in the upper cortical layers. B: In normal hearing cats, and also after chronic electrostimulation, the pattern involves deep layers. C: The earliest latencies of synaptic activity move to shorter values and become more similar after chronic electro-stimulation (grey color) compared to naive animals (red color). Early cochlear implantation therefore normalizes the activity in the primary auditory cortex. Figure from Prog Brain Res 2006.Brain_plasticity_files/ProgBrainRes_Color_2006_3.pdfBrain_plasticity_files/ProgBrainRes_Color_2006_3.pdfshapeimage_5_link_0shapeimage_5_link_1
Decoupling & Prediction codingTop-down_decoupling_in_deaf.htmlshapeimage_6_link_0
Additional information:
Examples of top-down effectsExamples.htmlshapeimage_8_link_0
Corticicortical couplingsFeedforward_%26_Feedback.htmlshapeimage_9_link_0