congenital Deafness - Sensory Deprivation
The brain stores and processes information in synaptic contacts. The synapses of the cerebral cortex show extensive development after birth: Synaptic densities increase first to reach a peak (“synaptic overshoot”) between 1st - 4th year of life. This peak in number of synapses is followed by a slow but extensive decrease extending into teenager years (synaptic elimination, “pruning”). During this process, children develop increasingly complex auditory and cognitive abilities. Is this process of synaptic selection dependent on experience, or is this process the sole consequence of the genetically-predetermined program? Research in the visual system with reversible blindness (eyelid suture) has shown that at least certain aspects of this process are dependent on experience. Unfortunately, there are no methods for reversible deprivation available in the auditory system: suturing the external auditory canal does not lead to complete deafness nor does it affect the perception of self-generated sounds. Pharmacological deafening is irreversible.
After cochlear implants became available for clinical purposes, the question of central effects of auditory deprivation has regained clinical and scientific interest: can one implant prelingually deaf children (deaf before birth or before acquiring language competence) and initiate the development of language through this neuroprosthesis? This is indeed possible, provided an early cochlear implantation has taken place (before age 4, today implantation is recommended even before age 2; Kral & O‘Donoghue,2010). Implantation in teen ages, however, does not provide the same success. We want to understand the neuronal mechanisms behind this process.
Our data demonstrate that the cortical microcircuitry within the cortical column is severely compromised by the lack of auditory experience (Kral et al. 2000). Even though some basic feature extraction does take place, the feature sensitivity is reduced and further processing steps within cortical neuronal networks is disrupted in congenital deafness. The starting point for learning is therefore compromised by these effects (reviewed in Kral & Sharma, 2012). 
The developmental sequence observed in the hearing auditory cortex is massively affected by deafness (Kral et al., 2005; Kral & O‘Donoghue,2010). Developmental delays, degenerative changes as well as modified maturational sequence have been observed in the „deaf“ auditory cortex (Kral & Sharma, 2012). The ultimate consequences were the deficits reviewed in Kral et al. (2006), Kral & Sharma (2012) and expressed in the decoupling hypothesis (read more). Recently, we observed that in the secondary (higher-order) auditory cortex, similar morphological deficits are observed as in primary auditory cortex, indicating that the effects described are at least partly replicated in many auditory areas (Berger et al., 2017). Our recent work suggests that specifically the corticocortical interactions are affected by congenital deafness (Yusuf et al., 2017), and thus most likely the exaggerated pruning in deafness relates to corticocortical synapses mainly. Here, top-down interactions are most affected (Yusuf et al., 2021). They relate to deficits in connectivity between infragranular and supragranular layer (Yusuf et al., 2022, Berger et al., 2017). 
>>> Read more on connectome model of deafness
Single-sided hearing (deafness) prevents these deficits, but reorganizes the auditory system, resulting in a central aural preference for the hearing ear (see >>>).


Left: Cortical activation after long-term electrostimulation through cochlear implants, Science 1999. Middle: Illustration of the location of cochlear implant in humans, Cochlear Corp., Basel, Switzerland. Right: Changes in morphology of cortical neurons during first years of life, J. L. Conel (1939-1967), for references see Int J Audiol 2007.

Development of synaptic activity in hearing (blue) and deaf (deaf) cats measured by current source density analyses. In hearing cats, cortical synapses are developed after birth and show a phase of net synaptogenesis and net synaptic elimination (pruning). In congenital deafness, development of synapses is significantly delayed (1) and the pruning is enhanced (2). Statistically significant differences (5% level) are shown by grey background. Data from Kral et al. (2005), figure from Kral & Sharma (2012).

Decoupling & Prediction codingTop-down_decoupling_in_deaf.htmlshapeimage_7_link_0
Additional information:
Examples of top-down effectsExamples.htmlshapeimage_9_link_0
Corticicortical couplingsFeedforward_%26_Feedback.htmlshapeimage_10_link_0
Cognitive effects in deafnessDeaf_Connectome.htmlshapeimage_11_link_0

Summary of the deficits observed in primary auditory cortex (A1) and secondary areas DZ and A2 based on morphological analysis of SMI-32 antibody staining and Nissl staining. In all auditory areas deep cortical layers showed similar dystrophic changes, indicating a dysfunctional cortical column. From Berger et al., 2017

Vocalizations of deaf catsVocalizations.htmlshapeimage_12_link_0