Many functions of the human brain are organized asymmetrically and are subject to strong population biases. Some tasks, like speaking and making complex hand movements, exhibit left hemispheric dominance, whereas others, such as spatial processing and recognizing faces, favor the right hemisphere. While pattern of preference implies the existence of a stereotypical way of distributing functions between the hemispheres, an ever-increasing body of evidence indicates that not everyone follows this pattern of hemispheric functional segregation. On the contrary, the review conducted in this article shows that departures from the standard hemispheric division of labor are routinely observed and assume many distinct forms, each having a different prevalence rate. One of the key challenges in human neuroscience is to model this variability. By integrating well-established and recently emerged ideas about the mechanisms that underlie functional lateralization, the current article proposes a general mechanistic model that explains the observed distribution of segregation phenotypes and generates new testable hypotheses.

Cerebral lateralisation of language processing leads to a right ear advantage in normal hearing subjects. The aim of this study was to present a systematic overview of the effect of implantation side on postoperative cochlear implant performance in patients with symmetrical severe to profound sensorineural hearing loss.
PubMed, Embase and The Cochrane Library databases.
Databases were searched from database inception up to 9 January 2017 for cochlear implant and side and all synonyms. Title, abstract and full-text of retrieved articles were screened for eligibility. Then, directness of evidence and risk of bias were assessed. For the included articles, study characteristics and outcome data (hearing and language development) were extracted.
2541 unique articles were screened, of which twenty were eligible for critical appraisal. No randomised controlled trials were identified. Twelve studies with a high directness of evidence remained for data extraction. Four of six studies including children with pre-lingual sensorineural hearing loss and four of seven studies investigating adults with postlingual sensorineural hearing loss found a right ear advantage in at least one outcome measurement related to cochlear implant performance.
The available evidence on the effect of side of implantation is of low quality, as study populations and outcome measures are heterogeneous. The majority of studies reveals evidence for a right ear advantage in prelingually deafened children as well as postlingually deafened adults. In view of the present evidence and as no left ear advantage was identified, we cautiously advise implanting the cochlear implant in the right ear when other prognostic factors do not favour the left ear and sensorineural hearing loss is symmetrical.