Dichotic listening and interhemispheric integration after callosotomy: a systematic review

The right-ear advantage (REA) for recalling dichotically presented auditory-verbal stimuli has been traditionally linked to the dominance of the left cerebral hemisphere for speech processing. Early studies on patients with callosotomy additionally found that the removal of the corpus callosum leads to a complete extinction of the left ear, and consequently the today widely used models to explain the REA assume a central role of callosal axons for recalling the left-ear stimulus in dichotic listening. However, later dichotic-listening studies on callosotomy patients challenge this interpretation, as many patients appear to be able to recall left-ear stimuli well above chance level, albeit with reduced accuracy. The aim of the present systematic review was to identify possible experimental and patient variables that explain the inconsistences found regarding the effect of split-brain surgery on dichotic listening. For this purpose, a systematic literature search was conducted (databases: Pubmed, Web of Knowledge, EBSChost, and Ovid) to identify all empirical studies on patients with surgical section of the corpus callosum (complete or partial) that used a verbal dichotic-listening paradigm. This search yielded ks = 32 publications reporting patient data either on case or group level, and the data was analysed by comparing the case-level incidence of left-ear suppression, left-ear extinction, and right-ear enhancement narratively or statistically considering possible moderator variables (i.a., extent of the callosal surgery, stimulus material, response format, selective attention). The main finding was an increased incidence of left-ear suppression (odds ratio = 7.47, CI95%: [1.21; 83.49], exact p = .02) and right-ear enhancement (odds ratio = 21.61, CI95%: [4.40; 154.11], p<.01) when rhyming as compared with non-rhyming stimuli were used. Also, an increase in left-ear reports was apparent when a response by the right hemisphere was allowed (i.e., response with left hand). While the present review is limited by the overall small number of cases and a lack of an appropriate control sample in most of the original studies, the findings nevertheless suggest an adjustment of the classical dichotic-listening models incorporating right-hemispheric processing abilities as well as the perceptual competition of the left- and right-ear stimuli for attention.