MEDICAL HYPOTHESES, vol.158, 2022 (SCI-Expanded)
Speech perception used to be seen as a solely an auditory phenomenon until the discovery of the "McGurk Effect" and in recent decades it has come to be understood as a multimodal, specifically, an auditory-visual process. Behavioural studies have shown that the magnitude of auditory-visual integration shows differences across ages and languages. A new question regarding the integration of auditory and visual speech information pertains to the case of mental disorders - a domain in which a significant paucity of data exits. The available studies (e.g., schizophrenic patients) generally point out to lessened levels of integration on both behavioural and cortical levels in respective clinical populations. A recent study with bipolar disordered individuals has shown that while those in manic and depressive episodes give similar behavioural responses to McGurk stimuli revealing an integration of auditory and visual speech information similar to the control group. However, in a visual-only condition (i.e., lipreading), compared to the control group and manic episode patients, those in the depressive episode show very impoverished levels of lipreading. We hypothesize that, during depressive episode of bipolar patients, abnormal lipreading is associated with impaired occipito-temporal connections in the dominant hemisphere, which are compensated by early activation of anterior cingulate gyrus and IFG, either through dominant or non-dominant hemisphere pathways. This can relay the signal in reverse fashion to the multimodal procession areas like posterior superior temporal sulcus, hence allowing audiovisual integration. This finding calls for cortical imaging studies to unearth the physiological processes involved in both lipreading and auditoryvisual speech processing in the bipolar disordered individuals. A full understanding of these processes will provide practitioners with secondary diagnostic tools and knowledge regarding this disorder.