The effect of the menstrual cycle on dichotic listening

Estradiol driven change in hallucination proneness across the menstrual cycle as studied with a white noise paradigm

The estrogen hypothesis for schizophrenia suggests neuroprotective effects of estrogen for the development of the disorder and for symptom severity, including auditory hallucinations. Furthermore, estrogen has shown enhancing effects on cognitive control, a function that is also implicated in auditory hallucinations. Whether estrogen affects the tendency to hallucinate in healthy participants, and the potential mediating role of cognitive control, has not yet been studied. Therefore, the current study aimed to test these relationships by using a white noise paradigm in combination with a N-back working memory task in which cognitive load could be manipulated. The paradigm used simulates a hallucinatory state by induction of negative emotions and drainage of cognitive resources. The simultaneous exposure to white noise elicit experiences of hearing voices (false alarms). In a between-subject design, forty-two participants were tested during the menstrual cycle in either the early follicular phase (low estradiol) or late follicular phase (high estradiol). A 2(Cycle Phase) x2(N-back task) ANOVA showed a main-effect of cycle phase on number of experienced hallucinations in the white noise task, with a significantly higher number of reported hallucinations in the early follicular phase. Furthermore, estradiol was found to predict number of hallucinations. No interaction effect of cycle phase and available cognitive resources was found. The results suggest an estradiol-related change in hallucination proneness across the menstrual cycle, but the idea that cognitive functioning mediates this relationship was not supported. Overall, the study supports protective effects of estradiol on hallucination proneness in line with the estrogen-hypothesis of schizophrenia, and that such effects are not specific to the disease.

Brain Lateralization: A Comparative Perspective

Comparative studies on brain asymmetry date back to the 19th century but then largely disappeared due to the assumption that lateralization is uniquely human. Since the reemergence of this field in the 1970s, we learned that left-right differences of brain and behavior exist throughout the animal kingdom and pay off in terms of sensory, cognitive, and motor efficiency. Ontogenetically, lateralization starts in many species with asymmetrical expression patterns of genes within the Nodal cascade that set up the scene for later complex interactions of genetic, environmental, and epigenetic factors. These take effect during different time points of ontogeny and create asymmetries of neural networks in diverse species. As a result, depending on task demands, left- or right-hemispheric loops of feedforward or feedback projections are then activated and can temporarily dominate a neural process. In addition, asymmetries of commissural transfer can shape lateralized processes in each hemisphere. It is still unclear if interhemispheric interactions depend on an inhibition/excitation dichotomy or instead adjust the contralateral temporal neural structure to delay the other hemisphere or synchronize with it during joint action. As outlined in our review, novel animal models and approaches could be established in the last decades, and they already produced a substantial increase of knowledge. Since there is practically no realm of human perception, cognition, emotion, or action that is not affected by our lateralized neural organization, insights from these comparative studies are crucial to understand the functions and pathologies of our asymmetric brain.