Background. The rate of binocular rivalry has been reported to be slower in subjects with bipolar disorder than in controls when tested with drifting, vertical and horizontal gratings of high spatial frequency.

Binocular rivalry occurs when conflicting images are presented in corresponding locations of the two eyes. Perception alternates between the images at a rate that is relatively stable within individuals but that varies widely between individuals. The determinants of this variation are unknown. In addition, slow binocular rivalry has been demonstrated in bipolar disorder, a psychiatric condition with high heritability. The present study therefore examined whether there is a genetic contribution to individual variation in binocular rivalry rate. We employed the twin method and studied both monozygotic (MZ) twins (n = 128 pairs) who are genetically identical, and dizygotic (DZ) twins (n = 220 pairs) who share roughly half their genes. MZ and DZ twin correlations for binocular rivalry rate were 0.51 and 0.19, respectively. The bestfitting genetic model showed 52% of the variance in binocular rivalry rate was accounted for by additive genetic factors. In contrast, nonshared environmental influences accounted for 18% of the variance, with the remainder attributed to measurement error. This study therefore demonstrates a substantial genetic contribution to individual variation in binocular rivalry rate. The results support the vigorous pursuit of genetic and molecular studies of binocular rivalry and further characterization of slow binocular rivalry as an endophenotype for bipolar disorder.perceptual rivalry | bipolar disorder | endophenotype | genetics | twins B inocular rivalry has been widely investigated in the visual sciences for more than 100 years. The phenomenon (Fig. 1A) is thought to engage a series of neural processes at different levels of the visual hierarchy (1). However, a detailed understanding of processing at each level and of interactions between levels has yet to be achieved. One aspect of binocular rivalry that has been studied extensively is its temporal dynamics. Several extrinsic factors are known to determine binocular rivalry rate and the strength of one stimulus over its rival (i.e., predominance). These factors include the contrast, spatial frequency, velocity, and semantic context of the stimuli (1, 2). Far less is known about the intrinsic factors that determine binocular rivalry rate when stimulus and ambient conditions are held constant, despite several studies showing that the rate of binocular rivalry varies widely between individuals but is relatively stable within individuals (3-8). The present study investigated whether there is a genetic contribution to individual variation in binocular rivalry rate.Our interest was motivated by a lack of focus on individual differences in binocular rivalry research and of models addressing such differences. We also were motivated by data (7,8) showing slow binocular rivalry may be an endophenotype for bipolar disorder, a condition with high heritability (0.59-0.85) (9, 10). A key feature of a putative endophenotype is that it should be a heritable trait (11). To examine the heritability of binocular rivalry, we employed the twin method and studie...