Our brain is divided into two hemispheres, which are linked through only a few connections. However, we do not seem to have a problem to create a coherent image of our environment our perception is not "split" in two halves. For the seamless unity of our subjective experience, information from both hemispheres needs to be efficiently integrated. The corpus callosum, the largest fibre bundle connecting the left and right side of our brain, plays a major role in this process. Researchers from the Max Planck Institute for Brain Research in Frankfurt investigated whether differences between individuals in the anatomy of the corpus callosum would predict how observers perceive a visual stimulus for which the left and right hemisphere need to cooperate. As their results indicate, the characteristics of specific callosal fibre tracts are related to the subjective experience of individuals.
In their study, Erhan Gen and colleagues used a motion illusion, called the "motion quartet", which can be perceived in two different ways. The "motion quartet" induces the phenomenon of apparent motion, where the impression of motion is caused by a sequence of static stimuli. This is similar to movies in TV or cinema, which consist of a sequence of still pictures that nevertheless generate a perception of natural dynamics. In the experiments, the stimuli are made up of four white squares in a rectangular arrangement. There are only two alternating movie frames with two pairs of diagonally opposing squares (upper left plus lower right vs. upper right plus lower left). In this case, observers see either horizontal or vertical motion; sometimes their perception switches between the two interpretations, although the stimulus remains unchanged.
Interestingly, it has been found that individuals predominantly perceive vertical motion when the distance between the four squares is equal and observers fixate at the centre of the quartet. Due to the organization of the vis
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