The relation between horizontality and rod-and-frame and vestibular navigational performance.

Sex differences on Piaget's water-level (horizontality) test are well established but poorly understood. In this article, correlates of female horizontality performance are systematically explored. Across the five experiments reported, it was found that female subjects who failed the water-level test (poor-horizontality female subjects) were selectively impaired on tasks that required processing information from the otolith organs. In Experiment 1, poor-horizontality female subjects were found to be impaired relative to good-horizontality subjects on the rod-and-frame test. In Experiment 2, a relation was found between female horizontality performance and the ability to process vestibular information in a passive transport task. Experiment 3 ruled out poor spatial updating as a mediating factor in this relation. The results of Experiments 4 and 5 indicated that poor-horizontality female subjects perform randomly on vestibular navigation because they cannot judge linear displacement under conditions of passive transport. The linear transport task is similar to the rod-and-frame task in that both require the central processing of otolith signals. It is proposed that one way to solve the water-level test is to imagine, on the basis of prior perceptual experiences, what the water level looks like inside tilted containers. Because of complex visual-otolith interactions, poor-horizontality female subjects may experience these events differently than good-horizontality subjects.