Heading direction is indicated by vestibular signals and patterns of image motion across the retina (optic flow). Heading precision improves when vestibular cues and optic flow are integrated. Such multisensory integration will be beneficial only if the two cues are produced by the same cause, that is, movement of the observer in a world-fixed environment. However, optic flow possesses an inherent ambiguity which poses a crucial challenge to heading perception: optic flow indicates the combined movement of the observer as well as movement of objects in the world. Using quantitative human psychophysics and mathematical modeling, here I test the hypothesis that the brain implements a decision process that infers the causes associated with vestibular and optic flow cues. According to this decision process, termed "causal inference", the two cues are integrated if it is inferred that a common cause produced them. Causal inference thus predicts that the multisensory heading percept will depend on the magnitude of the conflict between visual and optic flow heading information as well as the reliability of these cues. In addition, I will investigate the influenceof moving objects on heading perception within the causal inference framework, which predicts that the influence of moving objects on heading perception will decrease as the velocity of object motion in the world increases. A 6 degrees-of- freedom motion platform with attached large field-of-view stereo projection system and two-alternative-forced- choice methodology will be used for all experiments. I will develop mathematical models of the causal inference process and will rigorously compare the predictions of these models with the experimental data using Bayesian model comparison techniques. Findings of this research are important for understanding basic perceptual vestibular-visual interactions and opening new directions in the fields of basic and clinical spatial orientation psychophysics.
Understanding multisensory integration in self-motion perception is essential for treating spatial disorientation deficits prevalent in patients with vestibular and sensorimotor disorders. However, how information from visual and vestibular senses is integrated in healthy individuals is still a mystery which poses a major hurdle in understanding and treating perceptual deficits in these patients. The experiments proposed in this grant application aim at filling a very notable gap in our knowledge of heading perception, important for identifying and ultimately treating deficits in heading perception.