A visually-induced eyelid droop illusion as a classroom demonstration of cross-modality

Cross-modality, or the interaction between the different senses, has emerged as a fundamental concept in perceptual neuroscience and psychology. The traditional idea of five separate senses with independent neural substrates has been invalidated by both psychophysical findings of sensory integration and neurophysiological discoveries of multi-modal neurons in many areas of the brain. Even areas previously thought to be unimodal have been shown to be influenced by other senses, thus establishing multisensory integration as a key principle of perceptual neuroscience. There are several obstacles to students' understanding of the concept. First, everyday subjective experience is modal: one sees, hears, smells the world and is rarely aware that these seemingly separate impressions are in reality fully integrated with each other. Second, standard content in undergraduate classes and textbooks still emphasizes the modal model of the senses and their corresponding brain areas and rarely mentions cross-modal phenomena. Third, feasible classroom demonstrations of cross-modality are few, making it difficult to provide students with first-hand experience that would aid their understanding of the principle. This article describes an accessible and effective classroom demonstration of cross-modality between low-level vision, touch and proprioception. It consists in the illusion of eyelid droop in one eye when the other eye has been dark-adapted and when both eyes are exposed to the dark. The perceptual effect is dramatic and reliable. It illustrates cross-modality at a fundamental level of perception and might provide a means to help integrate the teaching of the concept into the standard content of undergraduate classes.