The Processing of What, Where, and How

Vision is the default sensory modality for normal spatial navigation in humans. Touch is restricted to providing information about peripersonal space, whereas detecting and avoiding obstacles in extrapersonal space is key for efficient navigation. Hearing is restricted to the detection of objects that emit noise, yet many obstacles such as walls are silent. Sensory substitution devices provide a means of translating distal visual information into a form that visually impaired individuals can process through either touch or hearing. Here we will review findings from various sensory substitution systems for the processing of visual information that can be classified as what (object recognition), where (localization), and how (perception for action) processing. Different forms of sensory substitution excel at some tasks more than others. Spatial navigation brings together these different forms of information and provides a useful model for comparing sensory substitution systems, with important implications for rehabilitation, neuroanatomy, and theories of cognition.