Representing colour through hearing and touch in sensory substitution devices

Visual sensory substitution devices (SSDs) allow visually-deprived individuals to navigate and recognise the 'visual world'; SSDs also provide opportunities for psychologists to study modality-independent theories of perception. At present most research has focused on encoding greyscale vision. However at the low spatial resolutions received by SSD users, colour information enhances object-ground segmentation, and provides more stable cues for scene and object recognition. Many attempts have been made to encode colour information in tactile or auditory modalities, but many of these studies exist in isolation. This review brings together a wide variety of tactile and auditory approaches to representing colour. We examine how each device constructs 'colour' relative to veridical human colour perception and report previous experiments using these devices. Theoretical approaches to encoding and transferring colour information through sound or touch are discussed for future devices, covering alternative stimulation approaches, perceptually distinct dimensions and intuitive cross-modal correspondences.