Aphantasia is inability to imagine images in mind. Affects 2.5 to 4 percent of population. Usually congenital. Can result from stroke, head injury, or depression. Opposite is hyperphantasia with vivid mental images. Galton discovered it in 1880. Ribot noted in 1897 some see words like ''dog'' instead of dog image. Rarely studied until Zeman's 2005 case of post-surgery loss. Zeman's 2015 paper defined congenital aphantasia where visualization never existed.