Some white cats suffer from congenital deafness caused by degeneration of the inner ear. This condition is associated with blue irises. In white cats with mixed-coloured eyes (odd-eyed cats), it has been found that deafness is usually limited to the ear on the blue-eyed side. White cats can have blue, gold, green, or copper coloured odd eyes. In one study of white cats with varying degrees of hearing deficiency, 72% of the animals were found to be totally deaf.