Rod monochromatism; an incomplete form

An incomplete form of rod monochromatism is described in a young man with normal visual acuity and absence of nystagmus or photophobia. ERG showed normal threshold sensitivity in white and blue lights but virtual absence of the first portion of response in red light. The sensory threshold curves lacked the typical rod cone discontinuity, whereas the curve after full dark adaptation was normal. It is suggested that the condition is due to atypical dysfunction of the cones.