Achromatopsia is a form of visual agnosia which results in the loss of color vision. The acquired form of this disorder results from a lesion (or lesions) in the V4 region of the brain.
Though there is no physical problem preventing the eye from responding to color correctly, or the brain from receiving this information, this lesion (or lesions) results in a disconnection with the language region of the brain. As such the individual loses the ability to identify and differentiate between colors.
There is also a genetic (hereditary) form of the condition, which is caused by an abnormality in the retina.