A 7-year-old boy is referred to you by the school nurse because of concerns about his vision. He apparently has been having difficulty in certain classes and has been noted to have erroneously colored some assignments. As a quick screening test, you show the child some Ishihara color plates, and he is unable to see the numbers on some of them, indicating that he has dyschromatopsia specifically discriminating between red and green. What cell type is most likely abnormal in this child's retina?