M4 (IMAGE) Johns Hopkins Medicine Caption An M4 intrinsically photosensitive retinal ganglion cells (ipRGCs) dyed in red. Credit Zheng Jiang, Ph.D., Johns Hopkins University School of Medicine Usage Restrictions None License Licensed content Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.