While human CLN1 patients present with retinal degeneration and the Ppt1-/- mouse model of CLN1 phenocopies the human disease, the electroretinogram (ERG) is effectively unaltered at 4 months in the mouse model (Lei et al., 2006), allowing for detailed study of the electrophysiological changes in the visual cortex associated with early disease states. Here, PPT1 is linked to retinal degeneration.