[1] Univ Penn, Dept Neurosci, Philadelphia, PA 19104 USA
[2] Max Planck Inst Hirnforsch, Max Laue Str 4, D-60438 Frankfurt, Germany
[3] Univ Tubingen, Inst Ophthalm Res, Werner Reichardt Ctr Integrat Neurosci, Otfried Muller Str 25, D-72076 Tubingen, Germany