p359fig10.07 The two bottom-up pathways from LGN to layer 4 of V1 can together activate layer 4 and contrast-normalize layer 4 responses.<BR>||  Bottom-up contrast normalization (Grossberg 1968, 1973; Sperling, Sondhi 1968; Heeger 1992; Douglas etal 1995; Shapley etal 2004).  Together, direct LGN-to-4 path and 6-to-4 on-center off-surround provide contrast normalization if cells obey shunting or membrane equation dynamics.