p359fig10.05 Activation of V1 is initiated, in part, by direct excitatory signals from the LGN to layer 4 of V1.<BR>||  How are layer 2/3 bipole cells activated? Direct bottom-up activation of layer 4.  LGN -> V1 layer 4.  Strong bottom-up LGN input to layer 4 (Stratford etal 1996; Chung, Ferster 1998). Many details omitted.