p359fig10.06 Another, albeit indirect, pathway from LGN exists that can also excite layer 4 of V1.  Why are not these two pathways redundant?  The answer, ultimately, how to do with how cortex learns, as well as with how it pays attention.  See the text for details.<BR>||  Another bottom-up input to layer 4: Why?? Layer 6-to-4 on-center off-surround (Grieve, Sillito 1991, 1995; Ahmedetal 1994, 1997).  LGN projects to layers 6 and 4.  Layer 6 excites spiny stellates in column above it.  Medium range connections onto inhibitory neurons.  6-t-4 path acts as on-center off-curround.