Perhaps the most fundamental prediction of the feedforward model is that simple cells should receive strong excitatory synaptic input from geniculate relay cells. It was found early on in the study of area 17 of the cat that simple cells lie predominately in layers 4 and 6 (Hubel and Wiesel, 1962, Shatz and Stryker, 1978, Kelly and van Essen, 1974, Bullier and Henry, 1979, Gilbert, 1977), the same layers in which the relay cells terminate (LeVay and Gilbert, 1976, Rosenquist et al., 1975). Electrical stimulation of the optic radiations or LGN, combined with intracellular measurements of the latency of the evoked EPSPs confirmed that most simple cells in layer 4 indeed received substantial monosynaptic excitation from relay cells (Ferster and Lindström, 1983, Martin and Whitteridge, 1984, Bullier and Henry, 1979). Cross correlation analysis on simultaneously recorded simple cells and geniculate relay cells also point to the presence of a direct connection between relay cells and simple cells (Tanaka, 1983, Reid and Alonso, 1995).