The feedforward model with push-pull inhibition and same-phase excitation is consistent with a number of observations obtained from intracellular recordings of simple cells and described above. First there is the motivating observation that excitation and inhibition are spatially opponent (push-pull inhibition). Second, the orientation tuning of inhibition to simple cells in the model is identical to that of excitation: Both peak at the preferred orientation and fall off to small values at the orthogonal orientation, as observed in vivo. Third, the model is consistent with the cortical inactivation experiments (Chung and Ferster, 1998, Ferster et al., 1996), which show that the orientation tuning of visually-evoked responses does not change when cortical cells are silenced, and show additionally that the cortex amplifies responses to the LGN inputs 2- to 3-fold. Fourth, the model explains why push-pull inhibition must be so strong relative to feedforward excitation. The behavior of Troyer et al. (1998)'s model conforms to that of simple cells in two other important ways. Iontophoresis of GABA antagonists into the visual cortex have long been known to degrade or even abolish orientation selectivity (Tsumoto et al., 1979, Sillito, 1975), whereas intracellular blockade of GABA inhibition in a single cell has little effect on the cell's orientation selectivity (Nelson et al., 1994). The model closely mimics these behaviors. Because it is a variant of the feedforward model and because it relies on the spatial organization of the geniculate input for establishing orientation selectivity, Troyer et al's model also exhibits a decrease in orientation tuning width with increasing stimulus spatial frequency (Jones et al., 1987, Hammond and Pomfrett, 1990, Vidyasagar and Sigüenza, 1985, Webster and De Valois, 1985). Again, the existence of interneurons with the proposed response properties is a key test of the model.