A Comparison among some Models of V1 Orientation Selectivity

Several models exist for explaining V1 orientation tuning. The modified feedforward model (MFM) and the recurrent model (RM) are major examples. We have implemented, at the same level of detail, these two models alongside a few newer variations, and thoroughly compared their receptive-field structures. We found that anti-phase inhibition in the MFM enhances both spatial phase information and orientation tuning, producing well-tuned simple cells. This remains true for a newer version of the MFM that incorporates un-tuned complex-cell inhibition. In contrast, when the recurrent connections in the RM are strong enough to produce typical V1 orientation tuning, they also eliminate spatial phase information, making the cells complex. Introducing phase-specificity into the connections of the RM (as done in an original version of the RM) can make the cells phase sensitive, but the cells show an incorrect 90° peak-shift of orientation tuning under opposite contrast signs. An inhibition-dominant version of the RM can generate well-tuned cells across the simple/complex spectrum, but it predicts that the net effect of cortical interactions is to suppress feedforward excitation across all orientations in simple cells. Finally, adding anti-phase inhibition used in the MFM into the RM produces a most general model. We call this new model the modified recurrent model (MRM) and show that this model can also produce well-tuned cells throughout the simple/complex spectrum. Unlike the inhibition-dominant RM, the MRM is consistent with data from cat V1 suggesting that the net effect of cortical interactions is to boost simple cell responses at the preferred orientation. These results suggest that the MFM is well suited for explaining orientation tuning in simple cells, whereas the standard RM is for complex cells. The assignment of the RM to complex cells also avoids conflicts 3 between the RM and the experiments of cortical inactivation (done on simple cells) and the spatial-frequency dependence of orientation tuning (found in simple cells). Since orientation-tuned V1 cells show a continuum of simple-to complex-cell behavior, the MRM provides the best description of V1 data.