All the above observations indicate that inhibition can sharpen the tuning of cortical neurons without being itself tuned oppositely to excitation, but rather by being as equally tuned as excitation, more broadly tuned or not tuned at all. Not surprisingly, the tuning properties of inhibition measured in principal neurons are consistent with the tuning properties of inhibitory interneurons. In some systems, interneurons and principal cells show similar stimulus selectivity in their firing (Cardin et al., 2007; Runyan et al., 2010), while in others cortical inhibitory neurons appear to be less sharply tuned than principal cells (Fig. 6) (Kameyama et al., 2010; Kerlin et al., 2010; Liu et al., 2009; Niell and Stryker, 2008; Poo and Isaacson, 2009; Sohya et al., 2007; Swadlow, 1988). One possibility that would account for the differences in interneuron tuning properties observed in different systems is that they receive convergent excitatory inputs from surrounding principal cells irrespective of their tuning properties (Bock et al., 2011). In other words, the tuning of an interneuron may reflect the average tuning of the network of excitatory neurons it is
