More recently, we have proposed that the primary visual cortex (V1) creates a bottom-up saliency map (Li,
1999a,
1999b,
2002; Zhaoping,
2005), despite the fact that its neurons are tuned to basic input features. In this theory, the most salient location is the spatial receptive field of the most responsive V1 cell to the input scene, regardless of the input selectivity of the neuron, as if the neurons are bidding for visual selection in an auction using their responses as a universal currency (Zhaoping,
2006; Zhaoping & Dayan,
2006). This proposal was partly motivated by the finding that a V1 neuron's response can be significantly suppressed by contextual inputs outside but near its receptive field (e.g., Allman, Miezin, & McGuinness,
1985; Jones, Grieve, Wang, & Sillito,
2001; Kastner, Nothdurft, & Pigarev,
1997,
1999; Knierim & van Essen,
1992; Lamme,
1995; Li & Li,
1994; Nothdurft, Gallant, & van Essen,
1999; Sillito, Grieve, Jones, Cudeiro, & Davis,
1995; Wachtler, Sejnowski, & Albright,
2003): The response to its preferred input feature, in orientation, color, or motion direction etc., is much more suppressed when there are similar rather than very different input features in the nearby context. Specific examples of such iso-feature suppressions include iso-orientation suppression (e.g., Knierim & van Essen,
1992), iso-color suppression (Wachtler et al.,
2003), and iso-motion-direction suppression (Jones et al.,
2001). The intra-cortical connections linking nearby V1 neurons (Gilbert & Wiesel,
1983; Rockland & Lund,
1983) are believed to mediate the suppression. As an instantiation of the theory, we showed that the responses of a physiologically based model of V1 that incorporates such connections can account for much of the behavioral data on visual searches and segmentation tasks, reflecting bottom-up saliency (Li,
1999a,
1999b,
2000,
2002). In particular, iso-feature suppression makes V1 response highest to feature singletons. For instance, the neuron responding to an orientation singleton in a background of uniformly oriented bars is typically the most responsive neuron since it is the only responding neuron to escape from the iso-orientation suppression experienced by the other neurons responding to the background bars. This most active neuron attracts attention to its receptive field, making the singleton salient.