The psychophysical effect of flankers may be caused by surround processing in visual neurons, whereby features outside of the classical receptive field modulate neural responses (Angelucci et al.,
2002; Chisum & Fitzpatrick,
2004). The amplitude of neural responses in the retina, thalamus, and visual cortex are normalized to spatially nearby contrast (Carandini, Heeger, & Movshon,
1997; Heeger,
1992; Shapley & Victor,
1979), likely due to lateral connectivity at each level. In many cases, oriented stimuli in the surround suppress activity and suppress most when surround orientation matches the driving stimulus (Bonds,
1989; Cavanaugh, Bair, & Movshon,
2002; Polat, Mizobe, Pettet, Kasamatsu, & Norcia,
1998). However, some cells in the appropriate contrast conditions increase their spiking activity when the orientation of stimuli in the surround matches the driving stimulus (Li et al.,
2006; Polat et al.,
1998; Sillito, Cudeiro, & Murphy,
1993). In many physiology experiments, oriented surround stimuli are presented in an annulus. However, physiological surround effects can depend on the angular position of the flanker with respect to the target orientation, specifically influencing geometric arrangements like collinearity (Cavanaugh et al.,
2002; Polat et al.,
1998). Cortical circuits are likely to be involved in orientation-specific surround processing (Chisum, Mooser, & Fitzpatrick,
2003; Das & Gilbert,
1999; Gilbert & Wiesel,
1989). It remains unknown if the orientation-selective circuits described in cats and primates are also found in rats. Rodents have orientation-tuned cells in V1 but lack orientation columns (Ohki, Chung, Ch'ng, Kara, & Reid,
2005; Van Hooser, Heimel, Chung, & Nelson,
2006).