Inhibition versus attentional momentum in cortical and collicular mechanisms of IOR.

Inhibition of return (IOR)-the automatic bias against returning attention or gaze to recently visited locations-is thought to have both collicular and cortical components and has been associated with the oculomotor system. Recently, distinct IOR mechanisms have been revealed that may have collicular and cortical origins: While standard luminance stimuli cause IOR in both manual and saccadic eye movement responses, "S cone" stimuli, which are invisible to the direct collicular pathway, caused manual IOR but not saccadic IOR. However, it has not been shown that the separate mechanisms are both inhibition of return, rather than facilitation due to attentional momentum or a visual motion transient. Here, we examined this question using four target and cue locations instead of two. Inhibition at the cued location predicts that responses for all noncued locations should be similar, whereas facilitation at the location opposite the cue predicts that the perpendicular locations would be more similar to the cued location than to the opposite location. Our results conform to the former prediction for both saccadic IOR and S cone generated IOR, demonstrating that both mechanisms of IOR are indeed inhibitory.