There is much physiological evidence that surround-suppressive mechanisms exist in MT/V5 (Allman et al.,
1985; Born,
2000; Bradley & Andersen,
1998; DeAngelis & Uka,
2003) and elsewhere in the visual system. This surround suppression undoubtedly serves an important function by reducing redundancy in a visual scene (Born & Bradley,
2005). Center–surround inhibition may explain the fact that the motion aftereffect (MAE) is weaker in the presence of a high-contrast surround moving in the same direction (Tadin et al.,
2003) but stronger if the surround moves in a sufficiently different direction (Falkenberg & Bex,
2007) and similar center–surround effects may explain the modulation of binocular rivalry by surrounds moving in the same direction (Paffen, Tadin, te Pas, Blake, & Verstraten,
2006; Paffen, van der Smagt, te Pas, & Verstraten,
2005). In this paper, we have not examined evidence for center–surround antagonism obtained using the MAE or binocular rivalry as a probe, although whether there exist any contrast sensitivity-dependent effects in these experiments obviously warrants investigation. It may well be that the effects of surround suppression have a psychophysical correlate in terms of the MAE or binocular rivalry, but it seems unlikely that these effects result from surround suppression in V5, since both of these techniques use long duration stimuli that are beyond the interval over which surround-suppressed neurons dominate (Churan, Khawaja, Tsui, & Pack,
2008). In addition, we do not think that the direction discrimination data, obtained with short duration stimuli, are necessarily a perceptual consequence of surround suppression. The very similar results presented here cast some doubt on whether the duration threshold technique of Tadin et al. (
2003) is actually measuring the action of surround-suppressed neurons in V5 (Churan, Khawaja, Tsui, & Pack,
2008; Pack et al.,
2005), since qualitatively and quantitatively similar effects appear to be possible at durations longer than those that preferentially activate these cells.