Aftereffects have been considered as a misinterpretation of the real world and an undesired side effect of adaptation (Krekelberg, van Wezel, & Albright,
2006). Does any adaptation have a beneficial effect on visual perception? One attempt to address this question is to investigate its effect on subjects' sensitivity to the adapting attribute, which can be assessed by the change of discrimination threshold. Although many experiments have been conducted to address this issue, evidence is equivocal and it is still difficult to draw a definite conclusion. Some studies have reported improved contrast discriminability after adaptation (Abbonizio, Langley, & Clifford,
2002; Greenlee & Heitger,
1988), while others have not (Maatanen & Koenderink,
1991). In the orientation domain, although the studies by Regan and Beverlay (
1985) and Clifford, Wyatt, Arnold, Smith, and Wenderoth (
2001) both reported a significant facilitation effect around the adapting orientation using a long-term adaptation paradigm, Dragoi, Sharma, Miller, and Sur (
2002) reported the opposite effect (i.e., impairment) after a brief adaptation. With respect to motion direction, Phinney, Bowd, and Patterson (
1997) reported a reduced discrimination threshold around the adapting direction using stereoscopic motion stimuli, while Hol and Treue (
2001) reported no adaptation effect using moving random dot patterns. Psychophysical and electrophysiological studies (Bex, Bedinghan, & Hammett,
1999; Clifford & Wenderoth,
1999; Krekelberg et al.,
2006) on speed adaptation found a slight enhancement of speed discrimination at the adapting speed, but Kristjansson (
2001) observed a similar beneficial effect only when subjects adapted to first-order, not to second-order, motion. Evidence about adaptation effect on spatial frequency discrimination seems to be more consistent. Two studies (Greenlee & Thomas,
1992; Regan & Beverlay,
1983) both reported little or no effect on spatial frequency discrimination around the adapting frequency. Taken together, these studies suggest that the functional role of adaptation in low-level vision is still far from being established.