Comparison of difference limen (DL; with s.e. bars) between the control group and the DXM group for amplitude discrimination with dual-site adaptation at times T0, T1, and T2 (see Methods for description). DLs are shown for two conditions of adapting stimulus duration: 500 and 2000 msec. Note that at T1, performance was significantly degraded for the DXM group under both conditions of adapting stimulus duration (p < 0.01). DLs show signs of recovering to baseline levels at T2, although they remain significantly elevated at that time (p < 0.01).