A moving grating oriented +/- 45 degrees to the vertical can be perceived at choice as drifting along a left-right or up-down directional axis. When the drifting stimulus is presented alone, direction discrimination thresholds are independent of the specified response-axis. However, they strongly depend on it when the moving stimulus is superimposed on a vertical or horizontal stationary grating. Facilitation is always obtained when the drift direction of the intersections of the two gratings ('blobs') is collinear with the response-axis (i.e. when the orientations of the stationary grating and of the response-axis coincide), while inhibition is observed in the 'noncollinear' cases (i.e. when the orientations of the stationary grating and of the response-axis are orthogonal). These results are generalized in a series of reaction time (RT) experiments where the stimulus configuration described above was set at suprathreshold contrasts and where the orientation/direction of the drifting grating was variable. RT increased when the angle between the response-axis and the direction of the drifting grating increased (uncertainty effect), whether the test stimulus was presented alone, or superimposed on the stationary grating. The uncertainty effect was, however, significantly decreased under 'collinearity' conditions. The attenuation of the uncertainty effect was proportional with the velocity of the blobs and about equal in amount to the RT decrease obtained through the manipulation of the velocity of the drifting grating when presented alone (velocity effect). This observation strongly suggests that both component- and blob/plaid-related information contribute to the directional perception of a compound stimulus and that they sum algebraically.