We measured psychophysical thresholds for the detection of four different optic flow components in the presence of a translational velocity. We also measured thresholds for detection of rotation in the presence of expansion and for expansion in the presence of rotation. Our stimuli consisted of sparse random dot patterns. Detection thresholds are similar for all four optic flow components. Thus, our experiments indicate that our subjects use a factor that is similar in all first-order flow components, namely the relative orientation of the velocity vectors. First-order components can be extracted independently of both each other and translational velocity.