Both electro-physiological and psychophysical studies point to the existence of detectors specialised for the analysis of optic Row. However, it is unclear whether these detectors are tuned to specific 'cardinal directions' (such as radial and circular motion), or whether they respond equally to ail directions of optic-flow motion, including intermediate spiral motions. Here summation and masking studies of motion coherence sensitivity are reported that suggest that optic Row may be tuned to radial and circular cardinal directions. Strong summation was found between two orthogonal directions of spiral motion. but much weaker summation between radial and circular motion. As orthogonal spiral motions always contain a common radial or circular component, the stronger summation for these motions implies that detectors are tuned to radial and circular directions. Similarly, the most effective masking stimuli (placed adjacent to bur not superimposed on the test stimuli) tended to be those in the radial or circular directions, even for spiral targets, further suggesting that flow-field motion is detected and discriminated by mechanisms tuned to these 'cardinal' directions. (C) 2001 Elsevier Science Ltd. All rights reserved.