Limitations of subjective estimation of differences method, which is used in modeling stimuli discrimination spaces, are discussed in the article. A new approach is suggested, that is based on using «pop-out effect» in visual search. The suggested method of measuring differences among stimuli is validated. The experiment shows that the efficiency of visual search (measured on its time and accuracy) is an adequate measure of differences among stimuli, which is correlated with subjective estimation of difference. It is shown that at visual search task oriented lines are discriminated in a three-dimensional spherical space.