Locating the singular point in first-order optical flow fields

The authors investigated how well participants were able to locate the singular point in an expanding or rotating flow field. The participants' responses gave insight into the magnitude and direction of possible deviations from the veridical point and into the reproducibility of the participants' answers. The stimulus window was circular with a radius of 10°. The performance of the participants improved with increasing expansion or rotation of the stimulus. When either the outer rim or the central part of the stimulus was blocked out, performance was not affected when singular points were inside the visible part of the stimulus. Singular points that were outside the visible part of the stimulus, however, were judged to be closer to the rim of the visible part than they actually were. Performance deteriorated with increasing eccentricity of the entire stimulus area (up to 20°, keeping the field size constant), whereas it was not affected by the eccentricity of the singular point within the stimulus. A drastic limitation of the presentation period had only a quantitative effect on performance.