Detecting a trajectory embedded in random-direction motion noise.

Human observers can easily detect a signal dot moving, in apparent motion, on a trajectory embedded in a background of random-direction motion noise. A high detection rate is possible even though the spatial and temporal characteristics (step size and frame rate) of the signal are identical to the noise, making the signal indistinguishable from the noise on the basis of a single pair of frames. The success rate for detecting the signal dot was as high as 90% when the probability of mismatch from frame-to-frame, based on nearest-neighbor matching, was 0.3. Control experiments showed that trajectory detection is not based on detecting a "string" of collinear dots, i.e. a stationary position cue. Nor is a trajectory detected because it produces stronger signals in single independent motion detectors. For one thing, trajectory detection improves with increases in duration, up to 250-400 msec, a duration longer than the integration typically associated with a single motion detector. For another, the signal dot need not travel in a straight line to be detectable. The signal dot was as reliably detected when it changed its direction a small amount (about 30 deg or less) each frame. Consistent with this, circular paths of sufficiently low curvature were as detectable as straight trajectories. Our data suggest that trajectory motion is highly detectable in motion noise because the component local motion signals are enhanced when motion detectors with similar directional tuning are stimulated in a sequence along their preferred direction.