DETECTING A TRAJECTORY EMBEDDED IN RANDOM-DIRECTION MOTION NOISE

Human observers can easily detect a signal dot moving, in apparent mot ion, on a trajectory embedded in a background of random-direction moti on noise. A high detection rate is possible even though the spatial an d temporal characteristics (step size and frame rate) of the signal ar e identical to the noise, making the signal indistinguishable from the noise on the basis of a single pair of frames. The success rate for d etecting the signal dot was as high as 90% when the probability of mis match 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 det ection improves with increases in duration, up to 250-400 msec, a dura tion longer than the integration typically associated with a single mo tion detector. For another, the signal dot need not travel in a straig ht 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 f rame. Consistent with this, circular paths of sufficiently low curvatu re were as detectable as straight trajectories. Our data suggest that trajectory motion is highly detectable in motion noise because the com ponent local motion signals are enhanced when motion detectors with si milar directional tuning are stimulated in a sequence along their pref erred direction.