Perceptual limits of common fate

We studied the perception of a coherently moving group of collinearly arranged dots ("target dots") that traveled orthogonally to their linear orientation within a background of noise dots moving in random yet straight directions at constant speed ("random-direction noise"). Using a 2-interval forced-choice task we obtained coherence thresholds equal to a signal-to-noise ratio of 1-2%. These thresholds are lower than the 4-10% reported in the literature suggesting that the collinear arrangement of the target dots, in addition to movement, provided form information. Weber's Law was found to hold 4-7 target dots. Overall, sensitivity was constant for a broad range of dot speeds up to at least 6.5 deg/s. Lifetime required for optimal perception was 430 ms, far shorter than the threshold duration of 1 s reported for randomly distributed (i.e., nonaligned) target dots [Vis. Res. 41 (2001) 1891]. Angular deviations from parallel between adjacent motion trajectories were tolerated up to 27 deg for divergence and up to 19 deg for convergence. Diverging motion was detected earlier (after 600-800 ms) than converging motion (>1 s). Forced-choice discrimination yielded a higher proportion of correct responses than the actual (i.e., conscious) perception of the coherently moving group of dots. Our results are consistent with findings from neurophysiological recordings and neuroimaging of motion-sensitive neurons in areas V1 and MT showing broad tuning curves for speed and direction of a moving visual stimulus.