Speed decrements are seen better than increments through small apertures

Five subjects observed a random dot pattern that moved horizontally within a rectangular, 40 deg x 4 deg invisible aperture. The long side of this aperture was either parallel or perpendicular to the direction of motion. A two-interval forced-choice procedure was employed for measuring the thresholds for detection of 100-ms speed increments and decrements superimposed on uniform motion of 1.1 s duration and base speeds of 8 deg/s, 16 deg/s, 24 deg/s, and 32 deg/s. The aperture orientation had almost no effect on the detection of decrements. With increasing base speed, the thresholds for increments within the perpendicular aperture rose markedly as compared to those for increments within the parallel aperture and to those for decrements. The results are interpreted in the context of facilitatory interactions between the motion detectors along the trajectory of motion of each individual dot. It is shown how, having rapid motion within the perpendicular aperture, increment detection can severely deteriorate owing to the short lifetime and the small visible path of the dots, whereas decrement detection may be almost unaffected.