Abstract
Three-motion-system theory proposes that, at the early stage, dynamic scenes are analyzed independently by three different motion systems; the first-order system extracts motion signals defined by luminance, the second-order system extracts motion defined by contrast modulations, the third-order system extracts motion defined by salience (Lu & Sperling, 1995 Vision Research, 35, 2697-2722). So far, most of the work regarding the three-motion-system theory has focused on one-dimensional, drifting sine-wave or square-wave gratings whose motion is defined only in the direction perpendicular to the grating orientation. Here, we extend the application of the three-motion-system theory to two-dimensional plaid stimuli that are composed of two equal-spatial-frequency sine-wave gratings. We show that a 20-30Hz plaid, which only activates first-order system, is perceived to move coherently in between the directions of the plaid’s two sine-wave components. Moreover, the perceived direction is perfectly predicted by a combinational rule that assigns weights to each sine-wave component according to a power function of their contrasts. The power is between 1.5 and 2.5 for different subjects (Sperling et al, 2020 Psychological Review, 127:305-326). At 1Hz which preferentially activates the third-order system, a plaid composed of two equal-contrast, equal-spatial-frequency sine waves is perceived to move coherently in the pattern direction (the motion direction of maxima and minima at intersections of the sine-wave components). At intermediate temporal frequencies (3Hz), a plaid is perceived to move, also coherently, in between the pure first- and the pure third-order system’s predicted directions. The perceived direction for the intermediate-temporal-frequency plaids reflects the additive combination of the first- and third-order system’s outputs. We demonstrate that the strength of the third-order system varies with contrast. The subjects’ perceived directions for 3Hz plaids reveal that the strength of the third-order system also is a power function of contrast with exponents between 2.9 and 4 for different subjects.