Motion of complex patterns is computed from the perceived motions of their components

A plaid pattern made by adding two gratings of the same spatial frequency, one moving 45 deg above the horizontal, and the other moving 45 deg below the horizontal, appears to move horizontally when the speeds of the two components are equal. If the apparent speed of the upward-moving component is reduced by a motion after-effect (MAE), the plaid appears to move obliquely downwards, unless the actual speed of the downward-moving component is reduced to match the (reduced) apparent speed of the upward moving component. This is consistent with the hypothesis that the visual system computes the motion of a plaid pattern in two stages, first estimating the motions of the components, and then combining them according to the intersection of constraints. An alternative explanation: that the vertical component of the plaid's motion is caused by an MAE in a horizontally oriented distortion product generated by non-linear transduction or transmission of the plaid, is ruled out by the finding that the adapting stimulus causes only a very weak vertical MAE.