Two mechanisms underlying the effect of angle of motion direction change on colour–motion asynchrony

Under appropriate stimulus conditions, judgments about the degree of temporal synchrony in sequences containing rapid alternations of colour and motion direction imply a large apparent delay of motion perception relative to colour perception. Whether this colour–motion asynchrony results from the relative processing delay of different visual attributes, or from inappropriate matching of time markers assigned to first-order change of colour and position has been the subject of recent debate. Colour–motion asynchrony is significantly weakened when the angle of direction change is reduced from 180° (direction reversal) to a smaller change in direction. Although this finding has been interpreted to favour the processing delay hypothesis, here we show that it is consistent with the time marker account. First, the reported dependence on the motion direction angle was particularly strong for random-dot stimuli, but our results indicate that this may reflect the introduction of an artefact, motion streaks, that allows subjects to make a colour–orientation synchrony judgement rather than a colour–motion synchrony judgment for direction change angles other than 180°. Second, when we used streak-free plaid stimuli, a certain amount of angle dependence remained regardless of whether we asked the observers to judge the apparent binding or synchrony of colour and motion direction changes. The degree of direction change also affected reaction times, but the effect of apparent asynchrony for a direct comparison of sequences of 90° and 180° motion direction changes was very small, if at all present. These findings with plaid stimuli are consistent with the time marker account; in that we allow that the direction change angle can affect the time course of the recruitment of neural responses to the new direction of motion, which will have a consequential effect on the temporal location of salient features in the sequence of motion changes.