The time course of the oblique effect in orientation judgments.

It is well known that maximal sensitivity to subtle orientation differences around a cardinal axis exceeds that around an oblique axis. In principle, this oblique effect in orientation sensitivity could either be constant across stimulus durations or could evolve as stimulus durations increase. To distinguish between these possibilities, we asked participants to judge subtle (4 deg) angular differences between pairs of gratings that were presented for various durations and masked to limit neural persistence. When the gratings were presented successively and for just 8.33 ms each, the ability to judge subtle (4 deg) orientation differences was already reliably better than chance, but comparable around cardinal and oblique axes. The oblique effect emerged only at subsequent stimulus durations, and increased across the tens of milliseconds after reliable (if modest) orientation sensitivity had occurred. These additional tens of milliseconds appear to be necessary but not sufficient for the oblique effect, which was absent at these durations when the stimuli were presented simultaneously rather than successively. Relative to simultaneously presented stimuli, successively presented stimuli generated a reduction in oblique orientation sensitivity, not an enhancement in cardinal orientation sensitivity. We believe the data suggest that the oblique effect in orientation sensitivity is a dynamic phenomenon that can be influenced by the neural events occurring between two successively presented stimuli.