Responding prior to all stimuli

[sliening]

[edited by moderator]

I am trying to set-up a modified stroop task. Each trial should run as such: fixation cross (500ms), color face (20ms-540ms), color mask.

The problem I have run into is that subjects are able (correctly) to respond while the face is being presented but before the mask. Currently, the subjects have to wait until the mask is presented in order to respond. Is there any way to allow for responses prior to all stimuli having been presented?