Visual stimuli.

<p>The visual target stimulus was a low contrast grating added to a background of random visual noise of 20% contrast. The grating could be presented counterclockwise or clockwise by 45° relative to vertical. In this figure, the visual target is shown at suprathreshold contrast, although in the experiment the contrast was much lower and was adjusted over trials to find the contrast detection threshold. In the non-target interval, only the noise was present, and the order of target and null intervals was randomized over trials.