Psychophysical stimuli.

The target is located above the fixation cross in all panels. Panel A. shows the no mask condition for which only the target was shown. Panel B shows the surround masking condition (mask orientation offset 0°). Panel C shows the overlay mask condition with a mask orientation offset of 0° (i.e. the mask and target are collinear). Under these conditions the mask and target contrast is summed and target detection is facilitated. Panel D shows the overlay mask condition with a mask orientation offset of 35° which suppresses the target (increases the detection threshold). Note that the targets are shown at high contrast in this figure for illustrative purposes.