It may be easier to see two things at the same time!

According to Reverse Hierarchy Theory (Hochstein & Ahissar, Neuron, 2002), high level cortical regions are responsible for pop-out - rapid detection of an element that differs greatly from surrounding elements in a single dimension such as color or orientation. With large-receptive field attention spread across the entire array, subjects detect presence or absence of targets with response times that do not depend on the number of distractor elements. We now asked what will be the speed and accuracy of detecting 2 such elements simultaneously. Subjects viewed a briefly presented 8 × 8 array of pink lines oriented at 55 degrees (or 60 deg.) followed by a masking stimulus after a variable Stimulus-to-mask Onset Asynchrony. On some trials, 1 or 2 of the elements were replaced by a pale green line of the same orientation, a pink line of orientation 35-40 degrees (or 30 deg.), or a line with both these changes. Subjects reported the number of odd lines, and their nature. Surprisingly, we found that subjects were more accurate at detecting-and-identifying 2 targets than single targets - for all types of odd elements. In addition, it was easier to report presence of two odd elements - one with an odd color and one with an odd orientation - than to report the presence of one odd element which differed from the distractors both in color and orientation. These results suggest that oddity is detected as a single whole so that arrays with a pair of targets are perceived as distinct unitary structures. The interdependence of detection of two pop-out elements supports the Reverse Hierarchy Theory notion that pop-out depends on high-level large receptive fields.