A new object captures attention--but only when you know which objects are old

According to the new object hypothesis (see, e.g., Yantis & Hillstrom, Journal of Experimental Psychology: Human Perception & Performance, 20, 95-107, 1994), an object appearing as a sudden onset captures attention because its appearance demands an immediate updating of visual short-term memory. This hypothesis was tested in three experiments using a procedure that allowed an object to be added to the display but, crucially, without incurring onset transients (Franconeri, Hollingworth, & Simons, Psychological Science, 16, 275-281, 2005). The latter showed that an object inserted in this fashion failed to capture attention. As a test of the new object hypothesis, this procedure assumes that the observers had encoded the display before the new object was introduced. If this assumption is not fulfilled, the new and the old objects cannot be distinguished one from the other. It was, however, unclear whether the encoding had taken place in the Franconeri et al. experiments. We showed that when circumstances were congenial to the encoding of the display before an additional object interposed, then the object successfully captured attention. But when the encoding of the initial display was either difficult or impossible, the additional object failed to capture attention.