Attention: Effect of Breakdown

Many encyclopedia entries and introductory chapters written about attentional selection start by noting that our environment is rather rich in sensory stimulation that cannot possibly be processed simultaneously and there has to be a mechanism (attentional selection) that deals with this bombardment of sensory stimulation by processing only the relevant information. This statement is valid, but what is the evidence for this implied limited capacity (i.e., the breakdown of attention)? Perhaps you have had the experience of walking into a crowded café trying to find an empty table while holding a tray with hot soup and a sandwich. After quite a bit of searching around, you finally find a table and sit down to enjoy your food. But just as you are about to bite ...