Most people have experienced the phenomenon of something catching their attention, only to discover that the person next to them completely missed what seemed obvious. Neuroscientists have also grappled with the question of why two people remember different and sometimes mutually exclusive details about the same scene. This century-old scientific question of visual processing has been partially answered in a new study published by researchers from the University of California, Davis, and Otto-von-Guericke University in Magdeburg, Germany. The results will be published in this Thursday's issue of the weekly journal Nature. The international team found that the way individual brains register and process visual information depends upon where individuals direct their attention. In other words, paying attention to one part of a complex scene effectively boosts those visual brain signals while weakening signals from other parts of the scene. The study is the first to successfully pair two functional brain-imaging techniques to describe a mechanism of human cognition. The researchers combined a modern noninvasive method that accurately measures when brain activity occurs with a method that pinpoints where the activity occurs. "Attention has powerful implications for our everyday perceptions," said Ron Mangun, a psychology professor at the UC Davis Center for Neuroscience and co-author of the study. "When people pay attention to something in the world around them, it changes the way their brains process that and other visual information coming into the brain." This is known as selective attention -- paying attention to one thing while ignoring others. Selective attention is an essential brain process that keeps us focused on the most important items at the moment but can also lead to failures of perception -- such as when we fail to detect a stop sign while concentrating on the road ahead. The study by Mangun, Hans-Jochen Heinze, professor of clinical neurophysiology at the Otto-von-Guericke University, and their colleagues shows that selective visual attention kicks in as fast as 80 milliseconds after the retina at the back of the eyeball sends an exact copy of the visual world to the brain for visual processing. In a brain region known as the "extrastriate visual cortex" in the lower back of each half of the brain, the selected visual stimuli receive enhanced visual processing. According to this study, the flow of information from the eye to higher brain areas is altered at this relatively early stage. "Experimental psychologists have known for more than 100 years that perceptions are influenced by desires, momentary needs and intent," Heinze said. "Our study punctuates very dramatically that this first happens in humans within the extrastriate visual cortex at a structure known as the fusiform gyrus." The researchers identified the time and place for this visual filter by combining two well-proven experimental techniques. One, involving "event-related potentials" (ERP), measures the rapid electrical fields produced outside the head by brain activity. Such electrical recording methods can measure the precise timing but not necessarily the precise location of the activity. The other, known as "positron-emission tomography" (PET), takes a picture of blood flow in the brain, telling researchers where brain activity took place but not exactly when. The two techniques represent noninvasive ways of studying the human brain. The findings of this study of 10 people converge with animal studies, helping to integrate knowledge of visual processing in animals with models of visual processing and attention in humans. The researchers suggest that combining different neuroimaging methods, such as PET and ERP recording, promises to be a powerful approach for defining when and where in the brain important higher mental functions take place. The results are also important in efforts to eventually diagnose and treat neurological diseases and disorders. In addition, the findings may have practical applications for human performance. The design of airplane cockpits, for example, incorporates knowledge about attentional mechanisms to prevent perceptual failure during highly demanding tasks. Understanding the basic brain mechanism of selective attention is important to complete the difficult puzzle of how we sense, perceive and act in a complex world, Mangun said.