Humans possess limited attentional capacity, enabling focus on small subsets of available information while filtering out unattended stimuli—even highly salient unexpected events. Simons and Chabris's landmark "invisible gorilla" study (1999) demonstrated that approximately 50% of observers fail to notice conspicuous unexpected events when attention focuses elsewhere, establishing inattentional blindness as fundamental limitation of human perception. This selective attention mechanism determines which interface elements users actually perceive regardless of visual prominence, making attention management critical for ensuring users notice important information.
Simons and Chabris's groundbreaking experiment (1999) revolutionized understanding of attention limitations. Participants watching a video of people passing basketballs failed to notice a person in a gorilla suit walking through the scene, stopping to beat their chest, and exiting—despite the gorilla appearing for nine seconds. Approximately half of observers experiencing inattentional blindness demonstrated that attention focused on specific tasks (counting passes) prevents perception of unexpected but visible events. This finding challenged assumptions that conspicuous visual information automatically captures attention.
The research established attention as selective rather than comprehensive. Humans don't process entire visual fields—they sample strategically based on current goals and expectations. When users focus on specific interface tasks (completing forms, reading content, searching for information), they develop "attentional tunnels" filtering out peripheral information regardless of size, color, or motion. This goal-directed attention explains banner blindness, change blindness, and other phenomena where users genuinely don't see prominent interface elements outside their attention focus.
Chabris and Simons's subsequent research (2010) demonstrated inattentional blindness operates across contexts—not just artificial laboratory conditions. Radiologists miss obvious anomalies in medical scans when searching for specific pathologies. Drivers fail to notice pedestrians when focusing on navigation. Interface users overlook critical warnings when concentrating on task completion. These findings establish selective attention as pervasive limitation requiring deliberate design accommodation rather than exceptional edge case.