Inattentional blindness is the failure to notice an unexpected but fully visible stimulus when attention is engaged by another task. Unlike change blindness (which requires a disruption to mask the change), inattentional blindness occurs even when the unnoticed stimulus is continuously present, fully visible, and well within the observer's visual field. It powerfully demonstrates that seeing requires more than just looking — it requires attending.
Classic Demonstrations
The phenomenon was named by Arien Mack and Irvin Rock (1998), who showed that when observers were focused on judging which arm of a briefly presented cross was longer, they frequently failed to notice an unexpected additional stimulus presented in the display. But the most famous demonstration is the "invisible gorilla" experiment by Daniel Simons and Christopher Chabris (1999): observers asked to count basketball passes among players wearing white shirts failed to notice a person in a gorilla suit walking through the scene — an event lasting nine seconds — at a rate of approximately 50%.
Inattentional blindness has serious real-world implications. Drivers focused on navigating may fail to see pedestrians or cyclists. Radiologists searching for lung nodules may miss a gorilla-shaped figure embedded in a CT scan (as demonstrated by Trafton Drew and colleagues in 2013, with 83% of radiologists missing it). Surgeons focused on the operative field may miss broader complications. These findings underscore that expertise does not eliminate inattentional blindness — even highly trained observers are vulnerable when their attention is focused elsewhere.
Factors Affecting Inattentional Blindness
Several factors influence whether an unexpected stimulus will be noticed or missed. Similarity between the unexpected stimulus and the attended items matters: if you are attending to white shapes, a black shape is more likely to be missed than a white one. Proximity to the focus of attention matters: stimuli closer to fixation or to attended locations are more likely to be detected. Distinctiveness helps: unique features like faces or one's own name are more likely to break through inattentional blindness. Perceptual load also plays a role: higher task demands increase inattentional blindness.
Implicit Processing
A key question is whether unnoticed stimuli are processed at all. Evidence suggests that they are — at least to some extent. Mack and Rock found that unnoticed words still produced semantic priming effects, and more recent studies have found evidence for implicit processing of emotional content in unnoticed stimuli. This suggests that inattentional blindness represents a failure of conscious awareness rather than a complete failure of processing, consistent with the distinction between attention-dependent awareness and pre-attentive processing.
Theoretical Implications
Inattentional blindness is a cornerstone finding for theories arguing that attention is necessary (or at least normally necessary) for conscious perception. It challenges the intuition that we are aware of everything in our visual field and supports the view that our phenomenal experience is much more limited than it seems. Combined with change blindness, it paints a picture of visual awareness as sparse, dependent on the current focus of attention, and prone to dramatic failures when attention is otherwise engaged.