What happened
You were asked to track a subset of moving targets while distractors shared the space. During high-load trials, an unexpected object crossed the display. Whether you reported it depended almost entirely on how much attention the counting task consumed.
Why it happened
Visual processing is capacity-limited. Task goals program a 'search template' that filters input before it reaches awareness. Objects that don't match the template can be fully processed by early vision yet never become conscious, perception without awareness.
Experimental design
Research protocol
Between-subjects (single critical trial); unexpected-stimulus detection
- Independent variable (IV)
- Attentional load of the primary task (e.g., count passes among one shirt color vs. both teams)
- Dependent variable(s) (DV)
- Detection of the unexpected object (hit vs. miss)
- Subjective awareness reports on the critical trial
- Measured constructs
- Selective attentionPerceptual awarenessTask-set maintenance
- Operational definitions
Inattentional blindness
Failure to report a salient, fully visible unexpected object when attention is engaged elsewhere
Attentional load
Difficulty of the counting or tracking task imposed on the observer
Unexpected stimulus
Object not described in pre-trial instructions and absent from practice
- Key terms
Inattentional blindness
Failure to notice an unexpected object in plain view because attention is occupied by another task (Mack & Rock, 1998; Simons & Chabris, 1999).
Change blindness
Failure to detect a change between two displays; distinct from inattentional blindness, which concerns a single display (Simons & Rensink, 2005).
Attentional set
The task-defined filter that determines which features or objects receive processing resources.
The original experiment
Year
1999
Researchers
Daniel Simons · Christopher Chabris
Sample
228 observers across four task conditions
Key finding
46% of observers failed to notice the gorilla; failure rate rose with attentional load.
Simons, D. J., & Chabris, C. F. (1999). Gorillas in our midst: Sustained inattentional blindness for dynamic events. Perception, 28(9), 1059–1074.
Where this shows up in the world
Radiologists miss anomalies outside the search template, a 2013 replication placed a gorilla in lung scans and 83% of radiologists missed it
Driver distraction: phone conversations produce inattentional blindness for pedestrians and signals
Eyewitness reliability: attention during an event bounds what can later be remembered
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