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Study Notes

Simons and Chabris (1999)

AS, A-Level, IB
AQA, Edexcel, OCR, IB, Eduqas, WJEC

Last updated 22 Mar 2021

Gorillas in our Midst: Sustained Inattentional Blindness for Dynamic Events

Background and aim: In order for us to detect change, attention is required. Change blindness is where individuals often do not detect large changes to objects and scenes from one view to the next. Inattentional blindness occurs when attention is diverted to another object or task and observers often fail to perceive an unexpected object, even if it occurs at the point of fixation. The aim of this study is to build on previous research into divided visual attention and to investigate inattentional blindness for complex objects and events in dynamic scenes.

Method: This study was a laboratory experiment with an independent measures design. The IV was the condition the participant took part in: 1- The Transparent/Umbrella Woman condition, 2- The Transparent/Gorilla condition, 3- The Opaque/Umbrella Woman condition, 4- The Opaque/Gorilla condition. Within each condition there were four task conditions: 1- White/Easy, 2- White/Hard, 3- Black/Easy, 4- Black/Hard. Therefore there were 16 different conditions. The DV was the number who noticed the unexpected event which was either the umbrella woman, or the gorilla.

Four video tapes, 75 seconds long were created, each tape showed two teams of three players, one team wearing white shirts, the other black shirts. Each team passed around a standard orange basketball to one another in a standardised order: player 1→ player 2 → player 3 →player 1 etc. Players also dribbled the ball and made various other movements. After 44 to 48 seconds of action, an unexpected event occurred for 5 seconds. In the Umbrella-Woman condition, a tall woman holding an open umbrella walked from on one side of the action to the other, left to right. In the Gorilla condition, a shorter woman wearing a full body gorilla costume walked through the action in the same way and the players continued as normally during and after. There were two styles of video, in which the unexpected person/gorilla was clearly seen (opaque) or was transparent.

228 volunteers (mostly students), known as ‘observers,’ took part in the study and were either given a large candy bar for doing so, or were given a fee for taking part in a number of studies. 36 participants’ data was removed, which led to 16 groups of 12 individuals. A controlled observation also took place, in which 12 different participants watched a video in which a gorilla thumped its chest. 21 experimenters carried out the research; procedures were standardised and followed a written protocol. Participants were tested individually and gave informed consent. Participants were told they’d be watching two teams of three players passing basketballs and they were instructed to pay attention to the white or black team (the black and white conditions) and to count the number of passes made or the number of bounce passes and aerial passes made by the attended team – these were the easy and hard conditions respectively. They were then asked to write down the number of passes they counted.

Following this task participants were asked some additional questions - Did you notice anything unusual in the video? Did you notice anything other than the six players? Did you see a gorilla/woman carrying an umbrella walk across the screen? Further details were asked for if they answered yes and no further questions were asked. They were also asked if they had taken part in a similar study or had heard of this phenomenon; if this was the case their results were removed. Debriefing then took place.

Results: Overall, 54% noticed the unexpected event and 46% did not. 67% of participants noticed the unexpected event in the opaque condition, compared to only 42% in the transparent condition. More participants noticed the unexpected event in the easy condition compared to the hard condition – 64% vs 45%. While the effect of task difficulty was greater in the transparent condition, the Umbrella Woman was noticed more often than the Gorilla overall (65% versus 44%).

Results from the controlled observation were that only 50% noticed the event (roughly the same as the percentage that noticed the normal Opaque/Gorilla walking event (42%) under the same task conditions).

Conclusions: We can conclude that individuals do have inattentional blindness for dynamic events. They will often fail to notice a seemingly obvious, but unexpected event if they are engaged in another monitoring task. The extent of inattentional blindness is dependent on the difficulty of the primary task. We are more likely to notice unexpected events if these events are visually similar to the events we are paying attention to. Without attention we have no conscious perception – objects can pass through the spatial extent of attentional focus, but still not be ‘seen’ if they are not attended to.


Data: Simons and Chabris collected quantitative data by calculating the percentage of people who noticed the unexpected event. This data allowed for comparisons across conditions and summaries to be made easily.

Ethical Considerations: There were no ethical concerns with this study. Informed consent was gained before the study and participants were debriefed at the end, where the video was replayed to them to prove the unexpected event had indeed occurred.

Sampling Bias: A large sample was used which means conclusions are more valid. They were also student volunteers, which is a comparatively quick and easy method to gain participants who are also motivated and interested to take part in the study. However, students are not a representative group of people, while volunteers have certain characteristics. This means the sample is biased and lacks population validity.

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