Did you ever arrange to meet a friend at a busy street corner, then rush up to a total stranger thinking it was your friend? Neuroscientists have a theory to explain why such potentially embarrassing mistakes occur. They probe the underlying perceptual and neural processes of visual search by studying how distracters affect performance of a visual search task. One might intuitively expect that as background noise created by distracters and errors increase, confidence in one's decision plummets. But in a new study published in PLoS Biology, Stefano Baldassi, Nicola Megna, and David Burr show that just the opposite happens. When they asked observers to search for a tilted target embedded in vertical distracters and estimate the target's tilt, the observers often overestimated the magnitude of the tilt--and did so with a high degree of confidence in their decision.
The authors used signal detection theory to make quantitative predictions about the probability that an observer will detect a target under cluttered conditions. SDT assumes the brain represents each element in a visual search display as an independent variable with its own noise. It also assumes that when the observer isn't sure which stimulus is the target, she monitors all stimuli, and performance suffers. Thus, increasing the number of distracters (trying to find your friend on a busy street or a document on a messy desk) increases the background noise of the visual system's representation while reducing the accuracy and reaction time of performing the task.
It turns out that SDT lends a logical prediction to the seemingly counterintuitive finding that observers make more high-confidence errors when confronted with clutter. The prediction flows from a "squeaky wheel gets the grease" rule about visual processing, called the "Sign Max Rule." In other words, since each stimulus generates a noisy internal representation, and subjects monitor all the distracters to search for the target, as the number of distracters increases, the chance of perceiving a distracter as being more tilted than the target increases, and confidence increases as well. This prediction bore out in the authors' experiments, as determined both by the observers' perceived magnitude and self-reports on their level of confidence about each decision. The authors conclude the visual system combines the outputs of noisy detectors and settles on the maximum signal.
These results suggest that the probability of being sure you saw something you didn't increases in chaotic environments, and could have far-reaching implications. The authors explain that while their study focused on "simple perceptual decisions about a single stimulus attribute," the same type of processes may also apply to complex cognitive tasks involving problem solving and memory. If people find themselves confronted with multiple events in a chaotic, confusing environment, they may decide about some aspect of the situation and be totally wrong even though they have full confidence in their decision. The consequences of such a phenomenon could be relatively trivial, explaining why professional athletes often end up wasting their time arguing questionable calls with an official. Or they could prove a matter of life and death, perhaps accounting for why eyewitness testimony can be unreliable--or why soldiers sometimes can't tell friend from foe in the heat of battle.
Citation: Baldassi S, Megna N, Burr DC (2006) Visual clutter causes high-magnitude errors. PLoS Biol 4(3): e56.
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