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 ones 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 targets 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 isnt 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 systems 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.
Paul Ocampo | EurekAlert!
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