Imagine that every day, you split a chocolate bar into two and offer one half to your friend. Even though you take care to divide the bar into equal pieces, your friend nearly always chooses the left half. Why is that? One possibility is that sensory bias in her visual system makes her perceive the left half of the bar to be larger than the right. But it is also possible that she does not see any difference between the two halves. Instead she simply decides to pick the left half because she prefers doing so.
The above example illustrates a key problem in studying perception. When asked to make a decision where there is no obviously correct answer such as deciding whether a painting is hanging perfectly straight people typically respond one way more often than the other. But does this response bias reflect biased perception or biased decision making?
Linares et al. have designed an experiment to tease apart these alternatives. Healthy volunteers had to decide whether gratings were tilted slightly upward or slightly downward. Almost all volunteers showed biases in their choice behavior in one of the two directions. To decouple sensory biases from ‘decisional’ biases, the volunteers had to press a particular key to select ‘upward’ on some trials, but ‘downward’ on others. This would not affect responding if the volunteers showed a decisional bias to press a key. But it would affect responding if the volunteers showed a sensory bias. The results revealed that both sensory and decisional biases influenced the volunteers’ choice behavior. However, sensory biases were more common.
People diagnosed with psychiatric disorders like schizophrenia often respond differently on perceptual tasks compared to healthy volunteers. Future studies should investigate whether this difference results from altered perception or altered decision making. This information could help narrow down the neural circuits affected by these disorders.