A new study reveals that while the lure of a reward sharpens our focus, it influences our choices through an entirely different neural pathway, challenging long-held assumptions about attention.
Imagine you’re foraging for berries in a forest. Your experience tells you that the bushes on the sunny side of the path often yield the sweetest, ripest fruit. Naturally, you focus your gaze there, your eyes scanning keenly for the deep red glint of a perfect berry. This is your brain on rewards: the promise of a tastier treat sharpens your attention, making you better at spotting your target. But what happens when you have to make a choice? Does the same brain process that sharpens your vision also push you to grab the berry?
For decades, neuroscientists have understood that the expectation of a reward is a powerful driver of behavior. It makes us faster, more accurate, and more motivated. However, a fundamental question has remained murky: how exactly does the brain pull this off? Does a reward simply turn up the volume on our attention systems, making us both perceive more clearly and act more decisively? Or are there different mechanisms at play? This is the puzzle that Sridharan Devarajan, an Associate Professor at the Indian Institute of Science, and his PhD student Ankita Sengupta set out to solve.
Their groundbreaking study, published in PLOS Biology, untangles two key components of this process: sensitivity and bias. Sensitivity is your ability to perceive something accurately—how well you can visually identify the ripest fruit. Bias, on the other hand, is your tendency to make a certain decision—your inclination to reach out and pick the fruit. Previous research often struggled to separate these two effects, as they usually happen at the same time. The IISc team designed a clever experiment to isolate them and see what was happening under the hood.
A Tale of Two Tasks
The researchers recruited 24 participants and had them perform a cognitive task while their brain activity was monitored with EEG and their eye movements were tracked. The participants looked at a screen showing two patterns of black-and-white stripes, known as Gabor patches, one on each side. After a brief moment, one or both of the patches would flicker, and its orientation might change. The participant’s job was to report if they detected a change.
This is where the experimental design got brilliant. The study was split into two distinct parts, each designed to manipulate either sensitivity or bias, but not both.
In the first task, designed to test sensitivity, the reward was tied to a specific location. For correctly identifying a change, participants would receive a fixed number of points for one side of the screen, but a variable (and potentially much higher) number of points for the other side. As the trials went on, participants learned which side offered a bigger potential payday. The result? Their performance on that high-reward side improved dramatically. Their eyes were drawn to it, and their ability to detect even minute changes—their sensory sensitivity—was heightened. Critically, their decision-making bias didn’t change; they were just better at seeing on the lucrative side.
In the second task, the team targeted decision bias. This time, the reward wasn’t tied to a location, but to a choice. For example, on one side of the screen, reporting
