A new study reveals that the brain uses entirely different circuits to handle how rewards sharpen our senses versus how they influence our choices—a discovery with major implications for understanding addiction and impulsivity.
Imagine you’re in a crowded market, searching for the vendor who sells the freshest, sweetest strawberries. Your eyes scan the stalls, instinctively drawn to the brightest reds and plumpest shapes. When you spot a promising batch, you make a quick decision to approach that vendor. This seamless act of focusing your attention and making a choice feels like a single, unified process. For decades, many neuroscientists thought the brain handled it that way, too. However, groundbreaking research from the Indian Institute of Science (IISc) reveals a far more nuanced reality: the brain treats them as two completely separate jobs.
A new study published in PLOS Biology demonstrates that the expectation of a reward splits our cognitive functions into two distinct pathways. One pathway sharpens our sensory perception, making us better at seeing the prize. The other influences our decision-making, making us more likely to act in a way that gets us the prize. Surprisingly, these two processes don’t even seem to talk to each other in the brain’s attention centers.
This discovery, led by Sridharan Devarajan, an Associate Professor at IISc’s Centre for Neuroscience, and his PhD student Ankita Sengupta, untangles a long-standing question in neuroscience. We’ve always known that rewards motivate us, making us faster and more accurate. But were these improvements in perception and decision-making driven by the same underlying neural machinery? Or were different systems at play? To find out, the researchers designed a clever experiment to isolate these two components.
A Tale of Two Tasks
The team recruited 24 participants and had them perform a cognitive task while their brain activity and eye movements were monitored using EEG (which measures brainwaves) and eye-tracking. The core of the experiment involved looking at a screen with two striped patterns, known as Gabor patches, on either side. After a brief moment, one or both of the patterns 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 researchers created two distinct conditions to separately test the two key components of attention: sensitivity and bias.
- Testing Sensitivity (How Well You See): In the first task, the reward was tied to a specific location. Detecting a change on one side of the screen always yielded a fixed number of points, while the other side offered a variable reward that could be much higher. Over time, participants learned to focus their attention on the potentially more lucrative side. This setup was designed to measure sensitivity—the brain’s ability to enhance its processing of a specific visual area to better detect changes.
- Testing Bias (How You Decide): In the second task, the reward was tied to a specific choice. For instance, on one side of the screen, saying “Yes, a change occurred” might be worth more points than saying “No,” regardless of whether a change actually happened. This condition was designed to measure decision bias—the tendency to favor one response over another because it’s more rewarding. It’s the mental equivalent of a loaded die, where you’re more likely to choose the option that pays out more.
A Surprising Neural Divide
The results from the first task, which tested sensitivity, were largely what scientists would expect. When a higher reward was associated with a specific location, participants’ performance improved. Their eyes were instinctively drawn to the high-reward side, and their EEG readings showed classic signatures of heightened visual attention. Specific brainwave patterns, like the lateralization of alpha-band power, confirmed that the brain was actively boosting its sensory processing for that area, effectively turning up the
