Healthcare (Commonwealth Union) – The observation to how animals seem to instinctively select the ripest fruits from a tree has fascinated many. This behavior, influenced by seeking out the most rewarding option, showed a valuable window into brain function. Animals do not just direct their focus toward the target that has most advantages —the location of the ripest fruit—but also make quick decisions to take action on that information. Researchers astonishingly found that behavioral and neural mechanisms behind this process have stayed relatively underexplored.
Sridharan Devarajan, who is an Associate Professor at the Centre for Neuroscience, Indian Institute of Science (IISc), joined by his PhD student Ankita Sengupta, set out to explore the way reward expectation influences attention. Their research, published in PLOS Biology, sheds new light on this intricate process.
It has long been understood that anticipating a reward shapes behavior. Individuals detect changes more quickly and respond with greater speed and accuracy when a reward is expected.
Despite the fact the most studies have explored the way reward anticipation has an impact on attention and decision-making, they have often fallen short of knowing whether these processes rely on the same or distinct brain regions as well as mechanisms.
To make up for the unavailability of this information, Devarajan and Sengupta devised an experiment to explore the way reward expectation influences two key aspects of attention: sensitivity (the capacity to visually mark the ripest fruit) as well as bias (the decision to engage on this knowledge). Prior research has found it hard to separate the effects of reward on sensitivity versus bias, making this study a crucial step to understand their interplay.
Sridharan’s study group conducted experiments with 24 volunteers, each completing a two-stage cognitive task.
In the initial stage, the aim was to examine how rewards influence sensitivity. Participants looked at a screen displaying two Gabor patches—smooth black-and-white striped patterns commonly utilised for attention studies—positioned on opposite sides.
After a brief gap of just 200 milliseconds, the patches began to flicker, and the orientation of one or both patches might shift. Participants had to indicate whether they noticed a change in either patch. Whenever they correctly reported a change, they earned a monetary reward.
This is where things became intriguing. The researchers deliberately manipulated the reward system: one side consistently offered the same number of points, while the other side’s rewards varied—sometimes higher, sometimes lower than the fixed side. Over time, participants picked up on this pattern through the rewards they received, which caused them to pay closer attention to and detect even subtle changes on the more lucrative side.
The second stage focused on how rewards affect decision bias. The setup was similar, but here the variation was in the payoff for answering “Yes” (indicating a change) versus “No” (indicating no change) on one side. As before, the “Yes” reward could be greater or smaller than the fixed side’s reward. This design nudged participants toward favoring the Yes choice when it was more profitable, and toward No when that yielded more points.
Throughout both stages, the team tracked eye movements and brain activity, allowing them to separately measure the two distinct components of attention.
Sridharan indicated that for the initial task, where sensitivity was the focus, participants had provided increased attention, and their gaze focused on the side linked to elevated rewards. However, this bias move was not altered. This showed that only sensitivity was modulated in this move.
In addition, established neural markers of attention—such as electrical activity patterns and rhythmic brain waves—indicated heightened sensitivity.
This was the first time, the findings reveal that sensitivity and bias appear to be driven by distinct neural systems, each influenced in its own way by the expectation of a reward. Furthermore, while sensitivity seems tightly connected to brain regions that control sensory attention, bias appears to be more closely tied to areas involved in decision-making.
