While playing a video game, trying to spot a hidden treasure that promises a huge reward tends to increase our focus and concentration. Your eyes scan the screen, your attention is razor-sharp, and when you spot a glimmer, you instantly decide to go for it. We intuitively understand that expecting a reward can make us pay more attention and influence our choices. But how exactly does this happen in the brain? Are the mechanisms that sharpen our focus the same as those that nudge us towards a particular decision? For years, researchers have grappled with this question, often finding it difficult to separate these two intertwined processes in laboratory settings.
Now, new research from the Centre for Neuroscience at the Indian Institute of Science (IISc), Bangaluru has shed new light on this fundamental aspect of human behaviour, revealing that our brains use distinct pathways for these seemingly related functions.
Did you Know? Your eyes are constantly making tiny, involuntary movements called microsaccades. Scientists in this study used these minuscule jiggles to figure out where people were paying attention, even when they weren't consciously moving their eyes. |
The team studied how reward expectation separately controls our spatial attention, or where we focus our senses, and our decisional biases, or how we lean towards certain choices. They discovered that when expected rewards varied across locations, it primarily boosted our sensory processing, making us more sensitive to stimuli in those rewarding areas. When rewards varied across choices (e.g., choosing yes versus no), it predominantly shifted our decision-making biases, making us more likely to select the more rewarding option.
Crucially, the study found that only the location-based reward expectation engaged the brain's established attention networks, whereas choice-based rewards activated different neural signatures related to decision-making. This means that while both types of reward expectation influence our behaviour, they do so through fundamentally different brain mechanisms.
To achieve this nuanced understanding, the researchers designed a clever two-alternative forced-choice task involving orientation change detection. Participants were shown two grating patterns, one in each visual field, along with a central cue indicating potential rewards. The experiment was divided into two main sessions, each manipulating reward expectation in a distinct way.
In the space-specific session, the reward for a correct response varied depending on which side of the screen the target appeared. For example, one side might consistently offer a higher reward than the other. The scientists hypothesised that this would make participants pay more attention to the higher-reward location, improving their ability to detect changes there. To measure this, they employed a concept from Signal Detection Theory known as sensitivity, which essentially quantifies an individual's ability to distinguish a true signal from noise.
In contrast, the choice-specific session maintained the overall reward level across locations but varied the reward based on the type of response chosen for a target on a specific side (e.g., receiving more reward for saying "yes" to a change versus "no" to a change). Here, the researchers predicted that participants would adjust their criterion (c), another Signal Detection Theory parameter, which reflects their willingness to make a particular decision. A low criterion means you're more likely to say yes, while a high criterion means you're more cautious.
To observe the brain's activity during these tasks, the team employed electroencephalography (EEG), a non-invasive technique that measures electrical brain activity through electrodes placed on the scalp. They looked for specific neural markers or brain signals to track the different sessions and the resulting activity.
In the space-specific session, participants indeed showed higher sensitivity at the more rewarding locations. All the attention-related neural and motoric markers (N2pc, P300, alpha lateralisation, microsaccades, and faster reaction times) were robustly modulated. This provided strong evidence that space-specific reward expectation directly engages spatial attention. This confirms that attention is a limited, conserved resource that is deployed competitively across our visual field.
However, in the choice-specific session, while participants clearly adjusted their decision criterion based on the reward structure, none of the spatial attention markers were activated. Instead, only the pre-stimulus alpha power, a marker of decisional bias, was modulated. This critical dissociation demonstrated that the brain employs distinct mechanisms: one for directing sensory attention to rewarding locations and another for biasing decisions toward rewarding choices.
This research significantly advances our understanding by directly addressing a challenge that has long plagued the field: the conflation of attention and decision-making effects. By meticulously designing a task that decoupled these two processes, the researchers were able to pinpoint the distinct neural underpinnings of each. Earlier work often found it difficult to tell if improved performance was due to better sensory processing or a biased decision. This study provides a clear answer, showing that reward expectation can influence both, but through separate cognitive and neural pathways. Furthermore, it directly tested and validated the hypothesis that decision-making prioritisation (criterion shifts) does not necessarily engage spatial attention, a point that was previously debated.
However, the study acknowledges certain limitations. The use of surface EEG, while excellent for temporal resolution (when things happen in the brain), has relatively low spatial resolution, meaning it's harder to pinpoint the exact brain regions involved. Future research using higher-resolution neuroimaging techniques like functional magnetic resonance imaging (fMRI) could provide a more precise map of these brain areas.
The findings nevertheless significantly improve our understanding of human behaviour. Clinically, this research could be vital for understanding and treating conditions where attention or decision-making is impaired, such as ADHD or addiction, by targeting the specific neural pathways involved. The work could also help in education, where attention and focus of the students are key. Ultimately, this work deepens our appreciation of the brain's remarkable flexibility and efficiency in navigating a world full of incentives, allowing us to make better choices and focus our attention where it matters most.
This article was written with the help of generative AI and edited by an editor at Research Matters.