New research published in Proceedings of the National Academy of Sciences This provides evidence that depressed patients experience visible changes in their hedonic expectations, making it difficult for them to enjoy rewarding activities. Scientists have discovered that a simple smartphone game can detect this change in just three minutes, offering the potential for a new way to quickly assess disease severity.
Depression is a widespread illness that affects millions of people worldwide. Despite the prevalence of this disease, doctors primarily rely on checklists of symptoms rather than specific measurements of underlying cognitive processes to diagnose symptoms. A key feature of depression is anhedonia, a condition in which a person loses the ability to experience pleasure from normally pleasurable activities.
Behavioral economists and psychologists suggest that how people perceive rewards depends on their decision-making reference point. Decision reference points serve as a baseline for internal expectations that determine whether an event is perceived as positive or negative. For example, finding a slice of pizza feels like a positive reward to someone who expects to have no food at all. If you were expecting a hearty five-course meal, you might be disappointed when you get just one slice of the same pizza.
Previous research has linked the processing of these expectations to the anterior cingulate cortex, a brain region involved in decision-making and emotion. Brain imaging studies have shown altered activity in this region in patients with depression. In some cases, doctors treat severe depression using deep brain stimulation, which sends electrical impulses to pathways that connect the anterior cingulate cortex and other brain areas.
A team of researchers led by Paul Glimcher and Dan Iosifescu at New York University Langone Health suspected that disruption of this reference point could cause symptoms of anhedonia. They designed a study to measure whether depressed patients have unusually high reference points and whether they have trouble adjusting these expectations when their environment changes.
To test these ideas, the research team recruited 120 adults. This group included 50 people with a clinically diagnosed major depressive disorder and 70 healthy adults without mental illness. Each participant engaged in two separate computer-based behavioral tasks designed to measure their internal expectations and how those expectations changed over time.
The first evaluation was a virtual foraging game in which participants collected digital apples from an animated tree. The design is based on the marginal value theorem, an ecological concept that explains how animals decide when to leave food behind. According to this theory, animals balance the energy gained from staying at a depleting food source against the time and energy required to move to a new food source.
Translating this into a human experiment, the goal was to collect as many apples as possible within a set time limit and earn a small amount of cash. At each turn, participants could choose to continue harvesting the current tree or wait for a movement delay (3 or 10 seconds) to move to a new tree. Each time participants harvested the same tree, yields decreased by an average of 12 percent.
Eventually, the diminishing rewards fall below the person’s decision-making criteria, prompting them to abandon the tree. Healthy adults tend to continue picking until they get 4 to 5 apples from a tree. People with major depressive disorder stopped harvesting much earlier. On average, they abandoned the tree after it had produced about eight or nine apples.
This behavior suggests that depressed patients’ baseline expectations of reward are approximately 50% higher compared to healthy participants. Interestingly, this difference did not imply that depressed patients had poorer economic performance. The researchers noted that the difference in total income between the two groups was not statistically significant, as the average income for both groups was $27. The difference was entirely in how patients valued diminishing returns, as they needed more apples to feel like staying on the tree was a positive choice.
Because depressed patients needed higher yields to feel rewarded, this task effectively identified their condition. The authors noted that this behavioral game can accurately distinguish between patients and healthy adults in just 3 minutes of gameplay. Additionally, individual quit points strongly matched a person’s depression severity as measured by a traditional clinical interview.
The second task investigated how participants adapted their reference points to changes in the environment. Participants received a $5 donation and were asked to bid on 30 common snack foods using an economic bidding process. This step measured baseline willingness to pay for compensation.
After these initial bids, participants entered an adaptation phase. They viewed and rated their 10 favorite or 10 least favorite snacks over 300 trials. This repeated exposure was intended to change their internal reference points by immersing them in highly challenging or less challenging environments.
After the adaptation phase, the researchers asked participants to re-bid the original 30 snacks in 90 trials. When healthy adults spent time evaluating high-value snacks, their internal reference points rose and their subsequent average snack bids temporarily decreased. After several rounds of bidding, healthy participants successfully relaxed their expectations and returned to their original baseline bid within minutes.
Depressed patients, like the healthy group, initially changed their bids according to the adaptation stage. However, their expectations became less flexible over time. Patients did not revert to baseline bids during the testing period and exhibited a rigid response to changes in the reward environment.
As study co-author Ardis Vittala said in a press release, “Depressed patients seem unable to successfully adapt their expectations in response to changing conditions. This gives us a hint as to what is wrong mechanically in their brains.” The inability to return the reference point to a normal state is evidence of broken cognitive mechanisms that may maintain feelings of anhedonia.
Although these findings represent a new way to measure depression, this study has several limitations and may be misleading. One limitation is that it groups together potentially different depression subtypes. Major depressive disorder includes a wide range of symptoms, and abnormalities in these reference points may apply only to certain subgroups who experience severe anhedonia. Readers should not interpret this study to mean that all depressed patients are rewarded in exactly the same way.
Current research also does not reveal whether these expectancy deficits also exist in other psychiatric disorders. Future research should investigate whether changes in baseline are specific to depression or also present in other disorders characterized by anhedonia. The scientists also want to test these measurements on a larger, more diverse population to see if the game can be used outside of clinical settings. Another direction for future research includes testing whether physical or cognitive effort helps modulate patients’ reference points.
Identifying this inflexible reference point opens new doors for targeted interventions. “For us, this looks like a therapeutic goal. We are already looking to see if behavioral treatments or drugs can modify this reference point stickiness,” Vittala explained in a press release. The authors suggest that measuring this cognitive mechanism could allow doctors to remotely match patients with the most effective treatments, saving time and reducing the need for repeated clinic visits.
The study, “Decision point pathology: Cognitive mechanisms and correlates of major depressive disorder in humans,” was authored by Aadith Vittala, Lulu Wu, Dongni Yan, David Liebers, Elizabeth Tell, Xiaotong Song, Damon Dashti, Kenway Louie, Candace Raio, Dan V. Iosifescu, and Paul Glimcher.
