People with depression often say that they experience the world differently from others. They commonly report changes in appetite, and sometimes say that “time seems to drag on.” And the English language contains many metaphors that link depression with color—we often say, for example, that someone with depression is “feeling blue.” A series of recent studies now offers preliminary evidence that depression may, indeed, alter aspects of perception.
In the most recent, mathematical psychologist Diana Kornbrot of the University of Hertfordshire and her colleagues recruited 46 students and used the Beck Depression Inventory (BDI) to gauge their mood. The BDI consists of 21 multiple-choice questions, and is widely used by psychiatrists to assess the severity of depression.
Kornbrot and her colleagues split the study participants into two groups, one with high BDI scores, corresponding to dysphoria, or mild depression, the other with low scores. They then asked the participants to listen to a series of sounds that varied from 3 seconds to 65 seconds in length, and to estimate how long they were as accurately as possible. Participants also were asked to produce sounds of varying lengths using a computer keyboard.
The researchers found a correlation between participants’ BDI scores and the accuracy with which they could estimate the length of the sounds they heard and produced: Those with higher scores made more accurate estimates than those with lower scores.
“What we see is that those with mild depression seem to be better at paying attention to short periods of time,” says psychologist Rachel Msetfi of the University of Limerick, a co-author of the study. The finding suggests that people with depression experience a time dilation effect, and lends support to the controversial notion of depressive realism, which posits that people who are depressed have a more accurate perception of reality than others.
Msetfi thinks that this time dilation might be related to one of the symptoms of severe depression. People with severe depression often experience helplessness, or the feeling that they are not in control of their lives, and this is usually accompanied by a feeling of guilt.
Time perception is crucial for agency, the sense that we are in control of our actions. Normally, our actions are followed very closely in time by their consequences; this can give us the sense that the two are causally related, and that we are responsible for the consequences of our actions.
Other work suggests that the brain actively compresses time to make action and cause seem closer together in time. This process, referred to as intentional binding, enhances our sense of agency. When the interval between an action and its effects is deliberately extended, people feel less in control of their actions. There is evidence that this process is perturbed in people with schizophrenia, and this may contribute to their propensity to misattribute their thoughts or actions to external forces.
“Subtle changes in the way we process time may contribute to the sense of helplessness [in people with depression],” says Msetfi. “If you have disturbances in the way you process time and context and the relationship between them, then you’re going to have some disturbance in your experience of everyday life and reality.”
Msetfi suggests that perturbations of what she calls the ‘psychological time-space continuum’ might be a general deficit that underpins other psychopathological conditions. “I don’t have any solid evidence to support that yet, though,” she says.
Another recent study, led by Petra Platte of the University of Würzburg, suggests that depression alters peoples’ perception of taste, too. Platte and her colleagues used film clips to induce sad, happy or neutral moods in 70 otherwise healthy participants. They used the BDI to assess the participants’ mood, and asked them to rate the intensity of sweet, sour, bitter, fatty, and umami stimuli, all of which were given at several different concentrations.
They found that mood modulated the participants’ perception of the stimuli—those with higher BDI scores could not differentiate between low and high fat concentrations after the induction of both negative and positive moods. The researchers suggest that this apparent deficit in taste perception might foster unhealthy eating habits in people with mild, subclinical depression.
It follows that we would expect to see a higher incidence of obesity in people who are depressed. Obesity and depression are indeed correlated, but the relationship between them is a complex and reciprocal one. It is by no means clear whether altered taste perception is what causes obesity in patients with depression.
In 2010, another group of German researchers reported that people with major depressive disorder (MDD) have reduced sensitivity to visual contrast compared with healthy controls. Emanuel Bubl of the University of Freiburg and colleagues recruited 40 patients diagnosed with MDD and 40 healthy controls matched for age and sex. They showed all the participants black and white checkerboard patterns, and used pattern electroretinography to measure the responses of their eyes. This technique uses electrodes placed on the cornea to measure the electrical activity of cells in the retina, with the size of the response indicating the extent of contrast gain.
Patients with MDD responded differently than the controls; they had markedly reduced contrast gain-related activity, suggesting that they were less sensitive to contrast. Within the group of MDD participants, the size of the responses was closely correlated with the severity of depression. The more severe the depression, the greater the reduction in contrast gain, and those who were taking anti-depressants also displayed slightly larger responses than those who were not.
Buble and his colleagues found that they could predict which of the participants had been diagnosed with MDD from the pattern electroretinography recordings alone, with an accuracy of greater than 90 percent. But they could not determine whether the effect they observed was specific to depression.
Several other studies published in recent years suggest that sensitivity to pain is similarly reduced in depression. Taken together, these findings suggest that depression alters basic perceptual processes across multiple sensory modalities.
But cognitive neuropsychologist Keith Laws of the University of Hertfordshire, who was not involved in any of the studies, has reservations about the findings. “In two of the studies, they’re looking at people who score within the normal range of the BDI,” he says. “These are completely healthy people, so it’s difficult to know what they mean in the context of people with depression.”
Moreover, the results are based largely on participants’ self-reports of the mood they were in at the time. Such reports are highly subjective in nature, and not necessarily accurate. And the studies divided participants into two groups according to their BDI scores, and then compared the averaged results of the groups, rather than those of individual participants. Consequently, the results from both groups overlapped significantly, making it difficult to draw any firm conclusions from them.
“It’s very interesting work, but I’m not altogether convinced by it,” says Laws, “and I think all of these studies need to be replicated and extended in order to determine whether depression actually does alter perception.”