Think eating is easy? Think again. According to the World Health Organization, more than 1.9 billion adults were overweight worldwide in 2016. Of these, over 650 million were obese—a number equivalent to roughly twice the population of the United States. And most of the world’s population lives in countries in which obesity kills more people than starvation or malnourishment.

There is nothing simple or easy about eating. Living with an abundance of food has not made us healthier. Quite on the contrary, worldwide obesity has tripled since 1975.

In light of those concerning trends, there has been growing research emphasis on the role of cognition in food intake. One focus of that type of research has been on an apparent attentional bias that we have for energy-dense food. The energy density of a food refers to the number of calories it contains per unit weight. Low energy density implies you can have a satisfying portion of those foods without stocking up on too many calories. High energy density implies the converse: have just a little and you’ve refueled with too many calories.

The implications of energy density can be considerable. Consider the two desserts below:

I reckon they look equally yummy, and they both contain about 215 kcal. However, the one on the left weighs 300g (11 oz), which is more than double the one on the right. This is because the mix of fresh berries, low fat plain yogurt and sprinkling of granola on the left weighs more than the strawberries and cream on the right.

In general, low-density foods tend to contain lots of water. The best low-density food items are therefore usually vegetables. It is difficult to gain weight simply by munching on celery. High-density foods tend to be free of water and contain lots of sugar or fat (or both). That’s why a single chocolate bar can set you back from your dietary goals.

Once you know this, would you still prefer a Snickers bar to a stick of celery? If so, why?

A recent article in the Psychonomic Bulletin & Review examined the involvement of visual attention in our food choices. Researchers Corbin Cunningham and Howard Egeth were interested in whether food stimuli might distract people from doing another task they are focusing on. Cunningham and Egeth reasoned that the extent to which a food stimulus “captures” our attention, contrary to the person’s intention, might differ with its energy density. Perhaps only highly desirable foods, such as chocolate bars, will capture our attention while we perform another task.

The figure below illustrates the procedure used by Cunningham and Egeth. On each trial, participants were shown a set of 4 characters in the center of the screen. Participants had to classify each of those characters, one at a time in ordinary reading order, as either a letter or digit by pressing a response key.

On a random half of trials, at some point during execution of the classification task (but always after the first response), a picture appeared on the screen. The picture was always irrelevant to the task but differed in its content: on one third of picture trials a non-food object appeared, whereas on the remaining trials the picture represented a food item of either high (1/3 of trials) or low (also 1/3) energy density. Care was taken to ensure that the three types of picture did not differ on potentially relevant variables such as color composition, contrast, brightness, size, and complexity.

Cunningham and Egeth created an “attentional capture” score to assess the amount of distraction during the task that used the response times on distractor-absent trials as a reference point. That reference point was subtracted from the response times on the remaining three types of trials, yielding a difference score that directly indicated the extent of attentional captures.

The black bars in the figure below show the data from their first experiment. It can be seen that the energy-dense foods captured attention more—that is, led to greater slowing—than the non foods or the low-energy food pictures.

In their second experiment, Cunningham and Egeth replicated the procedure of the first study exactly, except that they asked participants to eat two “fun sized” candy bars of their choice (e.g., Snickers, Kit Kat, or Reese’s Peanut Butter cups) before commencing the experiment. The researchers reasoned that if participants consume some high-energy food before the experiment, this might reduce the distractive power of those pictures.

The figure above also shows the results from that second experiment, identified as the light gray bars. As expected, the selective disruption associated with the high-energy food pictures disappeared.

In a final experiment Cunningham and Egeth set out to determine whether consuming the candy bars reduced the attentional-capture power only of high-energy food stimuli or whether the effect might generalize to other attractive stimuli that are known to capture attention. For example, pictures of emotional faces are known to capture attention and the question is whether that capture, too, might be eliminated by consuming candy bars.

The third experiment was effectively a combination of the first two studies, with participants assigned at random to a no-snack control and a two-candy-bar condition. In addition, the low-energy food pictures were replaced by emotional faces expressing fear or disgust.

The results are shown in the next figure.

The figure again shows the selective attentional capture due to the high-energy food pictures when participants did not consume any food immediately prior to the study. That effect disappeared when the participants ate two candy bars before the experiment, in which case all stimuli were equally distracting. (The latter result is somewhat surprising because the emotional faces were expected to capture attention to a greater extent than the non-food objects.)

So why does eating candy bars eliminate the attention-capturing power of energy-dense food? As Howard Egeth put it, “the answer has to do with a person’s motivational state. Recent research has shown that when an ordinary rewarding stimulus such as chocolate is devalued, attention is no longer orientated towards this rewarding-associated stimulus.” Likewise, eating some candy devalues its attractiveness because we no longer feel hungry or have a craving for sweet food.

This meshes well with another recent post by Heather Hill, which showed that when people are hungry, their ability to make fine-grained perceptual judgments about pictures of food items increases. The study by Cunningham and Egeth explores the flipside of this perceptual benefit: hunger increases the chances of our attention being captured by a food stimulus, even if that runs counter to our current intentions.

Psychonomics Article highlighted in this post:

Cunningham, C.A.  Egeth, H.E. (2017). The capture of attention by entirely irrelevant pictures of calorie-dense foods, Psychonomic Bulletin & Review DOI: 10.3758/s13423-017-1375-8.