Hungry Eyes See More Cookies: Food Deprivation and Perceptual Discrimination

Are you hungry, yet? How long has it been since you have eaten? Are you cranky and tired? I am as I write this piece.  I went on an unexpected food deprivation plan, thanks to a recent trip to Central America. I can’t wait to eat more than toast, applesauce, and bananas again.

Food deprivation produces several different states – a physiological state, a psychological state, and a motivational state.  Physiologically, our body responds to food deprivation by reducing metabolism through a number of different mechanisms while also sending signals to increase food intake (e.g., the experience of hunger).  Psychologically, food deprivation can produce negative emotions such as frustration and irritability, as well as deficits in cognition such as decreased attention or concentration.  Motivationally, food deprivation may lead to overindulgences when the opportunity arises or a willingness to work to attain access to food.

As an example, think about the last time you came home after a long day at work or chasing errands and children.  Were you irritable on the drive home because you kept catching all of the red lights or was the kids’ fighting in the backseat particularly annoying, and all you could think about was how hungry you were?  In fact, as soon as you get home do you head straight to the pantry or the refrigerator?  My children run through this sequence every day, followed by my “reminders” about “their eyes being bigger than their stomachs” as I watch them pile a bunch of junk food on their plates.

“Love looks not with the eyes but with the mind” – William Shakespeare (A Midsummer Night’s Dream)

The effects of temporary food deprivation on the ability of people to discriminate between different stimuli was the focus of a recent article in the Psychonomic Bulletin & Review. Researchers Noa Zitro-Emanuel and Tzvi Ganel of Ben-Gurion University in Israel investigated the visual perceptions of undergraduate students by asking them to discriminate between images that were very close in size and selecting the smaller of the two images.  This task relied on a perceptual phenomenon called the Just Noticeable Difference (JNDs), or the minimum difference that can be detected between two similar stimuli. The JND measures the perceptual resolution of the participants.  (This requires a procedure with many, many trials, which I am sure the participants loved!)

Look at the image below and answer the following question: Are they the same?

If you answered yes, then you fell into the perception trap. If you answered no, then you were able to detect the just noticeable difference. “Yes” folks, look closely at the water on the globe and the colors of the fox’s tail.

Now look at the next two images in turn.

Which one is the circle and which one is the oval?

Which cookie is smaller?

Could you tell a difference between the two ovals? Hopefully not, since they are both slightly oval and the same image duplicated.  (The same is true for the cookie.)

Using similar types of stimuli (the ovals, not the cookies), nearly a century ago Ivan Pavlov and other psychologists created “conditioned neuroses” in various subjects (dogs, cats, birds) using stimuli that were so similar that the animals eventually could not detect a difference between them, selecting one stimulus about half the trials and the other stimulus on the other half, or not selecting either one as they displayed signs of distress, frustration, and learned helplessness.

Zitron-Emanuel and Ganel were not attempting to create conditioned neuroses in their two experiments.  Rather, they were using established perceptual procedures to determine if participants would be better at detecting a smaller difference in size between food-based visual stimuli (cookies) when motivated by food deprivation than when discriminating between a neutral stimulus (like a circle) or when not food deprived. Examples of their stimuli are below.

Circling the Chocolate Chip Cookie. . .

In the first experiment, 72 female undergraduates had to determine which image in a pair was smaller (Panels a & b in the figure above). The key manipulation with this within-subjects variable was whether the image was food-based (i.e., the cookie, Panel a) or neutral (i.e., a circle, Panel b).  The principal between-subjects variable was whether the participant experienced a 3+ hour food deprivation or had eaten within the last hour (non-deprived).

Food-deprived participants were expected to detect very small differences between the sizes of the cookies as the deprivation state was predicted to motivate them to discriminate JNDs at smaller thresholds.  Control group participants were expected to perform at similar levels as they would with the neutral stimulus (the circle).

The results supported both expectations as seen in the next figure below.  A significant interaction was observed between food deprivation group and type of stimulus (food and control).  The difference between the two bars in the food-deprived group (on the left) was statistically significant, whereas it failed to reach significance for the non-deprived group.  But, the difference between the dark grey bars (food stimulus) was not different between the food-deprived and non-deprived groups.

… or Bouncing those Chocolate Pies?

To account for the reduced power inherent in a mixed factorial (one within-subjects and one between-subjects variable) design, and to generalize the findings to male participants, Zitron-Emanuel and Ganel conducted a second experiment with 45 female and 43 male students.

Another difference between the two experiments was the use of more complex stimuli (a chocolate pie – yummy! and a tennis ball – bouncy!) to control for the possible influence of the visual complexity of a cookie vs a simple circle.  Thus, participants in the first experiment may have counted the chocolate chips or perceived the covered surface area of chocolate chips to choose the smaller cookie (although this was probably difficult to do with the short presentation exposure).

In the second experiment, participants experienced both the food deprived and non-deprived conditions on different days and completed a similar task as in the first study, but with the stimuli presented in Panels c and d of the figure below.  Participants also had to indicate how hungry they were at the start of the session, using a 7-pt scale from 1 “replete – not hungry at all” to 7 “starving – extremely hungry”.

The changes made in this experiment led to more convincing results. Once again a significant interaction was observed, and in this experiment, significant differences were also observed between the dark grey bars of the two food deprivation conditions: deprived and not-deprived.  These differences can be seen in Figure 3 below.

“And above all, think chocolate” – Betty Crocker

Females are traditionally recruited for food deprivation studies given their higher prevalence of eating disorders.  By including males in the second experiment, the researchers were able to conclude that the male participants were as motivated by the food stimuli to detect smaller JNDs when food deprived as the female participants. Chocolate is extremely motivating for many creatures no matter their gender or sex. The fact that the findings were robust across both genders argues that this phenomenon is wired at a level that is not influenced by stereotyped perceptions or other biases.

The authors suggested that the decreased JNDs experienced by both genders while in a state of food deprivation may have been driven by attentional mechanisms.  That is, perhaps our brains are wired to increase our arousal and attention to “stimuli of relevant significance to the motivational state”.  Thus, when comparing chocolate chip cookies to simple circles or chocolate pies to tennis balls, especially when one hasn’t eaten in over three hours, chocolate (as a food source) is much more motivating and attention-getting than even something as fun as a tennis ball.

Psychonomics Article focused on in this post:

Zitron-Emanuel, N., & Ganel, T. (2017). The effect of food deprivation on human resolving power. Psychonomic Bulletin & Review, DOI: 10.3758/s13423-017-1296-6.


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