The overall goal of the following experiments is to objectively measure individual's motivation for different food rewards as well as the value they ascribe to them. In one experiment to examine motivation, a participant is required to work to win or see different rewards. To test subliminal motivation, a potential reward is presented so briefly that the conscious mind does not register it, and the participant's willingness to work for the subliminal reward is measured.
Another experiment requires the participant to place a monetary value on different food rewards, thus yielding a measure of value that can be easily understood and compared. These experiments are also done in an MRI scanner to correlate brain activity with motivation Experiments such as these can be used to demonstrate the potential effectiveness of selected drug treatments in binge eating and obesity. The main advantage of this technique over existing measures like self-report is that it allows us to objectively measure food related motivation and evaluation.
Individuals new to this technique may struggle because there aren't many examples of grip force tasks published in the literature and there are no full and clear descriptions of how to design and set up the tasks. We first had the idea for developing these methods when we were trying to design measures to examine the effects of a novel anti-obesity agent that alters the reward value of foods. And what these methods provided us with were objective measures of food reward that we could examine and see how they were affected by the treatment.
Set up the force transducer data acquisition system and stimulus laptop as described in the text protocol. Place the grip force transducer on the table and collect a baseline measurement to obtain a maximal voluntary contraction reference. The subject holds the grip force transducer with his arm in a comfortable position and squeezes the transducer as hard as possible.
Five times take the mean of the five exertions as the maximal calibration for the task. The relative reward application uses six images from three reward categories, high calorie food, low calorie food, and gender specific rewarding non-food, either female or male have the images independently rated for subjective liking by a healthy volunteer group. Create image pairs such that every image is paired with all the images from the other two categories.
Counterbalance the pairs such that each image appears as the default and non-fat image in an equal number of trials. The trial begins with the presentation of two images side by side. One is a large and clearly visible default image, and the other is a small indistinct non-default image set.
The force required to enlarge the non-fat image to its maximum size at 10%of the participant's maximal voluntary contraction. Next, explain the trial structure to the participant. Demonstrate that squeezing the transducer will increase the size of the non-fat image and simultaneously decrease the size of the default image.
Tell the participant that he can view images as he chooses depending on how much effort he exerts. Then leave the participant on his own to complete the task. The task will take about 22 minutes to complete and over the course of the task, the subject will evaluate 216 pairs of images.
Select three stimuli for the food motivation task, a savory food, a sweet food, and a neutral non-food item. Collect two images for each category, one for the conscious trial and the other for the subliminal trial. Using a graphics editing software tool such as Photoshop format all images to have the same luminance and the same patterned background and blur the picture edges Using a single pass, create a mask image by randomly scrambling all pictures and then blending them To create a composite image.
Explain the trial format to the participant. Mention that the presented image may be difficult to see on some trials and suggest that he follow his instincts. Tell the participant that when the fluid level graphic appears, he can squeeze the force transducer to win points towards the item just presented.
Explain that the feedback is unreliable and that he should judge his effort himself and not rely on the fluid level graphic. Begin the trial by presenting across in the center of the screen so the participant's eyes have something to fixate on. For conscious trials, present the mask for 200 milliseconds followed by the stimulus image for 200 milliseconds and ending with the mask for 100 milliseconds.
For the subliminal presentations, present the mask image for 200 milliseconds, the stimulus image for 33 milliseconds and the mask again for 267 milliseconds. When the response window displays the fluid level graphic on the screen, the participant exerts force on the grip force transducer as he chooses. The fluid level graphic provides real time, although unreliable feedback, continue to run the task in as many blocks as required.
Following the completion of the task, ask the subject how well the masking procedure worked. Also have the subject perform a forced choice discrimination by presenting the masked stimulus as in the main task followed by two options and asks the participant to indicate which one was just presented. Calculate the discriminability index as per signal detection theory.
If the masking has worked well, the discriminability index should be close to zero to perform the grip force tasks in an MRI scanner, the monitor screen in the control room is projected onto a screen inside the MRI scanner room. When in the scanner, the participant can see the projection via a mirror mounted on the scanner head coil to register the participant's response, A clench force rubber bulb is used that is connected to a pressure transducer located in the control room. Collect a baseline for the clench force bulb and test the connection between the bulb and the transducer hand.
The participant, the clench force bulb, and once the radiographer has prepared and positioned the participant in the scanner, the task is ready to run. Execute the task. As for any other FMRI paradigm, as the stimuli are displayed, the participant responds by squeezing the clench force rubber bulb.
The auction protocol involves a series of rounds each featuring one food item, photograph all food items on identical plates and backgrounds prior to the start of the task. Show participants the actual plates used in the images to provide an accurate sense of scale. Give the participants a fixed monetary budget.
For example, three pounds. Tell the participant that he will be taken through several rounds of the auction and each round will feature a different food item. He can place a bid on each round on a sliding scale that goes from zero pounds to three pounds in increments of 10 pence.
Inform the participant that only one round will be selected at the end of the auction as the round that counts. Therefore, he does not have to spend his three pound budget across different rounds, but can treat every round as if he still had three pounds to spend. Tell the participant that the computer will be bidding against him in each round.
If he outbids the computer on the selected round, he wins the food item and only has to pay the amount the computer bid and will be able to keep any remaining change. If however, the computer outbids or matches his bid, the participant does not get the food item, but still gets to keep the money. Explain to the participant that given these set of rules, the best strategy for bidding in this auction is to bid the amount closest to how much he would be really willing to pay for the food item on offer.
In a phase two study of a novel mu opioid antagonist at baseline on the relative reward tests, the exerted grip force by all subjects was significantly greater for the high fat food than the low fat food following treatment with either a placebo or mu opioid antagonist. The difference between the force exerted for high fat versus low fat food images is no longer significant for the subjects receiving the drug treatment. Even though the subjective liking for these images was higher, the correlations between the exerted force and liking ratings for the high fat food images are shown here again at baseline for both groups and separately at day 28.
The correlation between these two measures is seen at baseline and in the placebo group at the end of treatment, but is lost in the drug group. The force exerted to view rewarding non-food images in the placebo and drug groups and the liking ratings for the same images showed that there are no significant drug effects on the motivation towards or subjective liking for rewarding non-food images. In an experiment examining subliminal motivation for food subjects were first tested in a fasting state, then fed 30%of their daily calories and tested again in this sated state.
As shown here, the hunger ratings decreased after consumption of the food and the fullness ratings increased. In each test session. The subjects were shown masked images of food for either a relatively long time of 200 milliseconds or for an extremely brief time of 33 milliseconds, and they could squeeze a grip force transducer to win points towards the food just presented in the second test session.
After being sated, subjects exerted less for the food just consumed, but still squeezed for the other food regardless of awareness. In this FMRI study, subjects squeezed the grip force bulb in the scanner to win points towards items from three different categories, high calorie foods, low calorie foods, and neutral non-food. The subjects exhibited increased motivation for highly rewarding versus less rewarding foods.
This study tested the effect of satiety on the subject's willingness to pay for food rewards. It can be seen at the normalized mean bid decreased following satiation with the study meal. The decrease was seen for both foods that were similar to the food they were sated with and to those that were different.
Essentially, there was a general effect of satiation but no sensory specific satiation effect. After watching this video, you should have a good understanding of how to develop and use these grip force measures and the willingness to pay auction and to adapt them to answer your own research questions.
