Post-Movie Subliminal Measurement (PMSM), for Investigating Implicit Social Bias

Post-movie subliminal measurement can help answer key questions in understanding the neuro-processing underlying social perception, such as how contextual knowledge bias our implicit response toward others. Implicit bias is a very challenging phenomenon to investigate. By presenting stimuli for very brief periods of time, one can measure the implicit response of the brain that perceives conscious processing.

In our new paradigm, we are presenting movies to experimental subjects to shape their implicit bias. This can be relevant to researchers who are interested in investigating social perception and implicit bias amongst social groups that cannot be differentiated based on their appearances, such as political groups, religious groups, groups with different sexual orientation, and more. The following protocol will be demonstrated by Marita Kattelus and Johan Westo.

Upon arrival of the participants, they are briefed with the stages of the experiment, risks and usage of their data. The briefing is concluded with asking if they have any questions regarding the experiment, and that further briefing will be delivered after the experiment is done. The participant reads the briefing of the experiment and sign the consent form.

The participant is asked to remove all metal objects from their clothing, or preferably change to a metal-free lab cloth. The body of the participants should be scanned using metal detection to assure no metal was forgotten to be taken off. Standard contraindications to MRIs should be respected.

The participant is asked to enter the fMRI laboratory for a configuration of the video projector and audio system. Assure that the headphones are presenting the sound correctly. The participant lies down on an fMRI bed while a sample video is played to see if the picture is easily visible, and the audio level is comfortable and clear.

Should any complaints arise from the participant, adjustments should be made accordingly. Connect the fMRI-compatible eye tracking system. The tracking is used to assure that the subject is attentive during the experiment, and does not fall asleep or daydream during the fMRI scan.

Once the eye tracking is secured, start the calibration process to begin recording the eye movements. Inform the subject that the experiment is about to begin, and instruct the participants to relax as if they were to watch a TV program in his or her home. Remind the participant to follow the instruction slides that will guide them through the scan of the different steps.

Now, go ahead and start the fMRI scan. Move the participant to a different room where additional behavior data can be collected. Once all the data is collected, debrief the participant of the actual aim of the experiment, and answer any additional questions that might arise.

During the fMRI scan, the participant will be presented with a 30-minute audiovisual content, which includes instruction slides, four minutes baseline measurement, a 20-minute movie, four-minute post-movie subliminal measurement, and concluding slides. Note that the movie duration can be longer if needed. The four minutes of subliminal measurements, baseline and post-movie, are identical.

The participant should not be informed of the nature or purpose at this stage, and should be presented with the following instruction. Soon, you will see a calibration clip. This clip is meant for calibrating the MRI scanner for your responses.

The clip is only four minutes long, and will look like white noise on a TV screen. Please keep your eyes fixated at the mark in the center of the screen until notified otherwise. Following the instruction slide, a fixation cross will appear in the middle of the screen for two seconds, followed by the four-minute stimuli for the baseline subliminal measurement.

The four-minute subliminal stimuli contain white noise virtually divided into 16 blocks of 15 seconds each. The 16 blocks of white noise contain two types of blocks, a rest block, followed by a condition block. To create the four-minute subliminal stimuli, start with a 15-second white noise clip.

The white noise also serves as a masking stimulus for the face that is being flashed. Therefore, use a dynamic white noise clip that has big distortion and movements. Once you have the 15-second dynamic white noise clip, go ahead and create the 15-second condition block by inserting the 10 flashes of the protagonist’s face in it.

The subliminal flashes should be at a 40-millisecond duration, starting at the onset of the condition block inserted every 1, 500 millisecond. For an image of the protagonist’s face, you can search for an image of the actor on the internet, or you can take a frame from the movie. When using the internet, make sure the actor looks similar to his appearance in the movie.

Face images taken from the internet or the movie should have a neutral face expression, big, clear, and well-lit, as well as facing the camera. Adjust the image by centering it in the middle of the screen. Make sure there are no salient things in the background of the face.

If so, go ahead and cut them out or mask them to create a neutral image. Finally, flip the image from color to black and white before inserting it. Once you have the rest block and the condition block ready, duplicate them to create the four-minute subliminal stimuli.

By the end of the process, you should have a four minutes containing 16 blocks each of 15 seconds. Representative results. In the following section, we will share some of the strong and weak results using post-movie subliminal measurement from our recent study.

Here we investigated implicit bias among 14 homosexual and 15 heterosexual subjects toward the protagonist of the movie Priest. The 20 minutes edited version tells the story of a Catholic priest in a struggle between his devotion to his faith, and his desire to be loved by another man. To analyze the differences between the homosexual and heterosexual group, we ran a two-way mixed effect ANOVA, which compares the differences in bold responses between the face after watching the movie, versus the baseline.

In line with the hypothesis, the results revealed significantly larger bold signals in the homosexual versus heterosexual subjects in response to the face of the homosexual protagonist. Although the heterosexual group also showed significance in multiple regions including right amygdala, left insula, and bilateral central sulcus, the results were not strong enough to survive correction. This implies that in-group and out-group bias might differ in their strength, and therefore, it is advised to avoid using small sample groups when conducting group comparisons.

This video shows you how our method can be set up in an fMRI study. The post-movie subliminal measurement can also be used with other newer imaging modalities. When using this method, it’s important to choose a movie that is engaging, professionally executed, and relevant to your topic of investigation.

Luckily, a quick search on the internet can help you find any movie depicting any social group in almost any language. This what makes movie characters a great candidate as a stimulus to investigate social perception and implicit bias.