This protocol describes the use of movies to investigate brain mechanisms underlying implicit social biases during functional magnetic resonance imaging. When the face of a protagonist is presented after a movie subliminally, it evokes an implicit response based on knowledge of the protagonist gained during the movie.
New knowledge is continuously gained from a social environment that can influence how people respond to each other. Such responses often occur implicitly, at a subliminal perceptual level, and related brain mechanisms can be experimentally isolated by presenting the stimuli quickly. Subliminal presentation of faces that belong to different ethnicity groups, races, or gender has been shown to be successful in investigating social implicit responses. However, many implicit responses are based on knowledge previously gained about the faces (e.g., sexual orientation, political views, and socioeconomic status) and not solely on physical appearance. Here, a novel method called post-movie subliminal measurement (PMSM) is presented. When watching a socially engaging movie, a spectator gains knowledge about the protagonist and becomes familiar with his/her identity and world views. When the face of the protagonist is presented subliminally after the movie, it evokes an implicit neural response depending on what is learned about the protagonist. With a vast number of movies available, each depicting a variety of people with different identities, the PMSM method enables investigation of the brain's complex implicit biases in a manner that resembles real-life social perceptions.
Recent studies show that initial social judgment becomes formulated within the first 32–100 ms of meeting another person1,2,3,4,5,6,7. Subliminal presentation of faces has been used extensively to investigate implicit biases towards different ethnic and racial groups (e.g., by presenting Caucasian American and African American faces that differ in skin color to subjects from both groups)8,9,10,11,12,13,14. However, social groups are also characterized by factors other than physical facial characteristics15.
Facial perception has been shown to be highly sensitive to contextual cues (i.e., body posture16, eye-gaze direction of the face17, a priori knowledge about the face18, visual background of the presented face19, presentation of the face separately or with other faces20). These factors can all affect facial perception. Weiser and Brosch21, in their extensive review, suggested to investigate facial perception in more naturalistic settings by ensuring that laboratory experiment is similar to real-life environments. Indeed, even simple tasks, such as recognizing people, have been shown to be more accurate when presented with video footage closer to real-life perception than when using static images22.
During the last several decades, brain imaging studies have proven that video clips can be successfully used to study realistic social perception23,24,25,26,27,28,29. The presented method is based on results from these studies and additional findings demonstrating that movie narratives can temporarily transport viewers to the world of a protagonist30. The protocol combines movie viewing with subliminal stimulus presentation as an alternative method to investigate implicit social bias formation under naturalistic conditions.
The protocol for this novel approach, post-movie subliminal measurement (PMSM), is presented here. When watching a socially engaging movie, the spectator gains knowledge about the protagonist and becomes familiar with his/her identity and world views. In contrast with other narrative art forms, movies are unique in that they present a compelling, rich, and complex story over a short time period. Furthermore, audiovisual and cinematic properties of movies synchronize brain activity across spectators23,25,29,31. Thus, it is helpful to ensure that subjects are presented with the information in a considerably similar way.
The PMSM method shows that when the face of a protagonist is presented subliminally after the movie (vs. before), implicit neural responses are successfully evoked. These responses depend on knowledge that the viewer gains about the protagonist's character with respect to his/her implicit social views. As there is a vast number of movies available that depict a variety of social characters, the PMSM method enables investigation of the brain's complex implicit views in a manner that is close to real-life social perceptions.
The protocol was approved by the Aalto University Research Ethics Committee.
1. Participant screening and preparation
2. Procedure outside MRI
3. Procedure inside MRI
NOTE: During the fMRI session, participants are presented with a 30 min audiovisual content, which includes instruction slides, 4 min pre-movie subliminal measurement (for baseline), a 20 min movie, 4 min post-movie subliminal measurement, and concluding slides. In this section, follow the protocol to become familiar with the steps needed to create different parts of the stimuli as well as the order of presentation. Since the flash of the face during the subliminal portion has a 40 ms duration (a duration of a one video frame), it is possible to use an off-the-shelf video editing program (e.g., AVID media composer software or Adobe Premiere Pro editing software) to create the subliminal stimuli as well as edit the movie, if needed. When presenting the stimuli in the correct order using locked timing, use a software that is compatible with fMRI stimuli presentation (e.g., Presentation software, Neurobehavioral Systems Inc., Albany, California, USA).
Presented here are some results using PMSM from the published article by Afdile et al.34. Here, implicit bias was investigated among homosexual and heterosexual subjects (15 heterosexuals, 14 homosexuals) toward the protagonist after realizing he is homosexual in the movie, making him an "ingroup" to the homosexual participants and "outgroup" to the heterosexual participants. In line with our IAT results, this factor was shown to be significant in both groups, in which the heterosexual subjects were implicit in favor of heterosexuals over homosexuals, and homosexual subjects were implicit in favor of homosexuals over heterosexuals (mean heterosexuals = -0.26, mean homosexuals = 0.3, t = 3.72, p < 0.01). Both groups were significantly different from zero (homosexual: p = 0.0059, heterosexual: p = 0.043).
Our results revealed significantly larger differences in the homosexual vs. heterosexual subjects in response to the face post-movie in the bilateral superior frontal gyrus (sFG), right temporal parietal junction (rTPJ), anterior cingulate cortex (ACC), bilateral frontal pole (FP), and medial prefrontal cortex (mPFC). Figure 2 depicts the "strong" and "weak" representative results after performing the PMSM in homosexual and heterosexual Finnish participants.
Figure 1: Illustration of the PMSM stimulus structure. (A) 4 min baseline measurement (pre-movie subliminal measurement) with flashes of the protagonist's face. (B) 20 min movie depicting the story of the protagonist. (C) 4 min post-movie subliminal measurement [replication of (A)]. (D) Illustration of a 1 min clip from the 4 min subliminal measurement consisting of four blocks (rest, condition, rest, condition). The rest of the blocks are 15 s of dynamic white noise. The condition block contains 10 flashes of the protagonist face at a duration of 40 ms with intervals of 1,500 ms of white noise. This figure has been modified from Afdile et al.34. Please click here to view a larger version of this figure.
Figure 2: Representative results. BOLD signals in response to the subliminal presentation of the homosexual protagonist's face after viewing the movie. Left side: representation of strong results from homosexual vs. heterosexual (p < 0.01, cluster corrected). Right side: heterosexual vs. homosexual, significant but weak results that did not survive correction (p < 0.05, uncorrected). This figure has been modified from Afdile et al.34. Please click here to view a larger version of this figure.
This paper outlines the novel method for investigating the implicit brain using a post-movie subliminal measurement referred to as PMSM. In a recently published study, this method has shown that 1) implicit brain response is dynamic and 2) there is continuous learning from the social environment as well as formulation of judgment based on contextual knowledge (and not solely based on facial characteristics). Therefore, the proposed PMSM method can provide an alternative to the classical method when investigating implicit bias (e.g., when presenting faces that belong to different ethnical, gender, or race groups). The PMSM brings the experimental setting closer to real-life social perception, in which the results are based on naturalistic viewing.
The presented protocol uses fMRI techniques; however, it is also possible to conduct PMSM using other neuroimaging measures including electroencephalography (EEG) or magnetoencephalography (MEG). The experimental design presented here is for a two-group comparison; however, there are no limitations when using PMSM for multiple group or within group comparisons. Furthermore, it may be possible to extend PMSM to measuring the implicit response to more than one face (i.e., both the protagonist and antagonist in a movie). This can further shed light on the investigated subject group (i.e., measuring the implicit response to two characters carrying opposing world views in a movie).
Researchers interested in investigating other aspects of social perception and their interactions with memory and bias, such as attention and emotion under naturalistic conditions, can take advantage of the fMRI data collected during PMSM to run various analysis (i.e., inter-subject correlation, functional connectivity, and model-based analyses of activity elicited by various events in the movie). However, it is important to have a hypothesis to start with as this can help in collecting additional behavior data during the experiment that can be very helpful in interpreting the results.
Furthermore, by editing the videos presented in PMSM it should be possible to investigate various aspects of attitude formation towards in/outgroup members (e.g., manipulate the conversation between ingroup and an out-group character to assess how intergroup bias effects how opinions and world views of in/outgroup members are perceived). Another advantage of PMSM is its feasibility to measure implicit bias towards groups that are hard to differentiate based on appearance. For example, by showing videos of interactions of natives vs. newcomers, PMSM can measure implicit bias towards newcomers. Afdile et al. provides further reading regarding the limitation of past implicit and priming method in comparison with PMSM34.
In acknowledgement of PMSM's limitations, the representative results show that implicit bias may not be symmetrical across groups (e.g., among non-conflict social groups ingroup bias might be a stronger response than outgroup bias). This is reflected in our representative results, in which 14 homosexual subjects showed a robust ingroup implicit response to the face of the homosexual protagonist. In contrast, results from the 15 heterosexual subjects were not strong enough to survive correction.
Although this is not purely a PMSM limitation, and it is possible that using other neuroimaging methods may have shown stronger results in both groups, it is advised to use a larger number of participants when conducting PMSM with fMRI. Furthermore, a limitation in PMSM can be found in the number of faces that can be tested, as movies carry a finite number of significant characters within the story, especially in short movies. Although PMSM may be closer to life-like social perception, there must be caution in interpreting the results and drawing generalized conclusions (in comparison with more simplified task paradigms that have repeated tasks with a high number of conditions). PMSM should be chosen for cases in which it best suits testing of the hypothesis.
A critical step in the PMSM method is the choice of the movie. There are inter-individual differences in the level of how easily people identify with characters and get immersed or transported to the world of the story35,36. However, there are several approaches that can overcome this challenge. For example, movies that have been commercially successful tend to be highly structured (through cohesive editing style) and have simple and coherent inner logic to follow, which are two important factors that increase the engagement of the viewer37,38.
Furthermore, documentaries or movies depicting topics that are relevant to the viewer can further increase the transportability (see Green)39. Another strategy is choosing a movie of a genre that the experimental subjects will enjoy watching40. Successful choice of a movie can increase the efficiency of PMSM and also provide additional data for those who are interested in learning how the implicit brain formulates its judgment by analyzing brain activity during movie viewing.
The authors have nothing to disclose.
This work was supported by Academy of Finland, grant numbers [259952, 276643]. We would like to thank Mikko Sams for the supervision and Enrico Glerean, Jussi Alho, Anna Äimälä for helping with the data, Johan Westö for helping with the visualization as well as Marita Kattelus and Toni Auranen from Advanced Magnetic Imaging (AMI) Centre, Aalto NeuroImaging, Aalto University, Espoo, Finland for their help and support.
3T Siemens MAGNETOM Skyra | Siemens Healthcare, Erlangen, Germany | MRI device, using a standard 20-channel receiving head-neck coil | |
Avid Media Composer | https://www.avid.com/media-composer | Video editing software used to create the stimuli. | |
EAR-tip | Etymotic Research, ER3, IL, USA | Earplugs compatible for MRI | |
FSL software | https://www.win.ox.ac.uk/, version 5.0.9 | Software used to analyse the data. | |
Panasonic PT-DZ110X projector | Panasonic Corporation, Osaka, Japan | The stimuli were back-projected on a semitransparent screen | |
Presentation software | Neurobehavioral Systems Inc., Albany, California, USA | Software used to present stimuli during the fMRI scan | |
Sensimetrics S14 insert earphones | Sensimetrics Corporation, Malden, Massachusetts, USA | Auditory stimulation was delivered through Sensimetrics S14 insert earphones |