Participants
20 healthy volunteers took part in the experiment (age range 19-33, mean = 24.25, SD = 4.33). Informed and consent standing to the HUG ethic committee, they have all been paid 30 francs. The group comprised 10 women (age range 19-33, mean = 23.7, SD = 4.37) and 10 men (age range, 19-33, mean = 24.8, SD = 4.44).
Mainly students at the University of Geneva, with normal or corrected-to-normal visual acuity, and were all right handed (standing to the Edinburgh Handedness Inventory of Oldfield, 1971). The quotient of lateralization of all subjects was above 0, which indicated a right lateralization (min. = 8, max = 20, mean = 13.9).
The exclusion criterion was their history of psychopathology or neurological diseases at the exception of a subject, suffering of hypertension. Three participants have token medicament such as Beta-blockers, ACE inhibitors and anti-calcium (for the hypertension subject) and two others Zyrtec and Spasmo Canulase. All these drugs are supposed to reduce the reaction capacity. The participants had slept 7.33 hours on average, 7 subjects had estimated the night shorter than usual, one as longer than usual and the others as normal. The consumption of nicotine, tea or coffee was considered as normal by 14 subjects, as more than usual by 2 subjects and less than usual by 4 subjects. Three of the 20 participants considered smoking less than usual. 5 participants had consumed alcohol in the 24 hours prior the experiment, but the hours between their last consumption and the onset of the experiment was never inferior to 7 hours.
According to the rewired literature about the influence of anxiety in the recognition and detection of emotional stimuli of Staugaard, anxiety is supposed to modulate behavioral and ERPs responses to emotional stimuli. For this reason an anxiety test (Self-Evaluation Questionnaire of Spielberger) was submitted, before the EEG recording. No subjects presented an anxiety level susceptible to influence the EEG results
Material
The stimulus material was taken from Ekman & Friesen’s set of pictures and others similar databases (fig. 5). Black and white pictures of actors, presenting, respectively, happy, angry, fearful and neutral faces. The stimuli were modified, thanks to Adobe Photoshop 11, eliminating hair, ears and non facial boarding of faces. The aim was to give an oval shape to all the faces. The background was black for every stimulus.
The same scrambled faces (fig. 5) were used as masks for the masking paradigm, backward masking, that prevented the visual access to the targets, preserving the same physicals parameters (Di Lollo, Enns, & Rensink, 2000). A luminance analysis was made to standardize this variable and exclude luminance effects on face detection and recognition. The size of the square was 6X6 cm, the distance from the screen of 114 cm and the visual angle of 9’.
There were 40 stimuli (20 faces representing males and 20 representing females) for each emotional condition (angry, fearful, happy and neutral). The total amount of stimuli was 160 and each stimulus was presented 10 times for a total of 1600 stimuli and 1600 masks. Stimuli were divided in ten sequences of 160 stimuli each. The sequences were matched. Sequences were identical for each subject, but the presentation was randomized:
• In each sequence, half of the stimuli were presented subliminally and in the other half supraliminally.
• In half of the sequences the subjects had to respond to positive emotions (happy and neutral) and in the other half, they had to respond to negative ones (fearful and angry).
• In half of the sequences they had to respond with their right hand and in the other half with the left hand.
All these stimuli were transformed in bitmap images and they were presented by e-prime™ software.
Fig. 5. Example of normal and scrambled faces representing an angry and a neutral face used in the experimental study.
Procedure
Pre-experimental study
Before initiating the EEG experimental study, we had done a pre-experimental study to select emotional faces which represent with greater accuracy the four emotions chosen for the EEG experiment (happy, neutral, angry, and fearful).
This had been done because the emotional faces came out from different databases. The 51 subjects (28 Females and 23 Males) were submitted to an
e-prime™ software presentation representing the four emotions (from 30 to 40 for each category) and had to choose between the six basics emotions standing to (Ekman & Friesen, (1971): anger, happiness, fear, surprise, disgust and sadness.
EEG experiment
Subjects were seated comfortably in a moderately dark room with the monitor screen positioned approximately 114 cm in front of their eyes. Pictures were presented in a randomized order in the center of a computer monitor. During the examination, participants were requested to minimize blinking. They were required to observe stimulus during behavioral task and ERP recording. In the subliminal condition, it was emphasized that sometimes the target faces would be difficult to see, but the subjects were requested to concentrate as best as they could on stimulus, and to answer with less delay as possible the categories they were requested to pay attention to (positive or negative ones). A training test was made to familiarize them with the task.
Prior to recording ERPs, subjects were familiarized with the overall random order of all emotional stimuli presented in the successive experimental session. The images were passing by very fast.
The EEG experiment was divided in two evaluations:
Behavioral experiment
The behavioral part was the evaluation of correct versus incorrect responses in an attended and unattended condition (selective attention paradigm) and in a subliminal and supraliminal condition (awareness paradigm). During every presentation (total of ten parts), the subjects had to respond to a specific category that was positive (happy and neutral faces) and negative (angry and fearful faces).
There were four buttons and the response types were matched between the hand and the button to push. Before each of the ten sequences a screen explained if they
had to respond to the positive or negative faces, with which hand and with which button. Further, the experimenter verbally reiterated the procedure to follow. The subjects sometimes believed that they would be exposed to challenging masks and expressed their feelings about not being able to easily detect the emotions implied in the stimuli. However, when such a masked image did occur, they were asked to give an answer even if they were not entirely sure about the response (first impression). Moreover, although these fears are legitimate, it should be noted that before the EEG experiment the subject was submitted to an exposition of images representing the four emotions and a trainer was present. The experimenter could thus check whether the subject had well understood the task.
Fig. 6. Example of a sequence with the fixation cross and of the presentation of fearful faces in a supraliminal (287 msec. mask 13 msec.) or subliminal (13 msec., mask 287 msec.) way. The participant had to answer to each face presented as quickly as possible;
he/she had to determine whether the face was the target (positive, neutral or happy;
negative, angry or fearful) by pushing on the corresponding button during the inter stimuli interval (ISI, 1500 ms).
EEG recording (technique)
The EEG recording was taken in a Faraday cage, with a Geodesic system (Electrical Geodesics, Inc., USA) at high density. The data was recorded in a continuous way with a sample of 1000 Hz, starting from 256 electrodes equidistant placed covering the scalp of the subjects and with a vertex reference.
Each impendency was maintained fewer than 50 kΩ.
Analysis was made with Cartool (http://sites.google.com/site/fbmlab/cartool, 3.40 versions). The recording was filtered for the noise between 0.001 and 50 Hz.
During the averaging, the epochs contained artifacts (electric noises, muscular noises) or ocular movements were rejected. The epochs surmounting 100 µV were also eliminated. After averaging the electrodes of cheeks was interpolated (passage from 256 to 204 electrodes) and after that the electrodes that were excluded before. Grand-averaging for every of the 16 conditions were calculated.
The independent variables of the study are:
• The type of the emotion (fearful, angry, happy or neutral);
• The selective attention condition: if the stimulus it’s a target (T, positive, happy and neutral or negatives, angry and fearful) or not (NT, negative, happy and neutral or positives, angry and fearful);
• The presentation mode: if the stimulus were presented in a subliminal (sub, 13 msec., mask 287 msec.) or in a supraliminal (sup, 287 msec., mask 13 msec.) way.
• Three Regions of Interests (ROIs) for the N170 and the P2 component (temporo-occipital left, central and right) and one for the P300 component (parietal).
•
The dependent variables of the study are:
• The accuracy and the reaction times in the behavioral responses;
• Difference in peak components (N170, P2, P3) The controlled variables were:
• The gender, the age, the hand laterality The matched Variables were:
• The order of the presentation of the stimuli which varied randomly
• The hand which activated the buttons
Subliminal/supraliminal presentation, the backward masking procedure
The short stimulus presentation in the preattentive (subliminal) condition prevented the subjects from having a clear cognition of the stimulus. In the current study we employed an objective threshold for pre-attentive condition. It was defined by an identification procedure, the case where stimulus is perceived by the subjects not more than 50 % of the times (Liddell et al., 2004). According to the signal detection theory (SDT, Macmillan & Creelman, 1990), when detection threshold sensitivity is at chance level, it is unlikely that there is conscious awareness of the stimulus. In the current study we employed an objective which was defined by the stimulus duration, where the stimulus is perceived by the subject in 50 % of the cases (Merikle, Smilek, & Eastwood, 2001). The behavioral study and post-hoc briefing comparison confirmed that subjects were unable to detect target stimulus in the subliminal condition. During the experiment, we used a masking procedure. Each facial stimulus (target) was presented for either 13 (subliminal) or 287 (supraliminal) msec., followed by a scrambled face presented for 287 (subliminal) or 13 msec. supraliminal (Bràzdil et al., 1998 paradigm in Liddell et al., 2004). The duration of each sequence was 5.5 minutes, for a total of an hour of experimentation considering short pauses between sequences). The short stimulus presentation in the subliminal conditions prevented the subjects to have a clear cognition of the stimulus, but it allowed a semantic elaboration of the emotional faces. Target and mask pairs depicted the same individual. In total there were 1600 target-mask pairs in each threshold conditions (each facial expression was presented forty times). The conditions were counterbalanced between subjects (Bernat, Bunce, & Shevrin, 2001).
The ROIs
Depending on the component analyzed, different Region of Interests (ROIs) were created with different electrodes.
N1 and P2 (occipito-temporal), P3 (parietal)
Tree Region of Interest (ROIs) were created in order to compare left temporo-occipital posterior, right temporo-temporo-occipital posterior and middle temporo-occipital posterior with the three variables studied (emotional expression, type of presentation, selective attention).
Fig. 7. Three occipito-temporal ROIs used for the analysis of the N170 and P2 component. On the right part the electrode numbers are; TP10 191 201. On the central part; 122 123 01 125 126 135 136 Oz 138 147 02 157 158 1 and in the left part; 83 93 TP9
One region of interest (ROI) was created to observe the P3 component on the three variables studied (emotional expression, type of presentation, selective attention).
Fig. 8. Parietal ROI created for the analysis of the P300 component. The electrode numbers were: P1 89 100 129 130 P2 Pz POz.
Analysis
During the analysis phase, only the correct responses were retained (Lougheah et al., 2008). All results have been analyzed thanks to Excel and Statistica 8. For behavioral and ERP analysis an analysis of variance (ANOVA) was applied. See details (number of factors and levels) in the chapter results.
For the source localization we used Cartool (3.40 versions). In particular, the inverse solution retained was the local autoregressive average (LAURA, Grave de Peralta et al., 2001).