Participants were 24 undergraduate students (four men) at Maastricht University (average age 23.1 years; range 18–35). They read and signed an informed consent, and received course credits for their participation. The experiment was approved by the ethical committee of the Faculty of Psychology.
Every participant was asked to bring two passport photos: one of a sibling and one of a good friend. The persons on the two photos had to be of the same sex. These photos were scanned and converted to grayscale. Two additional versions of each picture were produced: one with a dot on the right cheek, and one with a dot on the left cheek (see Fig. 1 for an example). Stimulus size was 49 × 66 mm and viewing distance was 1 m.
Experimental Design and Procedure
Participants were allocated to groups of three. For each member of a group, stimulus material consisted of the two pictures they brought plus the four pictures the other two participants had brought. Each participant completed two conditions. In the face condition, participants had to classify each face based on familiarity. Participants were instructed to acknowledge recognition of one of the two familiar faces by pressing one of two buttons placed under their left and right index fingers, respectively, and pressing the other button for all unfamiliar faces. For half of the participants this entailed acknowledging recognition of their sibling, while for the other half it entailed acknowledging recognition of their friend. They were explicitly instructed to deny recognition the other familiar face, by classifying it as unfamiliar.Footnote 1 In the dot condition, we isolated mere recognition while ensuring that the faces were indeed processed. To achieve this, two versions of each picture were produced: one with a dot on the right cheek, and one with a dot on the left cheek (see Fig. 1 for an example). In this dot condition, the participants were instructed to press the left or right button in correspondence to the location of the dot. The order in which the participants completed the face and dot condition was counterbalanced.
Each trial started with the presentation of a picture, which was shown until the response button was pressed, with a maximum of 2,500 ms. Feedback was given if no response was given after 2,500 ms (‘too slow!’) or if the response was incorrect (‘wrong!’). Each response was followed by a blank screen of a 2,100 ms duration, after which the next picture was presented. The face condition consisted of 12 practice trials that served to familiarize the participants with the procedure, and 432 trials that were presented in three blocks of 144, with a break in between blocks that could be terminated by the participant. Thus, each face was presented on 72 trials. In the dot condition, left and right button presses were matched to the face condition, meaning that in 12 of these trials the dot was on one side, and in 60 on the other. The dot condition also consisted of one practice block with 12 trials and three blocks of 144 trials.
Data Acquisition, Reduction and Analysis
EEG data were recorded from four midline sites (Fz, Cz, Pz, Oz) and the right mastoid (A2), using Ag/AgCl electrodes, glued to the scalp with 10–20 conductive gel. All leads were online referenced to the left mastoid (A1). Horizontal and vertical electrooculograms (EOGs) were recorded using electrodes placed laterally to both eyes as well as below and above the left eye. EEG and EOG electrode impedances were below 5 and 10 kΩ, respectively. All signals were amplified using Contact Precision Instruments amplifiers. EEG was amplified 20,000 times, EOG 4,000 times. The signal was filtered online (0.1–30 Hz bandpass), and digitized at 200 Hz. All leads were off line re-referenced to an average of A1 and A2. Eye blink artifacts were reduced using a regression based method (Semlitsch et al. 1986) performed on the continuous data. After this, epochs were extracted from the continuous data, lasting from 100 ms before until 1,200 ms after stimulus onset. To ensure a reliable artifact rejection, these epochs were baseline corrected, after which all trials containing amplitudes exceeding ±75 μV and all trials with an incorrect or too slow (>2,500 ms) behavioral response were removed. Remaining trials were then baseline corrected on the pre-stimulus interval, and averaged per stimulus type. All trials on which an unfamiliar face was presented were pooled into one average.
P300 was measured using the peak–peak method described by Rosenfeld (e.g., Rosenfeld et al. 2006). Firstly, the maximal positive 100 ms segment average was determined in the 300–800 ms window. This was defined as the peak P300 amplitude. Next, the maximal negative 100 ms segment average following this positive segment was determined. Peak–peak P300 amplitude was defined as the difference between these two segments. It has repeatedly been shown that this peak–peak method outperform a typical base-peak measure in a CIT paradigm (e.g., Soskins et al. 2001). Therefore, this peak–peak P300 measure was used as the dependent variable in an analysis of variance. As P300 is generally largest at Pz, we limited our analysis to this site.
Results and Discussion
Analysis of the behavioral data in the dot condition revealed no difference between the familiar and unfamiliar faces in terms of error rates (F(1,23) = 1.1, p = .31; M familiar = .96, SD = 0.03, M unfamiliar = .97, SD = 0.03) or reaction times (F(1,23) = 1.2, p = .30; M familiar = 736.5, SD = 103.6, M unfamiliar = 754.0, SD = 130.9). The ERP waveforms elicited by the familiar and the unfamiliar faces in the dot condition are given in Fig. 2 (left panel). A repeated measures ANOVA comparing the two familiar faces including order as a between subjects factor revealed no significant effects, meaning both familiar faces elicited a comparable P300. These two were therefore averaged. Comparison of this average P300 with that elicited by the unfamiliar faces showed that familiar faces elicited a larger P300 than unfamiliar faces (F(1,23) = 37.1, p < .001). These results indicate that mere recognition was sufficient to elicit a P300.
To contrast the P300 elicited by mere recognition with that elicited by active concealment of recognition, it is also of interest to compare the P300 elicited by the familiar faces in the dot condition with that elicited by the familiar face of which recognition was denied in the face condition. P300 amplitudes for familiar and unfamiliar faces for both conditions are plotted in Fig. 3 [for the ERP waveforms of this condition see Meijer et al. (2007; Experiment 1)]. A repeated measures ANOVA on these values with condition (face, dot) and type (familiar, unfamiliar) as within factors and order as a between factor revealed no significant effect of order. This factor was therefore dropped from the analysis. The subsequent ANOVA revealed a significant interaction (F(1,23) = 13.2, p = .001). Post hoc testing showed that the P300 to the recognized face was smaller in the condition where participants classified according to dot placement compared to classification based on familiarity (F(1,23) = 11.1, p = .003), and no difference between the irrelevant stimuli.
The results indicate that, even under the instructions to respond to an irrelevant dimension, familiar faces still elicit a P300. This means that in this case, mere recognition was sufficient to elicit a P300. This P300 was, however, smaller than when participants were instructed to classify based on familiarity. To replicate the finding of a P300 due to mere recognition, we conducted a second experiment using autobiographical stimuli, again instructing the participants to respond to an irrelevant dimension.