MATERIALS AND METHODS

• Subjects : Eighteen right-handed subjects (nine males, 19-34 years old), with normal or corrected-to-normal vision and without any neurological or neuropsychological disorder, were paid for their participation.
  All were informed of the noninvasive recording technique and gave their written consent according to the Code of Ethics established by the World Medical Association (Declaration of Helsinki).

• Stimuli : All the stimuli (800) were grey-scale digitally scanned photographs of human faces, and were presented in a roughly front view. They included faces with glasses and faces without glasses. All the faces were of women, except the 40 target stimuli delivered in the gender discrimination task, which were male. Two wide age ranges were used: half of the faces were of young women (between about 25-50 years), and the other half of older women (over 65 years). A third age range was used for the 40 target faces in the intentional-age discrimination task and was of adolescent girls (between about 13-18 years). All the stimuli were matched for luminance and contrast, and were of equal size (100 x 120 mm). They were presented foveally, subtending a visual angle of 6.9°, for 300 msec at a rate of one every 1400 msec. A fixation cross was present at the center of the screen between each presentation. No stimulus was repeated except the 160 non-target faces delivered in the intentional-age discrimination task, which served also in the intentional-gender discrimination task.

• Experimental procedure : The experiment included 7 runs, in each of which the stimuli were delivered in blocks of 50. In each run, the subjects performed a mental oddball detection task and reported verbally, at the end of each block, the correct number of target stimuli counted; the target faces (20%) were delivered randomly among non-target faces (see Table 1). In runs 1 and 2 , referred to as No-discrimination condition, target items were faces with eyeglasses and non-targets were faces without glasses. All faces were of young women in run 1 and of older women in run 2. The order of presentation of runs 1 and 2 was counterbalanced across subjects. Run 3, referred to as Incidental-age discrimination condition, was similar to runs 1 and 2, except that the faces of young and older women were equally mixed.

• The rationale was that the level of processing required in run 3 was the same as in the two previous runs but, in addition, the stimuli could also be incidentally categorized according to their age (young/old). To prevent any interference of a possible (incidental) categorization process in the two previous ’one-class’ runs, run 3 always followed runs 1 and 2. In runs 4 to 7, the subjects had to process explicitly the faces according to their age or their gender: the targets were adolescent girls among either young women (run 4) or older women (run 5) in the Intentional-age discrimination condition, and either young men (run 6) or older men (run 7) among young or older women, respectively, in the Intentional-gender discrimination condition. Runs 4 to 7 followed run 3, and their order of presentation was counterbalanced between subjects.

• ERP recording : The EEG was continuously recorded at a sampling rate of 1000 Hz (0.10-200 Hz analogue bandwidth) from 32 Ag/AgCl scalp electrodes referred to the nose and placed according to the International 10-20 System (Fig. 1). Electrode impedance was kept below 5 k-Ohms. Eye movement artifacts were controlled via the two prefrontal electrodes (Fp1 and Fp2) and an electrode placed at the outer canthus of the right eye (YH). Trials in which the potential measured in any of those channels exceeded 100 µV were rejected. The ERPs were averaged separately for each stimulus type in each run over a 600-msec period including a 100-msec prestimulus baseline, and were digitally filtered with a bandpass of 0.1 to 20 Hz. located on the forehead, and the nose was used as the reference.

• Data analysis : Only the responses to non-target stimuli were analyzed. The ERPs elicited by young and older women faces in, respectively, runs 1 and 2, runs 4 and 5, and runs 6 and 7 were grouped, so that the average response from an equal number of stimuli (160 including 80 young and 80 older women faces) could be compared across the four discrimination conditions (No-discrimination, Incidental-age, Intentional-age and Intentional-gender discrimination).

First, the effect of age or gender processing on the latency and mean amplitude of the N170 ERP component was tested using two-way within-subjects analyses of variance (ANOVAs) with the factors: Type of task (No-discrimination, Incidental-age, Intentional-age, and Intentional-gender discrimination), and Hemisphere (left, right). The latency of the N170 component was measured at the electrodes of its peak value in the grand-average (T5/T6), and its amplitude was estimated as the mean potential value over ±20-msec time window around its peak latency (130-170 msec) at IMA, M1, T5 and their homologous sites on the right hemiscalp. The significance levels of the F-values were adjusted with Greenhouse-Geisser correction when necessary.

Second, we compared the responses elicited in the Incidental-age, Intentional-age and the Intentional-gender discrimination conditions, respectively, with the responses elicited in the No-discrimination condition. For each time sample and at each electrode, Student’s t-tests were performed comparing the responses between the No-discrimination condition and each of the three other conditions. On the basis of previous criteria (Rugg et al., 1995; Thorpe et al., 1996), effects were considered as significant when the differences had a significant amplitude (p<0.05) at at least two adjacent electrodes for at least 15 consecutive time samples. The significant differential effects were illustrated in Student’s t-maps.

Topographic maps were generated using a two-dimensional spherical spline interpolation (Perrin et al., 1989). The color codes of the potential maps were normalized to the peak voltage value (positive or negative) at the considered latencies.