MATERIALS AND METHODS

Table 1. Summary of the Experimental Design. The experiment was divided into seven runs, inducing four different discrimination tasks on human face stimuli (No-discrimination, Incidental-age, Intentional-age and Intentional-gender discrimination). In all conditions, the subjects performed an oddball detection task counting mentally the number of target stimuli delivered randomly among non-target stimuli.
Run Discrimination
task
Non-Target face stimuli
(Total number)
Target face stimuli
(Total number)
1 Young women without glasses (80) Young women with glasses (20)
No-discrimination
2 Old women without glasses (80) Old women with glasses (20)
3 Incidental-age Young and old women without glasses (160) Young and old women with glasses (40)
4 Young women (80) Adolescent girls (20)
Intentional-age
5 Old women (80) Adolescent girls (20)
6 Young women (80) Young men (20)
Intentional-gender
7 Old women (80) Old men (20)
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Figure 1. Distribution of the 32-channel electrode montage on the scalp. The ground was

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.