145-185 msec

In all the discrimination conditions, the responses elicited between about 145 and 185 msec significantly differ from those recorded in the No-discrimination condition at a number of fronto-central electrodes.

Figure 3. A. Scalp potential distribution of the N170 component around its peak latency (150 msec) elicited by non-target face stimuli in the No-discrimination condition.

B, C, D. Scalp potential distributions and corresponding Student t-maps of the differential effects of age or gender discrimination (B: Incidental-age, C: Intentional-age, and D: Intentional-gender) compared with the No-discrimination condition in four time windows (lines a-d). The maps are drawn at an illustrative latency indicated below each time window. The Student’s t-maps, in grey scale, display the scalp regions where the amplitude of the effect differs significantly from zero at the probability level indicated below each map. The difference patterns between the responses to age or gender discrimination tasks and the responses to the No-discrimination task reveal: (a) a negative potential field at early latencies (45-90 msec) over the central sites in the three age and gender discrimination tasks; (b) a positive pattern of activation around 145-185 msec latency over the fronto-central areas in the three age and gender discrimination tasks, with, however, smaller amplitudes in the Intentional-age condition; (c) a negative pattern of activation around 215-265 msec over the occipito-parietal regions in the Intentional-age and -gender discrimination tasks, but not in the Incidental-age discrimination task; (d) this last pattern of activation lasts up to around 400 msec with higher amplitudes in the Intentional-age discrimination task than in the Intentional-gender task.

In both the Incidental-age and the Intentional-gender conditions, the differences were highly significant (p<0.01 or p<0.001) from 145-150 msec to 180 msec over a wide fronto-central region (Fig. 4B, 1st and 3rd blocks; mean amplitudes over 145-185 msec: 0.81 µV and 0.87 µV, respectively; see Table 2). In the Intentional-age condition, the mean amplitude of the differences did not reach statistical significance when averaged over the whole 145-185 msec range and across all the selected electrodes (see Table 2), but significant effects were observed during a shorter period, from about 150 to 175 msec latency, at frontal electrode sites (Fig. 4B, 2nd block; mean amplitude over 145-185 msec at F3-Fz-F4: 0.50 µV, p<0.05). Figure 3B-D, line b, illustrates the topography of the effects at 165 msec latency showing, in the three conditions, a wide positive potential field over the fronto-central sites, spreading to the parietal sites with smaller amplitudes. A negative potential pattern can also be observed in the Incidental-age condition over the bilateral occipito-temporal sites, resulting from the slightly later peak latency of the N170 component in this condition.