Figure 4

Grand-average ERPs for each type of speller and each type of attention shown for 21 electrode sites. Responses to targets are given in magenta, responses to nontargets in black. Epochs in the range [-170, 670] are shown. In the overt attention condition, for both the Matrix and the Hex-o-Spell, there are two clear positive peaks at ≈ 250 ms (P2) and ≈ 400 ms (P3), with a broad scalp distribution centered at Cz. A positivity at ≈ 120 ms (P1) and a sharp negativity at ≈ 150 ms (N1) preponderates at parieto-occipital sites. Also, a N2 component (≈ 300 ms) is evident at these sites. In the covert attention condition, the response to targets versus nontargets is less differential. In comparison to the overt attention condition, amplitudes are smaller for targets and larger for nontargets. Furthermore, in this condition, P2 is pronounced only in the Hex-o-Spell at fronto-central electrode sites. The pattern of P3 amplitudes is similar to the one encountered in the overt attention condition. Note that the y-axis scaling is different for the two attention conditions.