*3.2. Electrophysiological Results*

#### 3.2.1. Anterior N2 (225–265 ms)

The ANOVA performed on the amplitude values of the N2 component revealed a significant main effect of attention (F(1,28) = 59.864, *p* < 0.0001, η2*p* = 0.68). The negativity was larger in response to non-target stimuli (−2.24 μV, SE: 0.63) relative to the target stimuli (−0.18 μV, SE: 0.72).

Furthermore, a significant interaction between attention and electrode factors (F(2,56) = 49.215, *p* < 0.0001, η2*p* = 0.64) was confirmed, with a larger frontal N2 elicited by non-target (compared with target) stimuli at all the electrode site considered (*p* < 0.001). Moreover, the N2 measured at AFp3h–AFp4h was significantly different from those measured at AFF1–AFF2 and F1–F2 in response to both target and non-target (*p* < 0.001) stimuli. At the same time, no difference in the N2 amplitude was found between AFF1–AFF2 and F1–F2 (*p* = 0.99) electrode sites. In addition, the N2 recorded at the AFp3h–AFp4h prefrontal sites (non-target minus target: −2.73 μV was more sensitive to the attentive modulation compared with the N2 at AFF1–AFF2 (non-target minus target: −1.85 μV) and F1–F2 (non-target minus target: −1.59 μV) frontal sites (see Figure 4).

**Figure 4.** N2 component. (**A**) The grand average ERP waveforms recorded at frontal sites. The correctly recognized targets (in red) relative to non-target stimuli (in blue) led to a reduction of the amplitude of the N2 response between 225–265 ms (area highlighted in green). (**B**) The topographic map (front view) of voltage distribution in the P300 time window (350–450 ms) computed as the difference wave target minus non-target. The positive values are represented in red, while the negative values are represented in blue. (**C**) The boxplots relative to the significant attention X electrode interaction. The N2 response to target (compared with no-target) stimuli was smaller at all electrode sites.

#### 3.2.2. Selection Negativity (240–280 ms)

The ANOVA performed on the amplitude values of the occipito-temporal selection negativity (SN) potential showed a significant main effect of the attention factor (F(1,28) = 8.512, *p* < 0.007, η2*p* = 0.23). The SN was more negative in response to target (4.32 μV, SE: 0.51) relative to non-target (4.85 μV, SE: 0.49) stimuli (see Figure 5).

**Figure 5.** Selection negativity (SN) component: grand average ERP waveforms. The figure illustrates the grand average ERP waveforms recorded at occipito-temporal sites. The correctly recognized targets (in red) relative to non-target stimuli (in blue) led to more negative values of the SN (selection negativity) response between 240–280 ms (area highlighted in green, which also corresponds to the area under the dotted curve). The top row shows electrode sites over the left hemisphere (P9, PPO9h, and PO7), while the bottom row shows those over the right hemisphere (P10, PPO10h, and PO8).

The SN was also larger over the left hemisphere (3.47 μV, SE: 0.46) compared with the right hemisphere (5.70 μV, SE: 0.57), as shown by the significant effect of hemisphere factor on the SN amplitude (F(1,28) = 46.414, *p* < 0.0001, η2*p*= 0.62).

Moreover, the significant attention by hemisphere interaction (F(1,28) = 16.756, *p* < 0.001, η2*p* = 0.37) revealed a more negative SN response to target (relative to non-target) stimuli over the left hemisphere (non-target: 3.92 μV, SE: 0.48; target: 3.02 μV, SE: 0.45; *p* < 0.0002), but not the right hemisphere (non-target: 5.78 μV, SE: 0.55; target: 5.62 μV, SE: 0.61; *p* = 0.64) (see Figures 6 and 7).

Finally, the main effect of the electrode factor (F(2,56) = 67.7041, *p* < 0.0001, η2*p* = 0.71) showed that the SN amplitude maximally peaked at P9–P10 electrode sites (3.38 μV, SE: 0.49) and gradually reduced at PPO9h–PPO10h (4.86 μV, SE: 0.50) and PO7–PO8 (5.52 μV, SE: 0.52), respectively.

**Figure 6.** SN component: boxplots. The figure illustrates the boxplots relative to the significant attention by hemisphere interaction for the SN component. The SN was larger over the left than the right hemisphere. Moreover, the SN was more negative in response to the target (in red) compared with non-target (in blue) stimuli over the left, but not the right hemisphere. This evidence suggests that selective attention processes for visual object recognition are left-lateralized.

**Figure 7.** SN component: topographic maps. The figure illustrates the back view of the topographic maps of voltage distribution in the SN time window (240–280 ms) relative to non-target stimuli (**A**), target stimuli (**B**), and the difference wave target minus non-target (**C**). The positive values are represented in red, while the negative values are represented in blue. A strong left-lateralized negative peak is visible at occipito-temporal sites (**C**).

#### 3.2.3. P300 (350–450 ms)

The ANOVA performed on the amplitude values of the P300 component showed a significant main effect of the attention factor (F(1,28) = 97.756, *p* < 0.0001, η2*p* = 0.78). The positivity evoked by the target stimuli (8.95 μV, SE: 0.77) was larger than that evoked by non-target stimuli (2.78 μV, SE: 0.46), as can be seen in Figure 8.

**Figure 8.** P300 component. (**A**) The grand average ERP waveforms recorded at centro-parietal midline sites. The correctly recognized targets (in red) relative to non-target stimuli (in blue) elicited a larger P300 response between 350–450 ms (area highlighted in green). (**B**) The topographic map (back view) of voltage distribution in the P300 time window (350–450 ms) computed as the difference wave target minus non-target. The positive values are represented in red, while the negative values are represented in blue. (**C**) The boxplots relative to the significant main effect of the attention factor.

#### 3.2.4. swLORETA Source Reconstruction (240–280 ms)

The swLORETA inverse solution investigated the cortical sources of the bioelectrical activity recorded over the scalp underlying selective attention processes for object recognition. For this purpose, the source reconstruction was applied to the difference wave obtained by subtracting the ERP evoked by non-target stimuli from those elicited by target stimuli in the SN time window (240–280 ms). A list of the estimated active electromagnetic dipoles can be found in Table 1. The main dipoles were located

in the medial frontal gyrus (BA 11) and right anterior cingulate cortex (BA 24). The uncus (BA 28/36) was also bilaterally engaged, together with the left middle/superior temporal (BA 22) and inferior frontal/precentral (BA 6/9) gyri (see Figure 9).


**Table 1.** List of estimated electromagnetic dipoles.

List of the electromagnetic dipoles estimated in response to target minus non-target stimuli in the SN time window (240–280 ms) according to swLORETA, with the relative Talairach coordinates. (Legend: Hem—hemisphere, MedFG—medial frontal gyrus, ACC—anterior cingulate cortex, IFG—inferior frontal/precentral gyrus, MTG/STG—middle/superior temporal gyrus, T—temporal lobe, F—frontal lobe, Lim—limbic system, BA—Brodmann's area, R—right, L—left).

**Figure 9.** Standardized weighted low-resolution electromagnetic tomography (swLORETA) source reconstruction of surface potentials in the SN time windows. SwLORETA performed on the grand-average waveforms of the difference wave (target minus non-target) in the time window of the SN component (240–280 ms). The sagittal (**A**), horizontal (**B**), and coronal (**C**) anatomical planes of the brain are shown. The engagemen<sup>t</sup> of limbic regions is visible, which includes the right anterior cingulate cortex (ACC, BA 24), medial frontal gyrus (MedFG, BA 11), and uncus, bilaterally (BA 28/36). Active dipoles in the left middle/superior temporal gyrus (MTG/STG, BA 22) and inferior frontal/precentral gyrus (IFG, BA 9/6) are also shown. The strongest magnitude values of the signal (nAm) are presented in red.
