On target selection as reflected by posterior ERP components in feature-guided visual search

Abstract

The N2pc event-related potential is a widely studied ERP component that reflects the covert deployment of visuo-spatial attention to target stimuli displayed laterally relative to fixation. Recently, an analogous ERP component, named N2pcb, has been proposed as a marker of the deployment of visuo-spatial attention to targets displayed on the vertical midline. Two studies that investigated the N2pcb component found analogous results, using however two different algorithms to compute the amplitude of N2pcb. One study subtracted the ipsilateral activity elicited by a lateral target from the bilateral activity elicited by a target displayed on the vertical midline, whereas the other study subtracted the bilateral activity elicited by target-absent displays from the bilateral activity elicited by a target displayed on the vertical midline. Here we show both algorithms estimate properly the N2pc as well as the N2pcb components. In addition, we explored whether the singleton detection positivity (SDP) component, a posterior bilateral positivity temporally concomitant to N2pc recently reported in studies using singleton search, could be observed in the present study in which a target was defined by a combination of features. Given that such component was indeed found using feature search, we named this component posterior processing positivity (PPP), and showed that bilateral activity elicited by target-absent displays is an adequate baseline for its correct isolation.

Keywords: ERP; N2pc; PPP; visual search; visuo-spatial attention.

FIGURE 1

Example of a 6‐frame MFP trial. A target circle (e.g., a green circle containing a bar in a specific orientation, as in the top left frame) could be displayed in a lateral position (3 or 9 o'clock) or in a midline position (12 or 6 o'clock). When a frame contained a circle in the target color, the bar inside this circle could be oriented as a target (target‐present frame; as in frames F1 and F3) or a non‐target (feature‐present frame, as in frame F6). To avoid clutter, the blank frames interposed between each two successive frames in the actual experiment have been omitted in this figure. The size of the stimuli is approximately to scale with the stimuli as displayed on the computer monitor used in the experiment. Hue and brightness of the stimuli have been chosen so as to maximize clarity of the depiction, and are only partially faithful to the stimuli used in the experiment (see Section 2.2 for the correct values).

FIGURE 2

ERPs elicited at PO7/PO8 electrodes. (Panel a) ERPs elicited by target‐present (solid lines) and feature‐present (dashed line) frames. (Panel b) ERPs elicited by target‐absent frames. The dashed rectangle in each graph indicates the time‐window for ERP amplitude analysis. Positive voltage is plotted upward. Contra = contralateral; Ipsi = ipsilateral.

FIGURE 3

Difference in ERPs recorded at PO7/PO8 generated by treating as baselines for N2pc/N2pcb amplitude calculation the ERP ipsilateral to a circle in the target color (Panel a), the bilateral ERP elicited by heterogeneous frames (Panel b), homogeneous frames (Panel c), and no‐color frames (Panel d). Dashed rectangles in the graphs indicate the time‐window for N2pc/N2pcb amplitude analysis. Positive voltage is plotted upward. Contra = contralateral; Ipsi = ipsilateral.

FIGURE 4

(Panel a) ERPs elicited at PO7/PO8 generated by subtracting the bilateral ERP elicited by target‐absent heterogeneous frames from contralateral and ipsilateral ERPs elicited by target‐present frames (i.e., frames including the target combination of colored circle and bar orientation). Positive voltage is plotted upward. Contra = contralateral; Ipsi = ipsilateral. (Panel b) Top‐viewed topographic map of difference ERPs in a 300–800 ms time‐window, indicated by the dashed rectangle in Panel (a), with contralateral and ipsilateral electrodes positioned over the left and right sides of the scalp, respectively.

FIGURE 5

Results of the ICA analysis of ERPs. Back‐viewed topographic maps (left) and corresponding time‐courses of the ICA components (right) with a lateralized source generated by subtracting from the average time‐course elicited by frames with a circle in the target color displayed in the left visual hemifield those displayed in the right visual hemifield (Panel a) and vice versa (Panel b). The dashed rectangles indicate the time‐windows explored for the isolation of ICA components with a likely lateralized origin (i.e., N2pc and SPCN). Positive voltage is plotted upward.

FIGURE 6

Results of the ICA analysis of ERPs. Back‐viewed topographic maps (left) and corresponding time‐courses of the ICA components generated by subtracting the time‐course elicited by target‐absent heterogeneous frames from the time‐courses elicited by target‐present and feature‐present frames. Black and red lines displayed along the x‐axis of the graphs on the right indicate a significant difference from 0 of the positive portion of the ICA time‐courses displayed in the corresponding color, while cyan lines indicate a significant difference between target‐present and feature‐present time‐courses in the indicated time‐range. Positive voltage is plotted upward.