Dissociation of visual C1 and P1 components as a function of attentional load: an event-related potential study

Abstract

The earliest cortical location at which attention influences visual processing is controversial. To address this issue, the C1 and P1 components of cue-elicited ERPs were examined in a spatially-cued task under high and low levels of attentional load (active vs. passive viewing). Cues were presented either to the left or to the right visual field in separate trials (unilateral presentation), or to both visual fields simultaneously (bilateral presentation). For the unilateral presentation, C1 (peak latency approximately 80 ms) was not modulated by attentional load, whereas P1 (peak latency approximately 120-140 ms) was larger for high-relative to low-load condition. Bilateral presentation of the stimuli enhanced the amplitude of the C1 component relative to unilateral presentation; however, the increase of signal/noise ratio of C1 revealed no attentional load effect on C1. Results show that attentional load modulates visual processing in the P1, but not in the C1 time range, regardless of the increased signal/noise ratio by bilateral presentation. While it remains unclear about the conditions under which a C1 attentional effect is reliably elicited, the present results suggest that the direct manipulation of attentional load under a voluntary attention task seems not crucial for eliciting C1 attentional effect.

Figure 1

Illustrations of the valid, neutral, and invalid trials (upper panel), and the cue and target stimuli (lower panel) used in the present study. For the unilateral (valid and invalid) cues, the cue stimulus appeared only in the left or right visual field. For the bilateral (neutral) cues, the cue stimulus appeared simultaneously in both the left and right visual fields. The perceptual load of the target stimuli was low or high with equal probability. The cue-elicited ERPs for the left, right, and bilateral cues were analyzed under active and passive viewing conditions, respectively. The ERPs elicited by the target stimuli were not analyzed in the present study.

Figure 2

The grand-averaged ERPs for the left (left column), right (middle column) and bilateral (right column) stimuli under the active (solid lines) and passive (dotted line) viewing conditions. The ERPs on two midline electrodes (PZ and POZ, top 2 rows) and two lateral electrodes (PO5 and PO6, bottom 2 rows) are shown. For the unilateral cue stimuli (left and middle columns), no C1 difference was found between the active and passive viewing conditions at PZ and POZ (top 2 rows). The increase of the C1 amplitude by bilateral presentation (right column, top 2 rows) did not reveal any attentional effect on C1. In contrast, the P1 attentional effect (active vs. passive viewing) was consistently observed on both midline and lateral electrodes, regardless of whether the stimuli were presented unilaterally or bilaterally. Therefore, a dissociation between C1 and P1 components as a function of attentional load was observed in the present study.

Figure 3

The scalp voltage maps of the C1 (80 ms) and P1 (130 ms) components for the left, right, and bilateral stimuli under the active and passive viewing conditions. The difference maps showed the scalp distribution of the difference waveform, which were obtained by subtracting ERPs under the passive viewing condition from ERPs under the active viewing condition. It can be seen that C1 shows no attentional effect (left panels), whereas P1 is larger under the active viewing condition (right panels). It is also clear that bilateral stimuli enhance the C1 component.