Written by︱Wang Jiaqi, Hong Xiangfei

Editor︱Wang Sizhen

Compiled by︱Yang Binwei

Humans can actively shift their visual attention to any position different from the point of gaze, which effectively facilitates the processing of information at the attended location while inhibiting the processing of information at non-attended locations[1,2]. The lateralization of the alpha rhythm in the occipital scalp region (8–13 Hz)—where the alpha energy in the attended visual field is significantly lower than that in the unattended side—is considered a typical EEG marker of visual spatial attention, potentially reflecting the inhibitory gating mechanism of attention on sensory processing[3–6].

On August 25, 2022, Professor Tong Shanbao from the School of Biomedical Engineering at Shanghai Jiao Tong University and Dr. Hong Xiangfei from the Shanghai Mental Health Center affiliated with Shanghai Jiao Tong University published a research paper titled “The effects of pre-cue posterior alpha on post-cue alpha activity and target processing in visual spatial attention tasks with instructional and probabilistic cues” in Cerebral Cortex.This study designed two visual spatial attention tasks with different cue strategies (instructional and probabilistic) and categorized subjects or trials into high alpha and low alpha groups based on the baseline alpha energy before the cue. The results showed that after the cue, the modulation of the alpha rhythm was closely related to the baseline alpha energy before the cue, under both cue strategies. However, the influence of baseline alpha energy on target stimulus processing may be related to the cue strategy. This study indicates that when investigating the functional significance of alpha rhythms in visual spatial attention, the baseline alpha before the cue and cue strategy are two significant factors that should not be overlooked.

In visual spatial selective attention tasks, the alpha rhythm in scalp EEG exhibits desynchronization (energy reduction) and lateralization modulation after the cue, potentially reflecting the regulation of attention on sensory information processing. However, there has been considerable controversy in recent years regarding whether alpha rhythm activity plays a crucial causal role in visual spatial attention tasks[7]. This question partly stems from insufficient understanding of the influencing factors of alpha rhythm in visual spatial attention tasks. By reviewing existing studies[8,9] and the team’s previous work[4,10], this study proposes the hypothesis that the baseline alpha energy before the cue and cue strategy (instructional, probabilistic) may be potential factors affecting the alpha rhythm in visual spatial attention tasks, which should be considered when explaining the functional significance of alpha rhythms.

To test this hypothesis, the study designed two visual spatial attention tasks with different cue strategies (instructional and probabilistic), collecting multi-channel scalp EEG data from 30 healthy young subjects in each task. Based on the baseline alpha energy at the occipital scalp electrodes before the cue, subjects or trials were categorized into high alpha and low alpha groups (Higher vs. Lower) at the between-subject or within-subject levels. Based on this, the study systematically examined the effects of baseline alpha energy and cue strategy on the alpha rhythm and target stimulus processing in visual spatial attention tasks. The results are as follows:

1、Behavioral Performance

The overall behavioral results from the two experiments indicate that all subjects completed the experimental tasks as required, utilizing spatial cueing to focus visual attention on the indicated side of the visual field. After categorizing subjects and trials into higher and lower alpha groups at the between-subject and within-subject levels, no significant differences in behavioral performance were found between the higher and lower alpha groups.

2、Overall EEG Results

The post-cue alpha activity in the occipital region (Figure 1A) indicates that visual spatial attention can lead to lateralization of alpha energy, meaning that the energy in the contralateral occipital region is lower than that in the ipsilateral region, with statistical significance observed 400-1200 ms after the cue. The ERP induced by the target stimulus (Figure 1C) reflects the modulation effect of visual spatial attention on the amplitude of the N1 component in the contralateral occipital region, with the effect of instructional cueing experiment (Exp 1) being stronger than that of the probabilistic cueing experiment (Exp 2). Additionally, in both experiments, a significant positive correlation was found between baseline alpha energy before the cue and the post-cue alpha lateralization index (Figure 2).

Figure 1 Overall post-cue alpha activity and target stimulus-induced ERP

（Source: Wang, el al., Cereb Cortex, 2022）

Figure 2 EEG power spectrum within 1000 ms before the cue and between-subject grouping

（Source: Wang, el al., Cereb Cortex, 2022）

3、EEG Results of Higher and Lower Baseline Alpha Subject Grouping (between-subject)

Only the higher alpha subject group demonstrated significant post-cue alpha lateralization, while the alpha (de)synchronization level of the lower alpha subject group was not significant (Figure 3), indicating that in visual spatial attention tasks, post-cue alpha activity is closely related to the baseline alpha energy before the cue. During the target stimulus processing stage (Figure 4), both higher and lower alpha subjects in the instructional cueing task (Exp 1) showed significant N1 amplitude attention modulation (enhancement), with no differences in the increase between the two groups; whereas in the probabilistic cueing task (Exp 2), significant N1 attention modulation was only observed in the higher alpha subjects, indicating that the target processing reflected by N1 may be jointly influenced by baseline alpha energy before the cue and cue strategy.

Figure 3 Post-cue alpha activity under between-subject grouping

（Source: Wang, el al., Cereb Cortex, 2022）

Figure 4 Target stimulus-induced ERP under between-subject grouping

（Source: Wang, el al., Cereb Cortex, 2022）

4、EEG Results of Higher and Lower Baseline Alpha Trial Grouping (within-subject)

Both higher and lower alpha trial groupings demonstrated significant post-cue alpha lateralization; however, the direction of alpha energy changes after the cue was opposite between the two groups: the higher alpha trial group showed significant desynchronization, while the lower alpha trial group showed significant synchronization (Figure 5). During the target stimulus processing stage (Figure 6), both higher and lower alpha trials in the instructional cueing task (Exp 1) exhibited significant N1 amplitude attention modulation (enhancement), with no differences in the increase between the two groups; whereas in the probabilistic cueing task (Exp 2), significant N1 attention modulation was only observed in the lower alpha trials, indicating that the significance reflected by baseline alpha energy before the cue may differ between within-subject and between-subject levels, with the former possibly representing direct excitability of the visual cortex (lower alpha representing higher excitability) and the latter possibly related to individual attention mechanisms (only higher alpha subjects exhibited post-cue alpha lateralization).

Figure 5 Post-cue alpha activity under within-subject grouping

（Source: Wang, el al., Cereb Cortex, 2022）

Figure 6 Target stimulus-induced ERP under within-subject grouping

（Source: Wang, el al., Cereb Cortex, 2022）

Conclusion and Discussion

In summary, this study reveals that in visual spatial attention tasks, the modulation characteristics of post-cue alpha rhythms largely depend on the baseline alpha energy before the cue, and this is true for both instructional and probabilistic cue strategies. However, the influence of baseline alpha energy on target stimulus processing may be related to the cue strategy. This somewhat indicates that alpha rhythms and their desynchronization/lateralization modulation may not be indispensable in visual spatial attention tasks. Nevertheless, due to the limitations of scalp EEG technology in spatial resolution and the lack of individualized brain imaging data in this study, it is not possible to accurately localize the alpha rhythms intracranially based on individualized brain structural imaging data, which limits further interpretation of baseline alpha energy in this study. Future research based on individualized brain structural imaging data and multi-channel scalp EEG/magnetoencephalography will help unravel the specific physiological significance behind alpha rhythms. Overall, this study highlights that baseline alpha energy before the cue and cue strategy are two important factors that should be considered in future related research.

Original link:https://doi.org/10.1093/cercor/bhac326

Wang Jiaqi (PhD student at Shanghai Jiao Tong University School of Biomedical Engineering) is the first author of this paper; Tong Shanbao (Professor at Shanghai Jiao Tong University School of Biomedical Engineering) and Hong Xiangfei (Dr. at Shanghai Jiao Tong University Medical School Affiliated Mental Health Center) are the co-corresponding authors. This paper received strong support from Professors Sun Junfeng (Shanghai Jiao Tong University School of Biomedical Engineering) and Li Chunbo (Shanghai Jiao Tong University Medical School Affiliated Mental Health Center).This research was funded by the National Natural Science Foundation (61601294, U20B2074, 61571295) and the Shanghai Natural Science Foundation (22ZR1453800), among other projects.

First author: Wang Jiaqi (left), Corresponding authors: Hong Xiangfei (middle), Tong Shanbao (right)

（Photo provided by: Hong Xiangfei/Tong Shanbao’s research group）

The research team has conducted a series of studies on visual spatial attention and its impact due to aging in recent years (Hong et al., NeuroImage, 2015; Hong et al., Sci Rep, 2017; Hong et al., Int J Psychophysiol, 2020; Hong et al., Hum Brain Mapp, 2020).

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