dc.description.abstract | Background: Visual Working Memory (VWM) is the crucial and complex cognitive function in our daily lives. Typically, estimates of VWM capacity can be measured by using feature binding method in which participants have to integrate multiple features of an object into a unitary representation. Several electrophysiological evidence has revealed the underlying neural mechanism and its correlation with the capacity of VWM. More specifically, the synchronization of alpha power has been found to be associated with the increasing load in the retention period of VWM tasks. It is in line with the inhibition hypothesis that synchronization of alpha power suppresses the irrelevant information in disengaged regions. However, more attention has been shifted to the role of the decreased neural activity in memory. It was mostly proposed that the decrease of alpha power is correlated with the heightened attentional state. On the other hand, several studies have shown that alpha and beta desynchronization, especially from the parietal sites, was strongly correlated with the processing of long-term memory encoding and retrieval. It was proposed that the desynchronization of alpha and beta power is related to the information richness. Therefore, the relationship between the desynchronization and VWM still remains unclear and more evidence is needed to clarify the role of decreased beta power in VWM capacity. Previously, studies have shown lateralized parieto-occipital negative slow wave (i.e. Contralateral Delay Activity, CDA) and lateralized alpha power modulation as neural correlates of VWM capacity that are highly correlated with the individual difference. However, the potential limitation of the traditional approach utilizing lateralized VWM task is that the process of spatial information and the binding process might be confounded. In addition, non-lateralized neural oscillatory processes might be constrained by the experiment design in which the lateralization of alpha frequency band is more pronounced. Hence, the present study adopted a modified version of the change detection task which combined the features of color and shape simultaneously. Moreover, the spatial information was considered as an irrelevant factor in order to dissociate the process of spatial attention and the binding process. In spite of this, previous studies have shown that the pattern of the neural correlates is different between high and low capacity individuals. Therefore, to observe the individual difference, Pashler’s K (Kp), which represented the number of items retained in memory, was adopted to estimate the capacity of binding-VWM. Also, an objective and stable criterion of Pashler’s K value as 2 (i.e. Kp = 2), rather than the traditional median, was utilized into the classification of the high and the low groups. To elucidate the neural mechanism behind the individual difference in binding-VWM capacity, Masking Empirical Mode Decomposition (Masking EMD) of Hilbert-Huang Transform (HHT), an enhanced EEG signal processing algorithm was further utilized for data analysis to achieve high temporal resolution in time-frequency domain. The current study aims to reveal the relationship between the alpha/beta desynchronization and the individual difference in binding-VWM. Based on previous findings reviewed above, we hypothesized that the performance in VWM is related to the alpha/ beta desynchronization and the pattern of electrophysiological activities in alpha/ beta frequency range would differ between low and high capacity subjects.
Method: 68 neurologically normal participants were recruited from National Central University for the current study. Owing to technical discrepancies such as more than two broken channels either with zero as the value of signal or consistently showed the unreasonable power which was 10 times greater than other channels and the insufficient of trials (less than 10 trials) in the high load condition after the process of artifact rejection, 10 subjects were excluded. The binding-VWM task with electroencephalography (EEG) measurement was employed. The binding paradigm was a modified change detection task adopted from Tseng and colleagues (2016). To identify the neural correlates underlying different load in high and low capacity individuals, we manipulated the load by changing the set size within a memory array. Set size four (SS4) and two (SS2) were corresponding to high and low load condition respectively. Participants were instructed to memorize the items presented in the study array. After a short-term interval, the test array was presented and participants were requested to judge whether the items in test array were identical to those in the study array with a key response. To estimate the capacity of binding-VWM, Pashler’s K value (Kp) was adopted in the current task. We utilized the value of Kp = 2 as the criterion for separating the subjects into groups of high or low capacity.
Results: In the present results, positive correlation between raw power of beta oscillations and performance was observed across time at occipital and parietal regions, indicating a task-irrelevant relationship which might be a baseline difference of the neural activity between individuals. To rule out the task-irrelevant interference in the following comparisons, we re-scaled the data from the baseline (-599 ms to -200 ms relative to stimulus onset). The current results presented the desynchronization of alpha and beta power. Importantly, the level of desynchronization within alpha and beta frequency bands were significantly lower in high load condition when compared to low load condition at right temporo-parietal, frontal and occipital regions in the binding-VWM condition. These results confirm the hypothesis that the information richness is reflected by the desynchronization of alpha and beta power. Furthermore, the decreasing alpha/beta power might reflect the reactivation of sensory features and correlated with the successful recall. The results then showed a negative correlation between beta activity and VWM capacity among left frontal regions, suggesting that the higher capacity was reflected by the lower amplitude of beta oscillations. Furthermore, these results demonstrated that only high capacity individuals but not the low capacity ones showed the neurophysiological pattern that the power of alpha/ beta oscillations significantly differed between load conditions across occipital, right temporo-parietal, and left frontal regions.
Conclusion: We argue that the observed desynchronization of alpha/beta oscillations reflects the information richness of a presented scene and is negatively correlated with the capacity of binding-VWM. Our results reveal the neural signatures pertaining to the set size difference circumscribed to posterior brain region whereas the individual differences refrain to the left frontal regions. According to current findings, we suggest that the general difference between loads among occipital and right temporo-parietal regions was more pronounced in high performers, indicating that the performance will improve when the left frontal regions were more engaged in the binding process. | en_US |