dc.description.abstract | The binding problem refers to the question of how multiple and different kinds of distributed information can be integrated in parallel to a unitary representation, such as an intact object, coherent perception, or even a memory episode. Beyond perception, the binding problem persists when one has to maintain such coherent representation of the world in short-term or long-term memory. The present study focuses on the neural oscillatory mechanism behind the binding processes that form and maintain color-shaped objects, which we refer to as Feature-Binding Visual Working Memory (Binding-VWM).
Although feature binding seems natural and automatic from our daily experiences, several previous studies have revealed that the processing of multi-featured object is not cost-free, both at the behavioral and neurophysiological level. Recent neuroimaging research has demonstrated that both the left temporal regions and the parietal cortex are involved specifically during the binding maintenance period. This is in line with several previous studies suggesting that the parietal region seems to play a vital role in visual grouping and binding-VWM. In addition, several electrophysiological studies have provided evidence showing that gamma frequency oscillation is related to WM load and is also relevant to feature binding and perception of coherent visual patterns. This motivates the hypothesis that feature binding in VWM may be mediated by the communication between temporal and parietal regions via gamma oscillations. Parietal regions here may provide the ‘glue’ that continuously maintain these bound features together even when objects are no longer in view. Causal evidence of how these brain regions operate together in storing multiple features of an object in VWM, however, has not been established with the use of correlational approaches. To this end, here we adopt gamma-frequency transcranial alternating current stimulation (tACS) to modulate the oscillatory signals between the left temporal and parietal cortices simultaneously to test the causality between their oscillatory pattern and binding-VWM performance.
In Experiment 1, the results confirmed previous research indicating the demanding nature of binding in VWM. In Experiment 2 there was a significant interaction between task condition (shape only vs. binding) and stimulation condition (out-phase gamma stimulation vs. sham), which was driven by the specific improvement via tACS in the binding condition but not in the sham condition. It is worth noting that the modulation effect was consistent across the on-line and off-line periods, suggesting that a carryover tACS effect was present even when stimulation stopped. The findings support the hypothesis that binding-VWM is established by the crosstalk between left parietal and temporal regions, in which gamma oscillation is a critical component. Moreover, our data demonstrate that the d’ profile of this modulation is not homogenous, but interacts with preexisting individual differences, where out-phase gamma tACS improved only low-performers’ memory. In Experiment 3 we applied in-phase gamma tACS over the same brain regions as Experiment2, but found no effect of tACS in both the single-feature and binding condition across low- and high-performers. These results suggest that the modulation effect from Experiment 2 is not a general effect of gamma tACS.
Taken together, our study showed that binding-VWM is demanding, but the low-performers can benefit significantly from out-phase gamma tACS over the left temporal and parietal regions, suggesting a possible mechanism of information transmission between these regions. It is likely that such neural interaction is what enables us to perceive and remember both the individual tree and the entire forest.
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