PURPOSE: To deal with a crowded visual scene, it is important that attention is allocated over time and space efficiently. Previous studies suggest that attention acts as a moving spot light dwelling on each location serially, whereas other studies reveal that attention can stay on multiple locations simultaneously. Interestingly, recent behavioral findings demonstrate rapid temporal fluctuations in attentional behavior, suggesting that attention shifts between two spatial locations rhythmically. However, the neural mechanism underlying the space-time distribution of attention remain largely unexplored.
METHODS: In the present study, we combined covert atten-tional paradigm and temporal response function techniques (TRF) to address the issue. EEG was recorded from fifteen human subjects as they were presented with 5-sec dynamic sequences at two spatial locations and were asked to attend to one of them. Notably, the visual sequences at the two locations were randomly modulated in luminance and independently manipulated, so that we can estimate the TRFs for attended and unattended visual sequences separately (Att vs. Unatt) from the same EEG response.
RESULTS: First, compared to Unatt condition, TRFs for Att condition showed an alpha-band (~10 Hz) power inhibition around 100ms, commensurate with previous findings that alpha activities represent inhibitory processes during attention. Second, the alpha inhibition did not display spatial specificity as found before (e.g., decrease on contralateral side and increase on ipsilateral side), suggesting that it may represent an object-level attention independent of space. We further examined TRFs when both visual sequences are in motion and revealed similar alpha inhibition, confirming its nature of object-based attention. Finally, the alpha inhibition was followed by a subsequent alpha enhancement, indicating attentional switching from attended to unattended object. Interestingly, this alpha switching pattern was modulated by task context. Specifically, the Att-Unatt alpha switching pattern became stronger with the decrease in attentional cuing validity (from 100% to 75% and 50%).
CONCLUSIONS: Our findings suggest that attention e?ciently and flexibly distributes over space and time to accommodate changing task demands. We propose that attention samples multiple visual objects in a rhythmic manner, by modulating and coordinating inhibitory alpha-band neuronal activities.