Unraveling the mechanism of semantic object-based attention: The role of top-down search strategies.

There is an ongoing debate between the sensory enhancement account and the attentional prioritization account concerning the mechanism underlying object-based attention. This debate remains unresolved because of the predominant use of geometric objects in previous studies, which made it difficult to experimentally dissociate the two accounts due to the prominent boundaries of these objects. The current study investigated the mechanism underlying semantic object-based attention by utilizing Chinese two-character words with different word frequencies and the event-related potentials (ERPs) technique to elucidate the ongoing debate. The behavioral results showed that the semantic object effect was observed only in the high-frequency condition. The ERP results revealed the following: (1) the N1 component was larger for the high-frequency condition compared with the low-frequency condition. However, there was no significant difference in amplitude between the N1 component elicited by invalid same object locations and invalid different object locations, irrespective of whether it occurred in high- or low-frequency conditions. (2) The P3 component elicited by invalid same object locations was larger than that elicited by invalid different object locations only in the high-frequency condition. (3) The N400 amplitude elicited by invalid same object locations was smaller than that elicited by invalid different object locations only in the high-frequency condition. These results suggest that in the absence of obvious object boundaries, the production of object-based attention is primarily driven by search strategies. Attentional prioritization, rather than sensory enhancement, is the dominant mechanism underlying object-based attention.