Attention is required for the perceptual integration of action object pairs.

Previous studies have demonstrated that functionally related objects are perceptually grouped during visual identification if they are depicted as if interacting with each other (Green and Hummel in J Exp Psychol Hum Percept Perform 32(5):1107-1119, 2006). However, it is unclear whether this integration requires attention or occurs pre-attentively. Here, we used a divided-attention task with variable attentional load to address this question. Participants matched a word label to a target object that was immediately preceded by a briefly presented, task-irrelevant tool that was either functionally related or unrelated to the word label (e.g., axe | "log" or hammer | "log"). The tool was either positioned to interact with the target object or faced away from it. The amount of attention available to process the tool was manipulated by asking participants to make a concurrent perceptual discrimination of varying difficulty on a surrounding frame stimulus. The previously demonstrated advantage for the related-and-interacting condition was replicated under conditions of no or low attentional load. This benefit disappeared under high competing attentional load, indicating that attention is required to integrate functionally related objects together into a single perceptual unit.