Incidental Encoding of Objects during Search Is Stronger Than Intentional Memorization due to Increased Recollection Rather Than Familiarity

Most memory is not formed deliberately but as a by-product of natural behavior. These incidental representations, when generated during visual search, can be stronger than intentionally memorized content (search superiority effect). However, it is unknown if the search superiority effect is purely quantitative (stronger memory) or also driven by differences in the degrees of recollection and familiarity, two hallmark processes supporting recognition memory. Here, we use signal detection modeling, introspective judgments, event-related EEG potentials, and eye tracking measures to answer this question. In a preregistered study, 30 participants searched for objects in scenes and intentionally memorized others before completing a surprise recognition memory test. Behavioral data from remember-know judgments and receiver operating characteristics indicate that search targets were more often recollected compared with intentionally memorized objects, whereas the two tasks did not lead to differences in familiarity. Surprisingly, the neural signatures did not fully align with the behavioral findings regarding recollection and familiarity. That is, both search targets and intentionally memorized objects elicited a more positive-going mid-frontal negativity peaking at around 400 msec post stimulus onset (FN400), which is associated with familiarity, as well as a more positive-going parietal late component (LPC), indicative of recollection. Both components showed no differences between tasks, indicating equal contributions of recollection and familiarity to remembering searched and memorized objects. Furthermore, the LPC was, as expected, sensitive to differences between recollected and familiar objects when these were intentionally memorized, but it was not affected by these differences for searched objects. Overall, our findings indicate that search superiority relies predominantly on increased recollection. The fact that established neural markers of recollection (LPC) behaved as anticipated for intentionally memorized objects but carried no predictive power for incidentally memorized objects implies that memories established in more ecologically valid tasks might involve neural processes different from those activated in commonly used settings that are more reductionist.