The effects of visual encoding speed on ERP markers of subsequent retrieval

Our memory for meaningful visual stimuli is remarkable: Even when we see thousands of images, each presented for a few seconds, we can later recognize them among new images with high accuracy and in detail (Standing et al., 1973; Brady et al., 2008). However, recognition suffers if the images are encoded at a speed of 2 images per second or faster (Intraub, 1980; Potter, 1976; Potter et al., 2002). Presumably, this happens because the encoding of each new rapidly presented image disrupts relatively slow short-term memory consolidation which is essential for the instantiation of subsequent long-lasting episodic memory. Here, we studied how encoding speed impacts EEG markers of subsequent recognition, namely, ERP Old/New effects, that is, differences between ERP responses to earlier presented (old) and never presented (new) stimuli. In each block, participants memorized sequences of 20 real-world object images at a slow or fast rate (one image each 1,750 ms or 250 ms, respectively). Their memory was then tested with an “old/new” recognition task combined with EEG recording. Our analysis focused on two ERP Old/New components typically distinguished in the literature (Curran, 2000; Paller et al., 2007; Rugg & Curran, 2007): earlier frontal, FN400, and later parietal, LPC. Although observers showed significantly worse recognition at the fast compared to the slow encoding condition, we found almost equally pronounced FN400 in both conditions. In contrast, the LPC was much larger in amplitude in the slow than in the fast encoding blocks. One interpretation of this dissociation can be that fast encoding speed selectively impairs recollection-based memory (which reflects in reduced LPE) but not familiarity-based memory (little effect on FN400). However, other interpretations (e.g., that slower encoding speed causes a stronger confidence signal reflected in LPE) can also be discussed.