Examining ERP correlates of recognition memory: evidence of accurate source recognition without recollection

Abstract

Recollection is typically associated with high recognition confidence and accurate source memory. However, subjects sometimes make accurate source memory judgments even for items that are not confidently recognized, and it is not known whether these responses are based on recollection or some other memory process. In the current study, we measured event related potentials (ERPs) while subjects made item and source memory confidence judgments in order to determine whether recollection supported accurate source recognition responses for items that were not confidently recognized. In line with previous studies, we found that recognition memory was associated with two ERP effects: an early on-setting FN400 effect, and a later parietal old-new effect [late positive component (LPC)], which have been associated with familiarity and recollection, respectively. The FN400 increased gradually with item recognition confidence, whereas the LPC was only observed for highly confident recognition responses. The LPC was also related to source accuracy, but only for items that had received a high confidence item recognition response; accurate source judgments to items that were less confidently recognized did not exhibit the typical ERP correlate of recollection or familiarity, but rather showed a late, broadly distributed negative ERP difference. The results indicate that accurate source judgments of episodic context can occur even when recollection fails.

Figure 1. Schematic depiction of events during test trials

For each test item, subjects first made an item memory confidence judgment, followed by a source memory confidence judgment. ERPs were recorded during the presentation of the test word, and classified according to the ensuing item and source memory responses.

Figure 2. Recognition memory ERP effects

(A) Topographic maps of mean amplitude differences between Hits and Correct Rejections for each 200ms latency window. (B and C) Mean ERPs for hits and correct rejections plotted for electrodes Cz (B) and Cp5 (C). (D and E) Mean ERP amplitude plotted as a function of item recognition confidence (collapsed across old and new items) for Cz (D) and Cp5 (E) effects. (F & G) Mean ERP amplitudes for correct rejections, source incorrect and source correct trials for Cz (F) and Cp5 (G).

Figure 3. Electrophysiological Correlates of Item Memory Confidence

Topographic maps of the item recognition confidence (N=25). Difference waves of ERPs at each confidence level (`2', `3', `4', & `5') compared to the ERP for the items rated `new' (i.e.: `1' responses) during the early (A) and late (B) latency windows of the putative correlates of familiarity and recollection, respectively. C) ERPs for each level of item recognition confidence at electrodes Cz and Cp5.

Figure 4. Source Memory Effects

(A) ERPs at electrode Cz for source correct, source incorrect and correct rejections. (B) Scalp topographies of the difference wave comparisons of `source correct' vs. `correct rejection' trials, and the `source incorrect' vs. `correct rejections' during the early period (400–600ms). (C) ERPs at electrode Cp5 for `source correct', `source incorrect' and `correct rejections'. (D) Scalp topographies of difference wave comparisons of `source correct' vs. `correct rejections', and `source incorrect' vs. `correct rejections' at Cp5 during the later period (600–800ms).

Figure 5. Correct source memory ERPs for items receiving high and low item recognition confidence

Time course of topographic maps of (A) mean ERP differences between high confidence item hits for which the source was correctly recognized (`Item5 +Source 4') and correctly rejected new items, and (B) mean ERP differences between low confidence item hits for which the source was correctly recognized (`Item 4 +Source 4') and correctly rejected new items. (C) ERPs at left parietal electrode Cp5.

Figure 6. ERP results of Correct Source Memory in Experiment 2

(A) Topographic maps of mean ERP differences between high confidence item hits for which the source was correctly recognized (`Item 5 + Source 4&5') and correctly rejected new items, and (B) between low confidence item hits for which the source was correctly recognized (Item 4 + Source 4&5') and correctly rejected new items. (C) ERPs at representative left parietal site Cp5. Compare to similar results found in Experiment 1 (shown in Figure 5).

Figure 7. Source ERPs when controlling for item confidence

In this analysis, confidence of new and old responses are held constant, comparing old items recognized with low confidence of “4” and low confidence source correct (i.e.: `Item 4+ Source 4', the same ERP condition plotted in Figure 5 and Supplementary Figure 1) vs. low confidence correct rejections (new items rated “2”). A) Topographic maps throughout the 1500 ms epoch. B) ERPs from representative electrodes from left frontal (F3), right frontal (F4), left parietal (P3), and right parietal (P4) are shown. The topographic locations of each site on the scalp are shown in red dots on the electrode map inset on site F4.

Figure 8. Source Correct vs. Source Incorrect ERPs

A) Topographic maps contrasting trials of `Item4 + Source4' vs. `Item4 + Source2' trials throughout the recording epoch. B) ERPs from representative electrodes from left frontal (F3), right frontal (F4), left parietal (P3), and right parietal (P4) are shown for trials in which subjects indicated accurate and inaccurate source memory for items recognized with low confidence. The topographic locations of each site on the scalp are shown in red dots on the electrode map inset on site F4.