Event-related potential correlates of item and source memory strength

Abstract

Event-related potential (ERP) studies of recognition memory have shown dissociations between item recognition and source memory, wherein item recognition is associated with the mid-frontal FN400 component, which varies continuously with item memory strength, while source memory is associated with the late parietal effect (LPC). There is current debate about whether source memory can vary along a continuum of memory strength or is a threshold process. The LPC has been shown to be generally sensitive to correct versus incorrect source judgments, but varying levels of "source strength" along a single dimension of source evidence have not been tested. The current experiment had participants encode novel visual objects in one of two different task contexts by performing either a conceptual or perceptual judgment about the object. On a subsequent memory test, participants made an old/new decision on a 4-point confidence scale followed by a source memory confidence judgment, in which they indicated their confidence about which task they had performed with the object at encoding. ERPs from the memory test were examined for electrophysiological correlates of both item and source memory strength. Item memory was associated with differences in the 300-500 ms time window, consistent with the timing of the FN400. Differences in the amplitude of the LPC were observed between correct and incorrect source decisions, consistent with previous findings. Comparing low and high confidence source decisions also revealed differences, suggesting that the LPC is also sensitive to variations in the strength of source memory.

Figure 1

Electrode montage, with the four clusters of electrode channels used for statistical analyses shown in bold.

Figure 2

The ERPs from four representative channels from each of the four electrode clusters, showing waveforms sorted by item (Old/New) confidence, regardless of the subsequent source memory decision.

Figure 3

The ERPs from correct item trials, separated by subsequent source memory accuracy, regardless of item confidence. The ERP from correct rejection trials is shown for comparison.

Figure 4

The ERPs from trials separated by item confidence, given a subsequent incorrect source decision. The correct rejections are not conditionalized upon source memory, since no source decision was collected on trials when the participant responded “New”.

Figure 5

The ERPs from trials sorted based on source confidence, with item memory held constant. These are trials on which the subject responded with high confidence to the item decision, separated by confidence in the subsequent source confidence decision. The ERP from correct rejection trials is shown for comparison. The scalp topography suggested a more central distribution, the electrode locations used in this analysis are bolded,

Figure 6

Scalp topographies from both the traditional and restricted confidence analyses. A) Topographic maps created from the mean R values from a four-point linear regression of mean amplitudes from 300–500ms across confidence levels at each electrode site. Shown are these topographies for the unrestricted Item strength analysis (left) and for the Item effects restricted to source misses (right). B) Topographies of the mean difference between items remembered with and without source (left) and the difference between high and low confidence source trials restricted to HC item trials (right), from 600–900ms.

Figure 7

A schematic of the experimental paradigm. 1) During the study phase, the word “meaning” or “complex” was shown prior to the object to indicate the encoding task to be performed on that object. 2) At test, item confidence was assessed with a delayed response on a 4-point confidence scale. 3a) If the item was judged “new”, a fixation cross was shown until the next trial. 3b) If the item was judged “old”, source confidence was assessed on a 5-point confidence scale.