The memory that's right and the memory that's left: event-related potentials reveal hemispheric asymmetries in the encoding and retention of verbal information

Abstract

We examined the nature and timecourse of hemispheric asymmetries in verbal memory by recording event-related potentials (ERPs) in a continuous recognition task. Participants made overt recognition judgments to test words presented in central vision that were either novel (new words) or had been previously presented in the left or right visual field (old words). An ERP memory effect linked to explicit retrieval revealed no asymmetries for words repeated at short and medium retention intervals, but at longer repetition lags (20-50 intervening words) this 'old/new effect' was more pronounced for words whose study presentation had been biased to the right hemisphere (RH). Additionally, a repetition effect linked to more implicit recognition processes (P2 amplitude changes) was observed at all lags for words preferentially encoded by the RH but was not observed for left hemisphere (LH)-encoded words. These results are consistent with theories that the RH encodes verbal stimuli more veridically whereas the LH encodes in a more abstract manner. The current findings provide a critical link between prior work on memory asymmetries, which has emphasized general LH advantages for verbal material, and on language comprehension, which has pointed to an important role for the RH in language processes that require the retention and integration of verbal information over long time spans.

Fig. 1

Grand average ERP waveforms to correctly identified test words, at all 26 scalp electrodes. At left are RVF/LH-studied hits (repeated at short, medium, and long lags) plotted with correct rejections; at right are LVF/RH-studied hits (repeated at short, medium, and long lags) plotted with correct rejections (note that test words appeared in central vision, and correct rejection waveforms are therefore identical in the two figures). Here, and in all subsequent figures, negative is plotted up. Latencies and amplitudes of early perceptual components (P1, N1, P2) are similar for all conditions, except for frontal P2s, which are larger for RH-studied hits. Old/new effects are visible at most electrode sites and for items studied in either VF, with greater positivity to hits than to correct rejections; this difference is strongest over centro-parietal sites and apparent in timewindows encompassing both the N400 and the LPC.

Fig. 2

Old/new difference waves across lag conditions as a function of study VF. Effect is shown at the midline parietal channel (MiPa), where the old/new effect and modulations thereof are strongest. At both short and medium lags, the old/new effect does not differ by study VF. (Note, however, the amplitude differences around 200 ms, which reflect P2 repetition effects exhibited only by RH-studied words; see Fig. 3) At long lags, the old/new effect is greater for LVF/RH-studied words throughout the recording epoch.

Fig. 3

P2 repetition effects at each lag, for RVF/LH-studied hits (first row) and LVF/RH-studied hits (second row). Effects are shown at the right medial frontal electrode site (RMFr), where P2 effects are prominent, with an abbreviated timescale (100 ms pre-stimulus to 500 ms post-stimulus onset). For RVF/LH-studied test words, P2 responses to hits and correct rejections did not differ at any lag. For LVF/RH-studied test words, in contrast, P2 responses to hits were more positive than P2 responses to correct rejections at all lags. (Note that correct rejections do not have a lag condition, but are overplotted at each to illustrate the repetition effect.)

Fig. 4

Dm effects for RVF/LH study words (left side of figure) and LVF/RH study words (right side of figure). Selected channels present the distribution of the Dm effect over left hemisphere, midline, and right hemisphere electrodes, at prefrontal regions, central regions, parietal regions, and occipital regions. For both VFs, encoding phase activity is predictive of later memory performance, in the form of greater central-posterior positivity for words that are later remembered at long lags as compared with that for words that are later forgotten.