The present results provide strong evidence for a unique memory benefit when items are both negative and also are presented directly to the RH (via LVF presentation). In this instance, even with brief presentation, episodic memory for the visual details of negative items is enhanced. By contrast, when the items are presented briefly and directly to the LH (via RVF presentation), or are presented briefly in the center of the screen (Kensinger et al., 2006
), no such memory benefits exist.
Given the proposed role of the RH in processing the visual specifics or relational details of objects, and also in processing emotional, and perhaps particularly negative, information, it makes sense that LVF/RH processing would lead to enhanced memory for the visual details of negative items. Nevertheless, this is one of the few studies to show such an effect of hemispheric processing on episodic encoding for objects (but see Berrini et al., 1982
; Evans & Federmeier, 2007
; Federmeier & Benjamin, 2005
; Hannay & Malone, 1976
for evidence that hemifield presentation can influence subsequent episodic memory for words), and the first to show that the magnitude of the emotion-related enhancement in episodic memory can be modulated by hemifield presentation. Thus, these results are intriguing in suggesting that the magnitude of memory benefit conveyed by emotional information may vary depending upon which hemisphere first processes information. The enhancement in memory specificity for negative information is greatest when the right hemisphere receives high fidelity information rapidly. This finding is broadly consistent with recent neuroimaging evidence suggesting that when the right fusiform and right amygdala are engaged during encoding, negative items are particularly likely to be remembered with specific visual detail (Kensinger, Garoff-Eaton, & Schacter, 2007c
). The present results suggest that this right-lateralized processing may be facilitated by giving the RH a “head start” in processing the information.
More broadly, the results of the present study further support a distinction between the types of processing mediated by the LH and RH. In the present study, when information was presented directly to the LVF/RH, memory for specific visual detail was enhanced, whereas when information was presented directly to the RVF/LH, information was more likely to be remembered with only gist information. This pattern of results is consistent with the proposal that the RH is specialized for processing the visual specifics or relational details of objects while the LH is recruited for the processing of more general or conceptual features of objects (e.g., Burgund & Marsolek, 2000
; Kosslyn, 1987
; Kosslyn et al., 1989
; Marsolek, 1995
). This finding also is consistent with recent event-related potential evidence that the neural processes involved in determining whether a test item is a perceptual match to a studied item are more likely to be involved if the item was studied in the LVF/RH than if it was studied in the RVF/LH (Evans & Federmeier, 2007
). Taken together with the results of the present study, these results provide convincing evidence that RH processing can lead to better retention of the details of studied information, making it easier to determine whether another item is a perceptual match to that studied item.
The fact that the effects of RH presentation on memory for visual specificity interacted with emotion, but the effects of LH presentation on memory for gist-only information did not, may reflect the lack of specialized emotional processing in the LH. As outlined in the introduction, many lines of evidence suggest that the RH may play a particularly important role in processing information with emotional meaning (e.g., Bourne, 2008
). However, it also has been proposed that there are valence-specific effects, with the RH specialized for processing negative emotion, and the LH optimized for processing positive emotion (e.g., Davidson, 1995
; Natale et al., 1983
; Sackeim et al., 1982
; Schaffer et al., 1983
). If the latter hypothesis were true, then it might be expected that there should be an interaction between emotion and subsequent memory in the LH as well, with the “gist” of positive items being more likely to be remembered when they are presented directly to the LH. At the present time, it is unclear whether our measure of gist-only recognition is insufficiently sensitive to detect interactions with emotion or whether no such interactions exist. It will be important for future studies to examine whether there can be interactions between emotional valence and subsequent episodic memory performance when information is presented directly to the LH.
There are a few limitations of the present study which should be noted. First, these findings are based upon a small sample drawn from a college campus, preventing us from examining whether the effects vary with age, gender, or other individual differences. Second, we saw only modest effects of direct hemispheric presentation on memory for neutral items (see ), though such an effect would be hypothesized to occur due to the RH role in processing exemplar-specific details. It is possible that the parameters of this study (e.g., stimulus presentation duration, encoding task, study-test delay) were not optimal for detecting the effects of direct hemispheric presentation on memory for neutral items. Alternately, it is possible that intermixing negative, positive, and neutral items changes the way in which neutral information is attended and processed (and see Gruhn et al., 2007
; Strange et al., 2000
for evidence), reducing the link between direct RH presentation and encoding of specific visual details. Future studies comparing mixed lists to blocked lists could address this possibility. More generally, future research will be needed to examine whether the effects of direct hemispheric presentation always are greater for negative information than for positive or neutral information, or whether the benefit for negative information can be modulated based on stimulus characteristics (e.g., the arousal level of the stimulus, whether the stimulus is a word or an object) or task demands (e.g., how attention is directed to the information).
In summary, the results of the present study indicate that when information initially is processed directly in the LVF/RH, there is a lasting benefit in memory for the visual details of negative objects. Thus, the RH specialization for processing both negative affect (e.g., Natale et al., 1983
) and exemplar-specific details (e.g., Marsolek, 1999
) has critical implications for the type of information that is encoded into episodic long-term memory. By contrast, when information is presented directly to the RVF/LH, there is an increased tendency to remember only the gist of the information, and this tendency is not influenced by the emotional valence of the information. These results emphasize the complex interplay between emotional valence, hemispheric processing, and memory specificity, and highlight the importance of considering differences in how the two hemispheres process information when examining the effects of emotional valence on memory.