The Memory Enhancing Effect of Emotion: Functional Neuroimaging Evidence

Emotional events are usually remembered better than neutral events. The anatomical and functional correlates of this phenomenon have been investigated in both animals and humans, with approaches ranging from neuropsychological and pharmacological to electrophysiological and functional neuroimaging. The present chapter reviews this evidence, focusing in particular on functional neuroimaging studies in humans, which have examined the effects of emotion on memory-related activity during both encoding and retrieval. The available evidence emphasizes the role of the amygdala, the medial temporal lobe memory system, and the prefrontal cortex. The chapter ends with a discussion of open issues and future directions. Among the multitude of stimuli comprising our environment, at any moment there are some stimuli that are more relevant to us than others, and thus are more likely to engage our processing resources. What is it, though, that makes some stimuli more relevant than others? To some extent, stimuli with virtually identical properties may still be preferentially processed because they better serve our current activities and goals. Nevertheless, some stimuli tend to “capture” our attention regardless of their relevance with respect to our present activity. This latter category comprises emotional stimuli, which as a result of evolution and ontogenesis have gained relevance that transcends that of present activities. What are the neural mechanisms though that allow for such privileged access to processing resources that can make us both emotionally resonate and behave accordingly by approaching something that is desirable, or avoiding something that is potentially harmful? Also, what are the mechanisms that make possible the transition from the activity-specific to the more general relevance of these stimuli? Particularly, what are the mechanisms underlying our ability to encode, store, and retrieve information about our experiences, which ultimately allow 6 The Memory Enhancing Effect of Emotion: Functional Neuroimaging Evidence 108 Florin Dolcos, Kevin S. LaBar, and Roberto Cabeza us to distinguish what may be approached from what should be avoided? Finally, how do alterations of these mechanisms contribute to clinical conditions, such as apathy, anxiety, depression, or post-traumatic stress disorders? These are only a few questions that humans have tried to answer for centuries, and neuroscientists have fervidly engaged in answering particularly during the last few decades. Consequently, a new field has emerged: the cognitive neuroscience of emotion. Different approaches ranging from neuropsychological and pharmacological to functional neuroimaging have attempted to define the neural mechanisms underlying the operations associated with different aspects of emotional processing in both humans and animals (reviewed in Damasio, 1994; Davidson & Irwin, 1999; Davis & Whalen, 2001; Lane & Nadel, 2000; LeDoux, 2000; McGaugh, 2004; Phan et al., 2002; Phelps, 2004; Zald, 2003). This chapter focuses on evidence concerning the neural correlates of the memory-enhancing effect of emotion, as obtained from functional neuroimaging studies of neurologically intact human subjects. In the following sections, we first introduce a few key concepts, and then highlight the role of functional neuroimaging techniques in revealing the neural correlates of the modulatory effect of emotion during encoding and retrieval of episodic memories (i.e., explicit memory for personal events; Tulving, 1983). The chapter ends with a discussion of future directions. CONCEPTS AND METHODS Recent developments in the cognitive neuroscience of emotion have led to important progress in identifying the nature of emotional phenomena and their relationship with other psychological processes, as well as in understanding their features and neural correlates. Among the various psychological phenomena involving emotional processing in humans, the modulatory effect of emotion on different stages of memory (e.g., encoding, retrieval) has received substantial attention in the literature. Concerning the main features defining emotional phenomena, a critical distinction in the emotion literature is the one between two orthogonal affective dimensions: arousal and valence (Lang et al., 1993; Russell, 1980). Arousal refers to a continuum that varies from calm to excitement, whereas valence refers to a continuum that varies from pleasant to unpleasant with neutral as an intermediate value (for methods to assess these dimensions, see Bradley & Lang, 1994). Specific emotions can be positioned within this dimensional space according to the specific arousal and valence levels associated with them. In addition, important progress has also been made regarding the neural mechanisms underlying the operations associated with various aspects of emotional processing. Among the brain regions most typically associated with emotional processing, the amygdala and the prefrontal cortices have been the most thoroughly investigated (e.g., Damasio, 1994; Davidson & Irwin, 1999; Functional Neuroimaging of Emotional Memory 109 Davis & Whalen, 2001; Lane & Nadel, 2000; LeDoux, 1996; McGaugh, 2000; Phan et al., 2002; Phelps, 2004; Zald, 2003), although the role of other brain regions such as the basal ganglia and insular cortex has also been studied (e.g., Davidson & Irwin, 1999). A significant contribution to this recent progress has been made by functional neuroimaging methods. Particularly important are non-invasive techniques, such as those involving hemodynamic (e.g., event-related functional magnetic resonance imaging: ER-fMRI) or electrophysiological (e.g., event-related potentials: ERP) measurements, both of which are assumed to be associated with changes in the neural activity underlying various psychological processes. Functional MRI typically involves measurements of blood oxygenation, and is characterized by excellent spatial resolution (at the level of millimeters), whereas ERPs typically involve recording of electrical potentials through electrodes placed on the scalp, and are characterized by excellent temporal resolution (at the level of milliseconds). These two techniques can thus offer complementary information concerning the brain mechanisms and the timing associated with various cognitive operations. An important advance in understanding the neural correlates of emotional memory has been made by application of event-related designs and the subsequent memory paradigm. One important feature of event-related designs is that recording of functional neuroimaging data (e.g., ER-fMRI/ERP) can be time-locked to “events” that occur either in the external environment (e.g., the onset of a stimulus) or in the participants’ minds (Picton, Lins, & Scherg, 1995; Rugg, 1995). In contrast to other functional neuroimaging techniques that use “blocked” designs (e.g., blocked fMRI and Positron Emission Tomography – PET), ER-fMRI and ERP data can be analyzed offline on a trial-by-trial basis and selectively sorted and averaged on the basis of subjects’ behavior. For instance, one may compare activity for items that are remembered vs. forgotten in a memory test, thereby establishing a direct link between brain activity and successful memory performance in individual participants. Comparing brain activity for remembered vs. forgotten items can be done both during the encoding (learning) phase and during the retrieval (test) phase. During encoding, one can identify regions showing greater activity for items that are subsequently remembered than for items that are subsequently forgotten in a later memory test (subsequent memory paradigm). The difference in activity between remembered vs. forgotten items during encoding is known as difference in memory or Dm effect (e.g., Paller, Kutas, & Mayes, 1987; Paller & Wagner, 2002). Brain regions showing Dm effects are assumed to mediate processes that lead to successful encoding of incoming information. The “remembered-minus-forgotten” difference can be also investigated during retrieval. In this case, the contrast identifies regions showing greater activity for items in which retrieval was successful (hits) than for items in which retrieval failed (misses). The activity difference between remembered vs. forgotten 110 Florin Dolcos, Kevin S. LaBar, and Roberto Cabeza items during retrieval is known as retrieval success (Prince, Daselaar, & Cabeza, 2005; Weis et al., 2004). One of the advantages of Dm and retrieval success analyses is that they can disentangle the effects of emotion on perception-related vs. memory-related activity. The effects of emotion on perception-related activity can be identified by comparing activity for processing emotional vs. non-emotional stimuli (for reviews, see Davidson & Irwin, 1999; Phan et al., 2002; Wager et al., 2003). In contrast, identifying the effects of emotion on memory-related activity requires isolating first activity specifically associated with memory processes, that is, successful encoding activity (Dm effect) and successful retrieval activity. For example, in a study of emotional memory encoding, one can identify the brain regions associated with encoding success, separately for emotional (emotional Dm = remembered emotional stimuli – forgotten emotional stimuli) and neutral stimuli (neutral Dm = remembered neutral stimuli – forgotten neutral stimuli). By contrasting the emotional Dm to the neutral Dm, the effects of emotion on successful memory activity can be identified. Similarly, during retrieval, one can identify the brain regions where successful retrieval activity is greater for emotional than for the neutral stimuli. In combination, these comparisons reveal brain regions that show an interaction between emotion and memory at different stages of memory processing. In the next sections, we will review evidence from functional neuroimaging studies investigating the neural correlates of the modulatory ef