Our neuroimaging results reveal two primary findings about the memory system engaged in the encoding of social information, one of which is related to encoding, and the other to processing social information. First, our findings converge with previous data indicating that social information is encoded by a distinct memory system, even when compared against another person-centered condition. However, we identify some selectivity to the role of the social memory system in that it is primarily engaged when encoding impressions formed intentionally with a social focus (IMP), rather than implicitly (SEM). Second, we found that the diagnosticity of impression information affected the engagement of the social system, but that this effect occurred regardless of the success of memory formation, and, contrary to previous findings, regardless of intentionality. Interestingly, our results do not indicate a role for medial temporal regions in the encoding of social information, further indicating the potential for the social memory system to rely on distinct mechanisms from other types of explicit memory. These findings will be discussed in turn.
Our first result suggests that the dmPFC supports encoding of first impressions when intentionally trying to form impressions, but not when incidentally forming impressions. Previous studies have highlighted Dm effects for social information versus non-social information (Harvey, et al., 2007
) or for encoding intentionally formed first impressions versus memorizing order (Mitchell, et al., 2004
). However, both of these studies contrasted against control conditions that did not require participants to make evaluative, person-centered judgments. Therefore, previous results may reflect the act of evaluation rather than a dedicated social process per se, or reflect differences in the attention devoted to evaluating a single individual. By contrasting a social task (form impression, IMP) versus a person-centered and evaluative task emphasizing the semantic rather than the social components (judging the location where a behavior occurred, SEM) we show in the present study that when forming and encoding first impressions intentionally, the dmPFC is recruited. We believe this advances our current knowledge because it better characterizes the involvement of the dmPFC in social evaluation as well as highlights its role in encoding first impressions into memory.
Somewhat surprisingly, this result is based on differences in deactivations, as opposed to previous studies finding activations (Mitchell et al., 2004
). Closer scrutiny of our neural data in the remembered tasks across both intentional and incidental encoding tasks reveals that differences across conditions seem to be driven by the forgotten trials, rather than remembered trials. We believe this pattern of de-activation is in line with the activity of the ‘task negative’ network of the brain or default mode network (Buckner, Andrews-Hanna, & Schacter, 2008
; Raichle et al., 2001
). Previous studies have found that activity of the default mode network hampers encoding such that deactivating it would support better memory performance (Daselaar et al., 2009
). While posterior regions, such as posterior cingulate, precuneus, and bilateral ventral posterior parietal cortices, have emerged more consistently in the literature, some work implicates anterior regions as well, such as anterior cingulate and medial prefrontal cortices, such that de-activation supports successful encoding (Kim, Daselaar, & Cabeza, 2010
Though the previous studies did not implicate dmPFC as part of the task negative network, it is possible that the social nature of our stimuli and the unique neural regions engaged to encode this type of information account for these differences. Previous studies (Daselaar et al., 2009
; Kim et al., 2010
) focused on the encoding of words, scenes, and faces, but did not incorporate stimuli relevant to socioemotional goals. Under such conditions, dmPFC could be associated with the network of regions deactivated during encoding and activated during retrieval. Deactivating this network to support encoding processes implies that focusing on internal processes detracts from the ability to focus on external stimuli and successfully encode them. In our task, this could mean that focusing too much on internal cognition, perhaps retrieving autobiographical memories of familiar individuals with similar appearance to the target stimulus or creating associations based on facial features alone, impairs one’s ability to form a memory trace of the face-behavior association presented in the study (see Shrager et al., 2008
for a discussion of similar effects in the hippocampus). This is consistent with our pattern of results showing less de-activation (greater activity) in the forgotten trials of the Impression condition, perhaps reflecting a failure to inhibit distracting internal associations that hamper successful memory formation. Although one might expect to see a similar pattern for Remembered vs. Forgotten trials in the Semantic condition (incidentally forming impressions), it may be that internal information is most interfering when one is focused on impression formation. Thus, when trying to intentionally form impressions, inhibiting interference is important for one to later remember externally-presented information leading to an impression. Future research explicitly testing the behavioral and neural effects of potential interference, such as from facial characteristics (e.g., Zebrowitz & Montepare, 2008
), during the encoding of face-behavior pairings would help to resolve this question, particularly if the effects vary based on the goals of the task (e.g., incidental vs. intentional impression formation).
Our second major finding is that diagnostic information that easily lends itself to forming an impression deactivates the dmPFC more than neutral information. We would expect that diagnostic information would engage regions implicated in forming impressions more strongly than neutral information regardless of orientation. This is because diagnostic information lends itself more easily towards forming trait inferences (as shown by Uleman (2008)
. However, Mitchell et al. (2006)
found that diagnostic information fails to engage the dmPFC more than neutral information when one intentionally forms impressions, and that diagnosticity differentially affects neural engagement only when one incidentally (unconsciously) forms impressions. Mitchell et al.’s interpretation is that when intentionally forming impressions everything is ‘diagnostic’ (even neutral information) but when unconsciously forming impressions only diagnostic information activates frontal regions associated with forming impressions.
In contrast to their findings, we find that both intentional and incidental impression formation engage the dmPFC such that it de-activates more for diagnostic rather than neutral information. In conjunction with our finding of better memory for diagnostic than neutral face sentences pairs, our data lend support to the notion that increased depth of processing may be associated with dmPFC activity for social tasks. This idea is consistent with previous behavioral studies showing that depth of processing contributes to better memory (Craik & Lockhart, 1972
; Craik & Tulving, 1975
), and in the social domain, that making complex judgments about peoples’ traits leads to better memory than simple judgments of their sex (Bower & Karlin, 1974
; Wenger & Ingvalson, 2002
). Again, our pattern of greater deactivation, rather than activation, for diagnostic information is surprising, but perhaps reflects the mismatch between behavioral information and facial appearance, which is more salient for diagnostic than neutral trials. Compared to prior studies that did not find this pattern, the need to integrate face and behavior could be more salient for one-shot impression formation tasks, as were employed in the current study.
Another possible explanation for differences from prior studies may lay in the different designs employed across studies. Though both used similar statements, our procedure was very different in that our study presented each face once, paired with a single unique sentence, whereas Mitchell’s (2004)
study presented each face paired with 10 different sentences. Pooling impression information across multiple trials may decrease the importance of diagnostic information on any single trial when intentionally forming an impression of an individual. Our use of a single actor - single behavior design may be more consistent with research on spontaneous trait inferences (STI) based on Uleman et al.’s (2008)
claim that in order to generate the most robust STI, one must be presented with a single or very few related behaviors and integrate these with an actor representation. In contrast, “integrating meanings and/or evaluations of one target’s many behaviors is less likely to occur spontaneously and requires high levels of relevant chronic goals” (p. 333). This argument indicates that a more naturalistic setting, in which we form impressions based on a range of behaviors, is not ideal for forming a lasting, distinct first impression. With a single defined behavior, there is evidence that first impressions occur spontaneously even in the absence of conscious effort to create an impression (Ambady, Krabbenhoft, & Hogan, 2006
; Todorov & Uleman, 2002
; Uleman, et al., 2008
Because our study presented only a single sentence for each person, this might have increased the perceived diagnosticity of each sentence, in contrast to studies with multiple sentences converging on a single trait. Distinct impression formation processes may be recruited when one is more concerned with updating and comparing impressions to current knowledge (e.g., Schiller et al., 2009
), as opposed to forming initial impressions (as in the case of our study). Although more research is needed to elucidate the underlying processes that contribute to encoding first impressions, we believe that our finding highlights the sensitivity of dmPFC to diagnostic information regardless of the state of mind one adopts. One would expect that a system that operates as seemingly automatic, like impression formation, (Todorov, et al., 2007
; J. Willis & Todorov, 2006
) would be sensitive to the type of information at all times, whether impressions are incidental or intentional.
Surprisingly, diagnosticity did not influence the role the dmPFC played in encoding impressions. This is particularly unexpected because the behavioral memory measures indicated that diagnostic information was better remembered than neutral information. While this suggests that we are better at encoding impressions that are based on meaningful behavior than impressions that do not contain trait diagnostic information, one might also expect intentionality to impact memory (e.g., Mitchell et al., 2004
). This was not the case for our data: intentionally and incidentally formed impressions were encoded equally well. However, orientation did affect the engagement of dmPFC during successful vs. unsuccessful encoding trials. This apparent inconsistency in behavioral and neural measures of memory may reflect the greater sensitivity of neural measures in some instances, allowing us to reveal a contribution of intentionality using neural measures. However, this potential for greater sensitivity may rely heavily on the selectivity of particular regions for specific processes.
Our results are consistent with those of Mitchell et al. (2004)
in indicating a lack of medial temporal contribution to the encoding of social information. This is surprising given the pervasive nature of MTL contributions to explicit memory (Shrager, Kirwan, & Squire, 2008
; Squire, 2009
; Tulving & Markowitsch, 1998
), although a small body of literature suggests that amnesics may be able to form new memories of impressions of others under some circumstances (Johnson, Kim, & Risse, 1985
; Todorov & Olson, 2008
). Importantly, MTL regions that did emerge in our contrast of Diagnostic > Neutral did not show a Dm effect. This was further tested using an anatomical MTL mask in order to have greater sensitivity to detect effects, and these results also indicate that MTL regions do not respond significantly or differentiate between social and non-social tasks or between information that is diagnostic compared to neutral information. Although we are limited in the inferences we can make based on a null finding, our data add to the growing evidence that social memory formation may not be MTL-dependent.
While our focus was on mPFC and MTL contributions to the encoding of impressions, our analyses also probed effects of orientation and content of information more broadly. While some of the regions implicated in intentional over incidental impression formation are consistent with prior studies (Schiller, et al., 2009
) that suggest a role of emotion (e.g., insula) and face processing (e.g., fusiform) in forming impression of others, our results also identify some novel regions, such as the anterior cingulate cortex (ACC). The ACC is known to be involved in decision making and conflict monitoring (Pochon, Riis, Sanfey, Nystrom, & Cohen, 2008
), which might suggest that intentional impression formation involves deeper processing and may allow one to be better able to account for ambiguity or inconsistency (e.g., a pretty face engaged in an ugly behavior). Notably, the reverse contrast (SEM>IMP) did not produce any MTL regions, suggesting our contrast was successful in contrasting social with non social evaluation while avoiding activation of classic memory networks.
While we achieve some success in differentiating conditions loading more heavily on social information, such as diagnosticity and intentional trait impressions, from those that invoke these processes less, it would have been helpful to have a true control condition that does not involve social information. This would have allowed us to further differentiate social from nonsocial, potentially allowing us to more directly investigate the role of the MTL in encoding nonsocial and social information. In addition, the relatively lengthy trials may have resulted in some blurring of the intentional trait impression condition and the unintentional semantic condition. The amount of time available in which to deeply process information, as well as the interspersing of trial types within a participant, may have encouraged participants to form impressions even on the semantic trials. Such a possibility may account for the lack of memory differences across these conditions.
In conclusion, our finding that encoding first impressions relies on the dmPFC only when intentionally trying to form impressions highlights the importance of orientation and the unique role played by this region when intentionally forming first impressions. It adds to our current knowledge in that it shows that this is true not only in comparison to nonsocial sequencing task but also when compared to a more nuanced person-centered evaluative task. In addition, we find a role for the dmPFC such that it processes diagnostic information which easily lends itself to impressions formation compared to neutral information. This highlights the role of the region as dedicated to forming first impressions and may indicate the importance of the task in engaging this region. Compared with previous findings, our results may suggest that the processes differ when impressions are based on single versus multiple behaviors. Further investigations will likely highlight the types of diagnostic information that most impact impression formation, the effects of multiple versus singular impression formation, and the role of these regions during retrieval of first impressions from memory.