3.3 Discussion
Our results showed modulation of the P2 in response to emotionally-laden
visually presented words, but not an EPN response in contrast to earlier
studies (cf Citron, 2012 for a review) which considered the EPN to be a
robust EEG signature. Indeed, several recent studies failed to
demonstrate a negative deflection at posterior sites during early time
windows for emotionally valenced words (Pauligk et al, 2019; Ku et al.
2020; Imbir et al. 2021). In addition, unlike previous research showing
larger P2 amplitude to either only positive words (Schapkin et al.,2000)
or only negative words (Huang & Luo, 2006), our results demonstrated an
enhancement of positive amplitude across the 220-300ms time window for
both positively and negatively-valenced words, compared to neutral
words. However, the effect was more widespread for negatively valenced
than positively valenced words. The P2 component is assumed to reflect
the early, automatic allocation of attentional regardless of
task-relevance of emotional context (González-Villa et al., 2014). Our
findings thus indicate that more attentional resources were allocated to
emotion stimuli during the recognition task, undoubtedly due to the
inherent importance of such stimuli. Our results add to literature on
the P2 effect for positive as well as for negative valence words
(Herbert et al.,2006). As noted above, this early effect was widespread
for negative words, whereas positive words elicited a larger P2
amplitude only over frontal-central sites. According to the automatic
vigilance model of emotion, humans have learned to avoid adverse
situations for survival in threatening environments. In contrast,
leaving a positive reward unattended would not incur a cost. The
mechanism that quickly detects undesirable messages thus developed,
leading to an attentional bias to negatively-valenced information (Estes
& Adelman, 2008). It is thus possible that such “negativity bias” of
emotion processing may have increased the P2 effect, extending to a
boarder cortical response to negative than positive words. The concept
of “negative bias” is in line with Rohr and Rahman’s (2015) results.
Although these authors reported an EPN, which exhibits a distinct
distribution and opposite polarity to our P2, they observed an EPN for
negative words, but not for positive words, suggesting that the
processing of negative words is prioritized over positive words at an
early perceptual stage.
In addition, our results showed reduced N400 amplitude with a
frontal-central distribution to words that carried negative connotations
in relation to neutral words. Interestingly, in roughly the same time
window, Grass et al. (2016) found that negatively connotated spoken
words elicited a greater anterior positivity compared to neutral words.
Recent, Leynes and Upadhyay (2022) suggested that distinct scalp
distribution of N400 can be associated with either relative or absolute
familiarity. They suggested that N400 detected at posterior sites
reflects integrating process to accumulated semantic knowledge based on
lifetime experience, while the frontally-distributed N400 (FN400) is
hypothesized to be sensitive to the exposure of current content, with an
attenuated negative wave to old relative to new items, indicating ease
of processing for words that were recently presented, for example during
a recognition task. Taken together, our results support the hypothesis
that valenced words were processed more easily than neutral words due to
the higher level of relative familiarity from previous auditory
exposure, as indexed by a reduced FN400.
In contrast, our positive stimuli failed to elicit a reduced FN400,
suggesting negligible facilitated processing during the recognition
task. This result could be explained by the “negativity bias” of
recognition memory in younger adults (Carstensen & DeLiema, 2018). That
is, while aging brains are prone to positive content, younger adults
tend to remember more negative than positive information. Taking this
into account, less priming resources of positive words were maintained
during initial encoding, leading to little or non-existent processing
facilitation for positive words. This pattern of EEG results does not,
however, match that for our behavioral data, which did not showed a
“negative bias” where both negative and positive words were recognized
better than neutral words. We can note that participants were not
required to respond within a given time frame, such that the conscious
retrieval of words may have overridden the facilitation often observed
for negative words.
GENERAL DISCUSSION
The aim of the current study was twofold. First, we explored how the
valence of auditory words, presented in an immediately prior experiment,
influenced the subsequent recognition of printed words. Second, this
design allowed us to directly compare the ERP response to the same
stimuli across auditory and visual format. In relation to the first
question, our results demonstrate a clear effect of valence/arousal of
auditory stimuli on the subsequent recognition of these stimuli
presented in written format. This was borne out by both behavioral and
ERP data. Our behavioral results revealed increased recognition for both
positive and negative valenced words compared to neutral ones, in
agreement with the existing literature (Kensinger & Corkin, 2003). The
neural response associated with the recognition of visually-presented
valenced words was revealed by two different components, which varied
according to polarity of words. We observed an increase in the P2
component, for both negative and positive words, although the effect was
more widespread for negative words. In contrast, we observed a reduced
N400 only for negative words in relation to neutral words. Overall, the
results suggest that more attentional resources were allocated to
negative words during recognition. It is possible that the relatively
young age of our participants contributed to the “negative bias” that
we observed in the ERP record (Carstensen & DeLiema, 2018).
Our ERP results align with those reported by Maratos and colleagues
(2000) for valenced words. They demonstrated reduced N400 amplitudes to
words with negative connotations compared to neutral words during a
surprise memory task, with a maximum amplitude at frontal sites.
However, the FN400 effect was found not only at the recognition phase
but also during the prior encoding phase. As such their results do not
support our claim that the effect arises from facilitated processing
during recognition. Indeed, the FN400 effect has been reported in prior
ERP studies including negative and positive words in diverse tasks. For
instance, in a lexical decision task, Kanske and Kotz (2007) reported a
reduction of N400 amplitude for concrete valenced words, relative to
neutral words, and the effect was stronger over anterior than posterior
sites. In an emotion-color Stroop task, valenced words elicited a
reduced N400 compared to neutral words, with a frontal distribution
(Sass et al., 2010). Together, these results indicate that the
facilitated processing of valenced words per se, as indexed by reduced
FN400, is likely due to higher-level inter-item association among
valenced words. Otherwise stated, words with emotional connotations tend
to prime one another due to their higher degree of semantic relatedness
(Buchanan et al. 2006). However, our results diverge from the
above-mentioned studies, in that we only found a reduced fN400 for
negative items. If said effect was only due to semantic relatedness
among stimuli, we would expect to observe a similar effect for positive
words, which we did not. Therefore, it is less likely that the FN400
effect found for our negative words stems from inter-item association.
Kaestner and Polich (2011) reported a similar pattern of “negative
bias” for facilitated processing during recognition in an ERP study
using pictorial stimuli. Participants were instructed to memorize a
series of pictures with high or low arousing, negative or positive
valence, and then asked to indicate if the image had been presented
previously. Behavioral results showed that d-prime (hit minus false
alarm) was higher for negative than positive stimuli, suggesting that
negative pictures were more salient and easier to detect than positive
pictures. EEG results showed different patterns across the first,
passive viewing task and the subsequent recognition task. During the
recognition phase, compared to positive pictures, negative pictures
showed larger effects of arousal, including enhanced positivity 200-400
msec and reduced negativity 450-650 msec post-stimulus. This effect,
however, was not found during the passive viewing task. Instead, they
reported a larger positivity 500-900 msec post-stimulus in response to
negative pictures with high-arousal level, compared to low arousal
level, reflecting a sustained allocation of attentional resources. Their
results suggest that the effects of emotional valence is dependent on
task demands and, critically, these findings support the idea that
stimuli with negative content may have a privileged status in memory
processing. Our results join this account and further demonstrate that
the negativity bias in recognition memory extends to lexical items.
In relation to the second question, we found clear differences in the
neural response to valenced words as a function of presentation format.
In line with the vast majority of studies, we did not find any evidence
of early ERP effects for auditory presentation, in contrast to written
words. As concerns later components, we again only found modulations, in
the N400 time window, for written words, with no reliable effects for
words presented auditorily. This question remains to be explored.
Indeed, whereas early effects are quite rare, previous work has shown a
modulation of the LPC (Hatzidaki et al., 2015) although it can be noted
that their results were limited to processing in specific contexts.
Overall, our results provide a compelling and complete picture of the
effect of valence on the immediate processing and subsequent recognition
of words as a function of presentation format. Our study provides
valuable, cross-linguistic evidence to the existing body of literature
and calls for further studies to examine how memory is affected by the
emotional content of words.