تفاوت جانبی‌گرایی در دمای پرده صماخ به حافظه اپیزودیک مرتبط است اما ناشی از خلق و خوی نیست

The present research examined the effects of pre-encoding and pre-recall induced mood on episodic memory. It was hypothesized that happy and/or angry mood prior to encoding (increasing left hemisphere activity), in tandem with fearful mood prior to recall (increasing right hemisphere activity) would be associated with superior episodic memory. It was also hypothesized that tympanic membrane measures (TMT), indicative of hemispheric activity, would change as a function of induced mood. Although subjectively-experienced mood induction was successful, pre-encoding and pre-recall mood did not alter memory, and only altered TMT in the pre-encoding fear and pre-recall angry mood induction conditions. Interestingly, baseline absolute difference between left and right TMT, a measure of differential hemispheric activity, regardless of the direction of that activity, was significantly positively related to number of total words written, number of correctly recalled words, and corrected recall score. This same TMT measure pre-encoding, regardless of specific mood, was significantly negatively related to false recall. Results are discussed in terms the HERA model of episodic memory, and in the nature of interhemispheric interaction involved in episodic recall.

The left and right frontal lobes are differentially involved in the experiencing of emotional/motivational state, with increased left hemisphere activity associated with positive/approach emotion/motivation, such as happiness, and increased right hemisphere activity associated with negative/withdrawal emotion/motivation, such as fear (e.g.; Davidson, 2002, Davidson, 2004 and Tomarken et al., 1992). Although there are other theoretical accounts of hemispheric lateralization of emotion/motivational state [see Shobe, 2014, for example and for review], examination and detailed discussion of these other accounts is beyond the scope of this manuscript. Instead, the current research assumes left hemisphere-happiness and right-hemisphere anxiety cortical lateralization, a conception that is well supported by other work (see Davidson, 2004 and Urry et al., 2004). There is controversy regarding lateralization of anger, with some suggestion that anger is a left hemisphere approach emotion (Carver & Harmon-Jones, 2009) while other work indicating it is a right hemisphere, withdrawal state (Zinner, Brodish, Devine, & Harmon-Jones, 2008). Given this controversy, anger is also investigated here, and it is hoped that the current work could help to illuminate the lateralization of anger. Interestingly, increased neuronal activity within one versus the other hemisphere results in a biasing of information processing, such that the more active hemisphere’s mode of ‘experiencing’ dominates the processing of incoming information (e.g.: Goldstein et al., 2010, Harmon-Jones, 2006, Propper, Christman, et al., 2013, Propper, McGraw, et al., 2013, Propper et al., 2012, Spielberg et al., 2011 and Seta et al., 2010). For example, increased performance on the Remote Associates Test (RAT) was found after left hand contractions, presumed to have activated global processing mechanisms in the right hemisphere, compared to following right hand contractions (Goldstein et al., 2010). Specifically, the RAT, considered a measure of convergent creativity, requires participants to find the commonality between three word roots. For example, individuals may be presented with the word trio of ‘falling’, ‘actor’ and ‘dust’, with the correct response being ‘star’. Superior performance following left, but not right, hand contractions was suggested to be the result of an increased right hemisphere neural activation, resulting in an increased spread of activation and superior creativity (Goldstein et al., 2010). Similarly, other research has also demonstrated that sustained unilateral motor activity increases hemispheric activation in the contralateral hemisphere, and that this increased activation is associated with increases in lateralized cognitive processes. For example, Schiffer et al. (2004), using fMRI, reported increased activity in the dorsolateral prefrontal cortex contralaterally to side of sustained unilateral gaze. In support of the notion that such sustained unilateral movements alter not only brain activity, but cognition as well, Propper et al. (2012), using methodologies for gaze identical to that of Schiffer et al. (2004), reported changes in semantic memory abilities as a function of sustained unilateral gaze. Presumably these changes in semantic memory were the result of alteration in hemispheric activity in (at least) dorsolateral prefrontal cortex, as a function of side of gaze. Work has also demonstrated that activity designed to increase lateralized hemispheric activation is associated with changes in affective state. For example, right hand clenching (left hemisphere activation) versus left hand clenching (right hemisphere activation) resulted in increased approach (e.g.: happiness, anger) versus withdrawal (e.g.: sadness, anxiety) emotional states, respectively (e.g.: Harmon-Jones, 2006, Peterson et al., 2008 and Schiff and Lamon, 1989). Additionally, the pattern of results in the unilateral hand clenching literature also supports models (e.g.; Davidson, 2002, Davidson, 2004 and Tomarken et al., 1992) of left hemisphere anger lateralization. Thus, induced unilateral hemispheric activity of one versus the other hemisphere is associated with the experiencing of a particular emotional/motivational orientation. Given that positive/approach and negative/withdrawal affects/motivations may be left versus right hemisphere (respectively) oriented (e.g.; Davidson, 2002, Davidson, 2004 and Tomarken et al., 1992).), it may be possible to alter lateralized hemispheric activity via changes in mood. Mood induction itself may therefore alter hemispheric activity (Flores-Gutiérrez et al., 2009, Schmidt and Trainor, 2001 and Tsang et al., 2001). Supporting this notion, Schmidt and Trainor (2001) reported changes in mood-congruent affect, as well as decreased alpha power (increased hemispheric activity) in left frontal areas in response to joyful and happy music, and decreased alpha power (increased hemispheric activity) in right frontal areas in response to sad and anxiety producing music. These results indicate that mood induction alters hemispheric activity and, furthermore, alters such activity in a manner consistent with theories of left lateralization of positive/approach and right hemisphere negative/withdrawal states (e.g.; Davidson, 2002, Davidson, 2004 and Tomarken et al., 1992). Given that music-induced mood induction alters hemispheric activity (e.g.; Schmidt & Trainor, 2001), and given that hemispheric activity is reflected in cognition, it is proposed that music-induced mood, thereby changing hemispheric activity, will be associated with changes in performance on tasks thought to be lateralized to the cerebral hemispheres. In support of this notion, mood induced changes in cognition may be associated with lateralized processes involving attention (e.g.; Ford et al., 2010). For example, Ford et al. (2010) reported that anger increased visual attending to rewarding information, an orientation that may support anger as an approach, left hemisphere motivational state in some contexts. The Hemispheric Encoding/Retrieval Asymmetry (HERA) model of memory proposes that left prefrontal regions are associated with encoding, and right prefrontal regions with retrieval, of episodic memories (Habib et al., 2003 and Tulving et al., 1994). Although this model has its detractors (e.g.; Lee et al., 2000 and Owen, 2003), as countered by Habib et al. (2003) the criticisms themselves do not necessarily invalidate the HERA model, which in itself also has heuristic value (e.g.; Tulving et al., 1994). Additionally, more recent work has provided results that have been interpreted as supportive of the HERA model (Babiloni et al., 2004, Griessenberger et al., 2012 and Okamoto et al., 2011). For example, Babiloni et al. (2004), in a re-analysis of data from a 2004 EEG study, found results supportive of the HERA model using nonverbal stimuli. Okamoto et al. (2011), using functional near-infrared spectroscopy, reported increased right hemisphere activity during recall for a taste, a result they interpreted as supportive of HERA. As suggested by others (e.g.; Cabeza, 2002, Habib et al., 2003 and Propper et al., 2012), the left hemisphere encoding/right hemisphere retrieval of episodic memory proposed by HERA may be influenced by a variety of factors, including stimuli material (i.e.; language or spatial-based; e.g.; Propper et al., 2012) and age (e.g.; Cabeza, 2002). The materials in the present study were language-based, and the participants were young adults; both factors which are associated with the patterns of brain activity predicted by HERA. Because increased activity of a given hemisphere is associated with domination of information processing by that hemisphere, increasing one versus the other hemisphere’s neuronal activity immediately prior to encoding, and immediately prior to recalling information, may influence recall ability. Specifically, increased left hemisphere activity during/prior to encoding, and increased right hemisphere activity during/prior to retrieval would be predicted by the HERA model to result in superior recall for episodic information. Previous work has supported this notion. Propper, Christman, et al., 2013 and Propper, McGraw, et al., 2013 reported that presumed increased left hemisphere activity in response to right hand clenching prior to the encoding of list words, and presumed increased right hemisphere activity in response to left hand clenching prior to recall, resulted in superior memory for list words relative to other hand clench conditions. These results suggest (a) differential hemispheric activity in frontal areas may bias cognitive processing and (b) episodic memory may be benefitted by increased left hemisphere activity during encoding and increased right hemisphere activity during retrieval. One purpose of the present research was to examine the effects of mood induction on hemispheric activity and on episodic memory. Happy and fearful mood induction were chosen as differential activators of the left versus right hemisphere, respectively. It was hypothesized that induced moods of happiness prior to encoding, and induced fearfulness prior to recall, would result in superior episodic memory, relative to other mood-induction encoding and retrieval combinations. An anger mood induction was also examined. As mentioned earlier, controversy in the literature regarding anger makes predictions regarding this emotion difficult (e.g.; Carver and Harmon-Jones, 2009 and Zinner et al., 2008), and another purpose here was to investigate anger lateralization. If induced anger in the present research causes a pattern of results similar to that found for induced happiness, anger may be similar to left hemisphere lateralized/approach motivational states. Conversely, if anger causes a pattern of results similar to that found for induced fear, then anger might be more similar to right hemisphere lateralized/withdrawal motivational states. Hemispheric activity was assessed via lateralized differences in tympanic membrane temperature (TMT). TMT is a relatively novel measure of lateralized hemispheric activation, and may be a simple, fast way of measuring general cortical activation non-invasively and without the expenses of imaging (e.g.; Helton, 2010, Helton and Carter, 2011, Helton, Harynen, et al., 2009, Helton, Kern, et al., 2009, Propper and Brunyé, 2013, Propper et al., 2010, Propper, Christman, et al., 2013, Propper et al., 2011 and Propper, McGraw, et al., 2013). TMT reflects hemispheric activity in frontal and temporal areas, and may be indicative of performance on cognitive tasks and of affective/motivational state (Cherbuin and Brinkman, 2004, Helton, Harynen, et al., 2009, Helton, Kern, et al., 2009 and Helton and Maginnity, 2012; see also Propper & Brunyé, 2013, for review of TMT-affect/motivation literature). The exact physiological mechanism by which the relationship between TMT and hemispheric activity occurs has been elusive (see Propper & Brunyé, 2013). In the most common prevailing perspective, decreased TMT is thought to be associated with increased hemispheric activation on the ipsilateral side of the brain. This finding has been interpreted as a result of a positive correlation between ipsilateral carotid artery and cerebral blood flow, and perfusion of TMT by the same cardiovasculature as that which perfuses the cortex, in conjunction with ipsilateral skull and artery related heat dissipation. That is, ultimately, blood flowing around the TMT is thought to be cooler than blood being actively recruited by the brain during neuronal activity (e.g.; Cherbuin & Brinkman, 2004). According to this model, the difference between left and right TMT is indicative of relative hemispheric activation within an individual (Cherbuin and Brinkman, 2004 and Helton, Kern, et al., 2009), with increased hemispheric activity being associated with decreased TMT ipsilaterally. On the other hand, Helton (2010) has suggested also that between-subjects differences in TMT measures may reflect individual differences in skull heat-build up between individuals, and may therefore result in a relationship between TMT and hemispheric activity such that increased TMT is associated with increased hemispheric activation on the ipsilateral side of the brain. Adding to the puzzle, other work has indicated that state versus trait variables may influence the relationship between TMT and hemispheric activity (see Propper & Brunyé, 2013). It should be noted that there is only one (of which we are aware) paper explicitly examining TMT measures and brain activity. Schiffer, Anderson, and Teicher (1999) used EEG and TMT within the same study. Due to the way the data were analyzed however, this latter research unfortunately could not address questions regarding the specific nature of the relationship between TMT and hemispheric activity. Thus, research examining TMT-hemispheric activity relationships have primarily used cognitive and affective measures to infer actual hemispheric activity, which may have added to the confusion in the literature. Additionally, some of the conflict in the literature may arise from the fact that the physiological mechanisms responsible for the relationship between TMT and hemispheric activation are not completely understood, and the association between increased TMT and decreased/increased ipsilateral hemispheric activity is likely modulated, and possibly reversed, by many factors; conflicts within this burgeoning literature await further explanation. Of particular relevance here, TMT measures have been associated with lateralized cognitive and emotional/motivational states (e.g.; Boyce et al., 1996, Boyce et al., 2002, Gunnar and Donzella, 2004 and Helton, 2010). For example, performance on a visuo-spatial task, presumed to rely on right hemisphere processes, resulted in decreased right TMT, while performance on a verbal task, presumed to rely on left hemisphere processes, resulted in decreased left TMT (Cherbuin & Brinkman, 2004). Regarding TMT and emotional/motivational state, Boyce et al. (1996) reported that increasing left side TMT (and therefore decreased left hemisphere activity) was associated with increased negative/withdrawal emotions, and behavior, while Boyce et al. (2002) reported that warmer left side TMT (and therefore increased left hemisphere activity) was associated with increased positive/approach emotions, and warmer right side TMT (and therefore increased right hemisphere activity) associated with increased negative/withdrawal emotions. Therefore, unfortunately, despite these (and other, see Propper & Brunyé, 2013) works, it is still not clear if an increased or decreased TMT is associated with an increased or decreased ipsilateral hemispheric activity. Despite conflict in the literature, it is clear that TMT measures are predictive of emotional/motivational state and of hemispheric activity, being significantly predictive (sometimes positively and sometimes negatively) with cognition (e.g.; Cherbuin and Brinkman, 2004 and Helton and Maginnity, 2012) and with measures of happiness and approach motivational states, as well as with anxiety, withdrawal motivational states (see Propper & Brunyé, 2013). Other work has demonstrated an association between increasing absolute difference between left and right TMT and increasing anger, indicating that increased difference in activity between the cerebral hemispheres, regardless of the direction of that difference, is associated with increased anger (Propper, Christman, et al., 2013, Propper, McGraw, et al., 2013, Propper et al., 2010 and Propper et al., 2011). Interestingly, such a finding has been suggested to be related to the variability in lateralization of anger; that is, if anger may be left or right hemisphere lateralized, depending on state and trait individual characteristics, then increasing difference in activity between the hemisphere, regardless of the direction of that difference, would be predicted to be related to the experiencing of anger (e.g.; Propper et al., 2011). Therefore, TMT can be considered an indicator of differential hemispheric activity, and is associated with both cognitive performance and with affective/motivational state. The following hypotheses were made: 1. Induced happiness prior to encoding and induced fear prior to recall will result in superior episodic memory. 2. If anger is an approach, left lateralized emotion, then anger will result in memory performance similar to that of happiness. Conversely, if anger is a withdrawal, right lateralized emotion, then anger will result in memory performance similar to that of fear. 3. If increased TMT is associated with increased ipsilateral hemispheric activity, then induced happiness will be associated with increased left hemisphere TMT, and induced fear with increased right TMT. If decreased TMT is associated with increased ipsilateral hemispheric activity, then the converse will occur. If anger is an approach, left lateralized emotion, then anger will result in TMT similar to that of happiness. Conversely, if anger is a withdrawal, right lateralized emotion, then anger will result in TMT similar to that of fear. 4. Induced anger will be associated with increased absolute difference between left and right TMT. 5. If increased TMT is associated with increased ipsilateral hemispheric activity, then increased left TMT prior to encoding, and increased right TMT prior to recall, will be associated with superior episodic memory. If TMT is associated with decreased ipsilateral hemispheric activity, then the converse will occur. 6. Other work has indicated that episodic memory benefits from increased interhemispheric interaction (e.g.; Christman et al., 2003 and Christman et al., 2004). If so, then relative activity of the hemispheres, measured via TMT, may reflect interhemispheric interaction. For example, there is considerable research indicating that interhemispheric interaction via the corpus callosum may at times be inhibitory in nature (Bloom and Hynd, 2005 and Palmer et al., 2013). Callosally-mediated ipsilateral inhibition of motor movements is well established (e.g.: Tazoe & Perez, 2013). Additionally, inhibition of sensory information occurs in cortical areas as well; interestingly, this inhibition may occur in tandem with exitation across multiple cortical levelas (Palmer et al., 2013). Finally, Homae (2014) suggests a developmentally mediated increasing inhibition of homologous cortical areas via the corpus callsoum. If callosal interaction is inhibitory (at least in those cortical laminae involved in memory), then increasing difference between the two hemispheres in neuronal activity, measured via TMT, may indicate increased inhibitory interhemispheric interaction, and TMT measures may therefore be predictive of episodic memory.