Intracranial markers of emotional valence processing ...

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Intracranial markers of emotional valence processing and judgments in music adef

Diana Omigie bcd

Baulac

ag

cdef

, Delphine Dellacherie , Dominique Hasboun bdef

, Claude Adam

a

, Sylvain Clément , Michel

ab

& Séverine Samson

a

Laboratoire de Neurosciences Fonctionnelles et Pathologies, Université de Lille, Lille, France b

Unité d’Epilepsie, Hôpital de la Pitié Salpêtrière, Paris, France

c

Service de Neuroradiologie, Hôpital de la Pitié Salpêtrière, Paris, France

d

Institut du Cerveau et de la Moelle Epinière, Social and Affective Neuroscience team and Centre MEG-EEG - CENIR, Paris, France

Click for updates

e

Université Pierre et Marie Curie-Paris 6, UM 75 and Centre MEG-EEG, Paris, France

f

CNRS, UMR 7225 and Centre MEG-EEG, Paris, France

g

Centre National de Référence des Maladies rares, Service de Neuropédiatrie, CHRU de Lille, France Published online: 11 Dec 2014.

To cite this article: Diana Omigie, Delphine Dellacherie, Dominique Hasboun, Sylvain Clément, Michel Baulac, Claude Adam & Séverine Samson (2014): Intracranial markers of emotional valence processing and judgments in music, Cognitive Neuroscience, DOI: 10.1080/17588928.2014.988131 To link to this article: http://dx.doi.org/10.1080/17588928.2014.988131

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COGNITIVE NEUROSCIENCE, 2014, http://dx.doi.org/10.1080/17588928.2014.988131

Downloaded by [Max Planck Institute for Empirical Aesthetics], [diana omigie] at 03:05 17 December 2014

Intracranial markers of emotional valence processing and judgments in music Diana Omigie1,4,5,6, Delphine Dellacherie1,7, Dominique Hasboun3,4,5,6, Sylvain Clément1, Michel Baulac2,3,4, Claude Adam2,4,5,6, and Séverine Samson1,2 1

Laboratoire de Neurosciences Fonctionnelles et Pathologies, Université de Lille, Lille, France Unité d’Epilepsie, Hôpital de la Pitié Salpêtrière, Paris, France 3 Service de Neuroradiologie, Hôpital de la Pitié Salpêtrière, Paris, France 4 Institut du Cerveau et de la Moelle Epinière, Social and Affective Neuroscience team and Centre MEG-EEG - CENIR, Paris, France 5 Université Pierre et Marie Curie-Paris 6, UM 75 and Centre MEG-EEG, Paris, France 6 CNRS, UMR 7225 and Centre MEG-EEG, Paris, France 7 Centre National de Référence des Maladies rares, Service de Neuropédiatrie, CHRU de Lille, France 2

The involvement of the amygdala and orbitofrontal cortex in the processing of valenced stimuli is well established. However, less is known about the extent to which activity in these regions reflects a stimulus’ physical properties, the individual subjective experience it evokes, or both. We recorded cortical electrical activity from five epileptic patients implanted with depth electrodes for presurgical evaluation while they rated “consonant” and “dissonant” musical chords using a “pleasantness” scale. We compared the pattern of responses in the amygdala and orbitofrontal cortex when trials were sorted by pleasantness judgments relative to when they were sorted by the acoustic properties known to influence emotional reactions to musical chords. This revealed earlier differential activity in the amygdala in the physical properties-based, relative to in the judgment-based, analyses. Thus, our results demonstrate that the amygdala has, first and foremost, a high initial sensitivity to the physical properties of valenced stimuli. The finding that differentiations in the amygdala based on pleasantness ratings had a longer latency suggests that in this structure, mediation of emotional judgment follows accumulation of sensory information. This is in contrast to the orbitofrontal cortex where sensitivity to sensory information did not precede differentiation based on affective judgments.

Keywords: Emotional evaluation; Musical valence; Amygdala; Orbitofrontal cortex; Intracranial electroencephalography.

The processing of emotional valence is well known to recruit both medial temporal and prefrontal brain regions. Indeed, the involvement of structures like the amygdala and orbitofrontal cortex with the processing of emotions has been repeatedly demonstrated in a range of

stimuli from facial expression to sounds (e.g., KrolakSalmon, Hénaff, Vighetto, Bertrand, & Mauguière, 2004; Omigie et al., 2014). However, while a large number of studies have investigated responses in these individual brain areas as a function of the stimuli’s

Correspondence should be addressed to: Diana Omigie, Institut du Cerveau et de la Moëlle Épinière (ICM), UMR7225/U1127, CNRS/ UPMC/Inserm, GHU Pitié-Salpêtrière, 47 Boulevard de l’Hôpital, Paris, France. 75013. E-mail: [email protected] No potential conflict of interest was reported by the authors. This study has received funding from the French Ministry for Higher Education and Research (Agence Nationale de la Recherche) [ANR-09-BLAN-0310-02] and the Institut Universitaire de France to S. S., the Regional Council of Nord-Pas de Calais (PhD scholarship) to D. D., and the program “Investissements d’avenir” [ANR-10-IAIHU-06].

© 2014 Taylor & Francis


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physical properties—properties that are, in turn, assumed to give rise to specific emotions—only a few studies have related processing in these brain areas with subjective and individualized experience as inferred from behavioral reports. Interestingly, it is increasingly held that activity in the amygdala, as with processing in the auditory cortex (Fishman et al., 2001), is based to a considerable extent on the acoustical features of stimuli. Bordi and LeDoux (1992) showed that some amygdala neurons in rodents are tuned to the highfrequency signals that may be observed in rodent distress calls while more recently, results from Kumar, Von Kriegstein, Friston, and Griffiths (2012) showed that acoustic features of aversive sounds modulate the connectivity from the auditory cortex to the amygdala. In contrast, mounting evidence suggests that activity in the orbitofrontal cortex may be more concerned with subjective emotional evaluation than processing the physical properties of a valenced stimulus. Indeed, several studies have now demonstrated that vastly differing levels of activation in the orbitofrontal cortex may be elicited by a given stimulus, depending specifically on how the stimulus is evaluated (De Araujo, Rolls, Velazco, Margot, & Cayeux, 2005; Grabenhorst, Rolls, & Bilderbeck, 2008). In particular, De Araujo et al. (2005) showed that when participants were required to rate the affective value of either a given odor or clean air, which on different trials was either labeled “cheddar cheese” or “body odor,” not only did participants rate the odor and clean air as more pleasant on trials when they were labeled cheese, but also participants’ pleasantness ratings correlated with activations in the medial orbitofrontal cortex. Critically, in the amygdala, modulation of activity as a function of the label was seen only for the test odor and not for clean air trials, unlike in the orbitofrontal cortex where the modulation was seen for both. Thus, the amygdala would appear to show a less reliable involvement in the mediation of subjective experience than the orbitofrontal cortex. The aim of the current experiment was to further examine this issue in the auditory domain, given that previous work has tended to be in the taste and flavor domain (De Araujo et al., 2005; Grabenhorst et al., 2008). In particular, we sought to disentangle the degree and timing with which the amygdala and orbitofrontal cortex show sensitivity to the physical properties of an auditory stimulus on the one hand, and the subjective experience of its emotional valence on the other. To this end, we examined

electrophysiological responses to “consonant” and “dissonant” chords, which participants rated for pleasantness. Consonant and dissonant chords are generally considered by listeners to be “pleasant” and “unpleasant” respectively. The perceived unpleasantness of dissonant sounds is believed to be due to physical properties of these sounds: They possess frequencies that activate regions of the cochlea that are too closely spaced to be resolved, thereby resulting in amplitude modulations that travel up the auditory nerve. This phenomenon, known as beating, is taken to account for the feeling of auditory roughness that listeners tend to find unpleasant (Plomp & Levelt, 1965; although see Cousineau, McDermott, & Peretz, 2012; McDermott, Lehr, & Oxenham, 2010 for an alternative theory regarding the source of dissonance’s aversiveness). However, despite these physical determinants, subjective responses nevertheless show a degree of variability due to factors such as expertise, culture, preference, and experience (Butler & Daston, 1968; Cazden, 1980; Lundin, 1947; McDermott et al., 2010; Vos, 1987), in turn making such stimuli interesting for examining correlates of subjective feelings. Specifically, by comparing results when trials are sorted by pleasantness judgments with results when trials are sorted by acoustic properties, one can address the different regions’ roles in responding to the physical properties of a stimulus and the mediation of a subjective experience toward it (Schön, Regnault, Ystad, & Besson, 2005). In the current study, we took advantage of the high temporal and spatial resolution of intracranial EEG recordings that may be collected from epileptic patients implanted with depth electrodes for presurgical evaluation. Participants were presented with consonant and dissonant chords, which they rated for pleasantness. We predicted that while the listeners’ pleasantness ratings would be influenced to a significant extent by the acoustic properties of the chords (with consonant chords being considered pleasant, and dissonant, unpleasant), some variability would nonetheless be observed. Further, based on previous evidence, we predicted that the amygdala might be initially more sensitive to the physical properties of the valenced stimuli, with reflection of affective evaluation only later being observed. In contrast, we predicted a greater role of the orbitofrontal cortex in mediating subjective experience. Specifically, we hypothesized that these differences of the two regions in terms of sensitivity to physical processing on the one hand and subjective experience on the other, would be seen in the latency of the onset and/or duration of differences between responses in these regions.

INTRACRANIAL MARKERS OF EMOTIONAL VALENCE PROCESSING

METHODS

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Pitié-Salpêtrière Hospital in Paris (agreement 19–07 issued on 26 March 2007).

Data from participants suffering from severe, pharmaco-resistant partial epilepsy, who had been implanted with depth electrodes for presurgical evaluation, were analyzed. Five participants (1 male; mean age = 32, SD ± 8.9 years) had contacts in the amygdala while three of them (1 male; mean age = 34, SD ± 11.53 years) had contacts in the orbitofrontal cortex resulting in a total of 12 contacts (eight in the left hemisphere and four in the right) in the amygdala across five participants and a total of 10 contacts (six in the left hemisphere and four in the right) in the orbitofrontal cortex across three subjects (Figure 1). The invasive exploration had been planned according to the brain locations expected to be at the origin of epileptic seizures. Electrodes were composed of 4–10 contacts, 2.3 mm long, 5–10 mm apart, mounted on a 1 mm wide flexible plastic probe (Ad-Tech Medical Instruments) and were stereotactically inserted using a Leksell frame. For each recording site, Cartesian coordinates were calculated after normalizing the anatomical cerebral MRI into Talairach space. Local field potential data were acquired with a Nicolet 6000 (Nicolet-Viasys) at a sampling rate of 400 Hz (bandpass: 0.05–150 Hz) and data were analyzed with bipolar montage in order to minimize the influence of distant sources. Experiments were approved by the Ethical Committee for Biomedical Research of Consonant Dissonant

Stimuli and task Stimuli were the same as those used in a previous study (Omigie et al., 2014). The notation software known as Sibelius 5 was used to create the three-tone consonant and dissonant chords: 50% of consonant chords were made of perfect major chords while the other 50% were made of minor chords. Dissonant chords were composed of two minor seconds or a combination of a minor and a major second. The duration of each chord was 1800 ms and the inter-stimulus interval varied between 300 and 700 ms. Participants were presented with the consonant and dissonant chords in eight blocks (four in piano timbre, and four in organ timbre) of 48 chords each, and consonant chords made up 50% of each block while the other 50% were dissonant chords. Participants were required to indicate, at the end of the presentation of each chord, how pleasant, on a scale of 1 (very unpleasant) to 4 (very pleasant), they found the chord.

Analyses Data were epoched from −300 ms to +1800 ms relative to the onset of the chords. Automatic epileptic-activity artifact removal was carried out by excluding: (1) trials whose maximum amplitude exceeded the mean amplitude of the trial by at least

AMYGDALA

Pleasant Unpleasant

ORBITOFRONTAL CORTEX

1.0 Acoustics Ratings

0.8

Time (secs)

* ______

0.6 0.4

Acoustics Ratings

* ______

0.6 0.4 0.2

0.2 0.0

0.8

Time (secs)

uV

Consonant Dissonant

Pleasant Unpleasant

____ 1.0

Time (secs)


iEEG recording

0.0 DURATION

ONSET LATENCY

DURATION

ONSET LATENCY

Figure 1. The location of all amygdala and orbitofrontal cortex electrodes contacts used in the analyses (top row), the mean of total duration of significant segments and the latency of the first such segment, when trials are sorted by physical properties and by pleasantness ratings in the amygdala (left) and orbitofrontal cortex (right) respectively. ERP traces from a sample electrode in the amygdala and the orbitofrontal cortex, when trials are sorted by acoustics and ratings. Significantly different segments are indicated in red below the traces.


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five standard deviations, and (2) trials from contacts which had more than 5% of trials excluded. Also, individual trials were checked for spikes and abnormal rhythmic activity. Analysis focused on the sensitivity of these different regions to physical properties of the stimuli versus subjective experiences of pleasantness by comparing the differential event-related potentials (ERPs) to chords when trials were sorted by acoustic properties, to the differential responses when the same chords were sorted by pleasantness ratings. For all chord categories, ERPs were baseline corrected to −300 ms to 0 ms relative to the onset of the chord. The relevant chord category pairs were then compared for significant differences using a two-step procedure: Firstly, sample by sample t-tests were carried out to identify segments of the ERPs in which at least 24 consecutive samples (60 ms) were significantly different at a criterion of p < .05. Secondly, segments were permuted (1000 times) and t-tests carried out again to assess the null distribution of the data and confirm the significance of the actually observed difference with respect to this null distribution. The value of 60 ms was chosen for two reasons: Firstly, to be in line with previous iEEG studies (e.g., Naccache et al., 2005) and secondly, because, as observed by Meletti et al. (2012), iEEG studies tend to report broad negative components in the amygdala from in excess of 100 ms to as long as several hundred milliseconds (Krolak-Salmon et al., 2004; Meletti et al., 2012; Oya, Kawasaki, Howard, & Adolphs, 2002). The use of a permutation procedure ensured that only those segments which were genuinely significant with respect to the null distribution would be isolated. We specifically examined, in the two regions of interest, the influence of sorting criterion (acoustic features, pleasantness judgments) on the onset latency and duration of differential responses.

RESULTS We used R (R Core team, 2012) to carry out all statistical analysis. Table 1 shows how listeners rated the different chords they were presented with. The lme4 package (Bates, Maechler, & Bolker, 2012) was used to perform a linear mixed effects analysis of the relationship between ratings and sound acoustics. We entered acoustics (consonant/dissonant) as the fixed effect and an intercept for subject as the random effect. We also used a by-subject random slope for the acoustics effect. We estimated p-values

TABLE 1 Showing the proportion of times “pleasant” (very pleasant and quite pleasant) and “unpleasant” (very unpleasant and quite unpleasant) ratings were given to ad-hoc (based on acoustic properties) consonant and dissonant chords. Mean pleasantness rating

Type of chords

Dissonant Consonant

Pleasant

Unpleasant

426 (44.37%) 650 (67.71%)

534 (55.63%) 310 (32.29%)

using the likelihood ratio test comparing the model with the fixed effect of acoustics to one without it. As predicted, the ratings which participants gave were influenced to a significant extent by the acoustic properties of the chords, whereby participants rated consonant chords as more pleasant than dissonant chords (consonant: 2.69 ± 1.04, dissonant: 2.30 ± 1.06; t = 2.54, df = p < .05; 1 = very unpleasant, 4 = very pleasant). However, also as expected, and as can be seen in Table 1, particularly with dissonant chords, the acoustic properties of the chords did not perfectly predict rating responses. Specifically, consonant chords were not always considered pleasant and dissonant chords were not always considered unpleasant. Barplots in Figure 1 shows the duration of significant differences between ERPs and the onset latency of these differences when trials were sorted by acoustic properties and pleasantness ratings. These measures were submitted to a 2 by 2 repeated measures ANOVA with region (Amygdala, Orbitofrontal cortex) as a between-subjects factor and sorting criterion (Acoustic properties, Pleasantness ratings) as a within-subject factor. With regard to duration of responses, the two-way ANOVA revealed neither the main effects nor the interaction between them to be significant (Region: F(1, 38) = 3.52, p = .07; Sorting method: F(1, 38) = 0.432, p = .51, Sorting method* Region = F(1, 38) = 1.92, p = .17). However, with regard to onset latency, while neither of the main effects was found to be significant (Region: F(1, 38) = 0.10, p = .75, Sorting criterion: F(1, 38) = 0.25, p = .62), a significant interaction between region and sorting criterion was observed (F(1, 38) = 12.07, p = .001), indicating a difference in onset latencies of ERP differences in the amygdala and orbitofrontal cortex as a function of sorting criterion. Simple post-hoc contrasts were carried out to observe the effect of trial sorting criterion on the two regions independently. These revealed that, in the amygdala, the onset latency of differences was earlier when chords were sorted by acoustic properties than when trials were sorted by



pleasantness ratings, (p = .01), while in the orbitofrontal cortex, the onset latency of ERP differences was earlier when trials were sorted by pleasantness ratings than when sorted by acoustic properties, (p = .04). Note that the latter effect would, however, not survive correction for multiple comparisons (p = .05/2). Finally, tests examining regional differences revealed that while the onset latencies of differential activity in the amygdala and orbitofrontal cortex when trials were sorted by pleasantness ratings was not significantly different (p = .09), the onset latency of the response in the amygdala was earlier than that in the orbitofrontal cortex when trials were sorted by acoustic properties (p = .01).

DISCUSSION The current study examined the extent to, and latency at, which the amygdala and orbitofrontal cortex tend to reflect either a stimulus’ acoustic properties or the evaluative judgment of the given stimulus. Previous work had shown that the perception of dissonance is determined both by a sound’s physical characteristics and various cultural and expertise factors that vary across listeners. Thus, we took advantage of the variability that listeners showed in pleasantness ratings to musical chords in order to examine whether activity in the amygdala and orbitofrontal cortex better or earlier reflects either the physical properties of a valenced sound or its affective evaluation. Our results showed that, first and foremost, preceding any potential role in the mediation of subjective experience, the amygdala is initially concerned with processing of the physical properties that differentiate positively and negatively valenced musical stimuli. This is in contrast to what is found in the orbitofrontal cortex, where sensitivity to the physical properties of the stimuli did not precede the reflection of subjective judgments. In a previous study, we showed, using functional connectivity analysis, a tendency for information to flow from the amygdala to orbitofrontal cortex during the processing of musically valenced chords (Omigie et al., 2014). Those results demonstrated that the amygdala is earlier than the orbitofrontal cortex in differentiating valenced chords as defined by the physical properties of these chords. However, in contrast to that study in which no subjective responses were collected, the current study required participants to report on their subjective experience of

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the individual stimuli. In doing so, it allowed us to relate processing in these two brain areas, not just with stimulus properties, but also with subjective and individualized experience of the stimuli. Specifically, it allowed us to show differences in the latency of the sensitivity of the amygdala and orbitofrontal cortex to acoustic properties and subjective experience of valenced stimuli. It also allowed us to demonstrate that the amygdala is, first and foremost, sensitive to acoustic properties of the valenced stimuli, and that the accumulation of sensory information may be what allows it to play a consequent role in the mediation of subjective experience. This finding allows us to better characterize the role of the amygdala relative to the orbitofrontal cortex. A number of studies have reported on the latency of processing in the amygdala and orbitofrontal cortex with some specifically demonstrating a dependence of processing latency on factors such as attention (KrolakSalmon et al., 2004) and stimuli expertise (James, Britz, Vuilleumier, Hauert, & Michel, 2008). The current study shows the observed latency of processing will depend on whether one contrasts the stimuli according to its physical properties or the participants’ emotional evaluation of the stimuli. Our results demonstrate that as with processing in the auditory cortex (Fishman et al., 2001), processing in the amygdala is based to a considerable extent on the acoustical features of the stimuli (Bordi & LeDoux, 1992; Kumar et al., 2012). The current results showing an earlier sensitivity to physical properties in the amygdala relative to in the orbitofrontal cortex support our previous finding of a strong flow of information from the former to the latter (Omigie et al., 2014). The finding that physical properties of the stimuli are reflected very early in the amygdala is in line with findings that activity in the emotion network reflects the physical components of a stimulus in addition to how it is affectively evaluated (Grabenhorst, Rolls, Margot, da Silva, & Velazco, 2007; Winston, Gottfried, Kilner, & Dolan, 2005). The current results are also important to consider in light of findings that some single neurons in the amygdala appear to be selective for perceived emotion (e.g., Wang et al., 2014). Such seeming discrepancies in what should be considered the amygdala’s particular role, however, may be accounted for in terms of the latency with which the different effects emerge. Wang et al. (2014) reported that selective effects (significant differences in the response to fear and happy faces) appeared on average 625 ms post onset of the stimuli. This latency is much greater than that which we showed in the physical properties-based analysis and is, rather, in line


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with what was found in the judgment-based analysis. We agree with those authors that “subjective perceptual decision of the facial emotion emerges over some window of time” (Wang et al., 2014, p. E3118) and argue that such late reflection of subjective responses in the amygdala is subsequent to its initial role in the accumulation and assessment of sensory information. Our findings here are interesting with respect to fMRI evidence in other domains showing a less reliable relationship between the amygdala and subjective experience as compared with the relationship between the orbitofrontal cortex and subjective experience (De Araujo et al., 2005). They are also in line with reports that tend to emphasize the role of the orbitofrontal cortex, rather than the amygdala, in making idiosyncratic beauty judgments (Ishizu & Zeki, 2011; O’Doherty et al., 2003). The orbitofrontal cortex is considered to play a critical role in the executive function system and its greater fidelity to ratings than physical properties may also be interpreted in terms of decision-making and cognitive control. In any case, our results suggest that the latency of differential processing in the orbitofrontal cortex may present an important index of higher order idiosyncratic evaluation. Specifically, the tendency for an earlier onset of activity in this region when trials that were considered as being pleasant and unpleasant were compared (relative to when trials that were sorted based on their acoustic properties were compared), suggests that the orbitofrontal cortex’s primary concern is the mediation of subjective feeling and value judgment (unlike the amygdala, which is also concerned with accumulating evidence based on stimuli properties). At this point, it may be noted that a large proportion of the trials that were considered pleasant were also acoustically consonant. Indeed, the extent to which the two effects can be disentangled is limited in the current study as the link between emotional judgments and acoustic properties of musical chords has a very strong biological basis. While the possibility of finding neutral musical stimuli is limited, the use of neutral stimuli in other domains, for example, in taste and smell, may prove useful in further isolating any effects that are specifically related to mediation of subjective feeling and value judgment. Also important will be for future studies to use parametric analyses based on several judgment categories (as opposed to the binary contrasts used here: “pleasant” vs “unpleasant”) to better demonstrate the extent to which amygdala and orbitofrontal cortex activity systematically reflect affective evaluation. In the current study, a rating scale of 1 to 4 was used to elicit

evaluative judgments. This use of only a few discrete categories combined with the participants tending to not use the whole scale (often using only two or three categories) and also, the presentation of an insufficient number of trials to overcome these limitations, precluded the implementation of such analysis. Future studies may seek to address this weakness in the current study by using a scale without discrete categories that would allow any number of post-hoc categories to be built (for example, by sorting trials by ratings and then splitting them into terciles or quartiles, etc.) and compared. There has been considerable debate in the literature regarding the relationship between emotion processing and brain laterality (Gainotti, 2012) and, thus, one limitation of the current study is its inability to contribute to this discussion due to insufficient electrode coverage of the two hemispheres. However, while our collapsing across electrodes from the two hemispheres may result in a somewhat crude description of activity in the amygdala and orbitofrontal cortex, we argue that such an analysis is acceptable given our main concern is not providing a comprehensive characterization of activity in either region, but contrasting the activity in both of them. It is also worth noting that a previous study failed to show an effect of brain lesion lateralization on emotional judgments of dissonant chords (Gosselin et al., 2006). Finally, it is important to emphasize that while we focus on the amygdala and the orbitofrontal cortex and present these regions as being more or less relevant to the processing of physical stimulus features and subjective experience, these regions do not work in isolation and are part of a dynamic system (Omigie et al., 2014) comprising many different structures each of which receive multiple bottom-up and top-down inputs (Haber & Knutson, 2010). Indeed, it has recently been suggested that a key top-down source of input in the orbitofrontal cortex is the lateral prefrontal cortex (Rolls, 2013) and more and more efforts are being made to identify the source and mechanisms of the top-down biased activation that controls attention to affective versus sensory processing (Ge, Feng, Grabenhorst, & Rolls, 2012). Further studies examining activity in several relevant regions will be necessary to better understand the precise ways in which different modes of processing arise. In sum, the current results provide insights into the relative and timed involvement of key structures in the processing of physical properties and the mediation of subjective experience of



emotional valence in music. We show that intracranial EEG provides a valuable resource in the characterization of the emotion network and suggest that future work could benefit greatly from this approach. Original manuscript received 25 July 2014 Revised manuscript received 11 November 2014 First published online 10 December 2014

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