TY - JOUR
AU - Ackermann,, Hermann
AB - Abstract In order to delineate brain regions specifically involved in the processing of affective components of spoken language (affective or emotive prosody), we conducted two event-related potential experiments. Cortical activation patterns were assessed by recordings of direct current components of the EEG signal from the scalp. Right-handed subjects discriminated pairs of declarative sentences with either happy, sad or neutral intonation. Each stimulus pair was derived from two identical original utterances that, due to digital signal manipulations, slightly differed in fundamental frequency (F0) range or in duration of stressed syllables. In the first experiment, subjects were asked: (i) to denote the original emotional category of each sentence pair and (ii) to decide which of the two items displayed stronger emotional expressiveness. Participants in the second experiment were asked to repeat the utterances using inner speech during stimulus presentation in addition to the discrimination task. In the absence of inner speech, a predominant activation of right frontal regions was observed, irrespective of emotional category. In the second experiment, a bilateral activation with left frontal preponderance emerged from discrimination during additional performance of inner speech. Compared with the first experiment, a new pattern of acoustic signal processing arose. A relative decrease of brain activity during processing of F0 stimulus variants was observed together with increased activation during discrimination of duration-manipulated sentence pairs. Analysis of behavioural data revealed no significant differences in evaluation of expressiveness between the two experiments. We conclude that the topographical shift of cortical activity originates from left hemisphere (LH) mechanisms of speech processing that centre around the subvocal rehearsal system as an articulatory control component of the phonological loop. A strong coupling of acoustic input and (planned) verbal output channel in the LH is initiated by subvocal articulatory activity like inner speech. These neural networks may provide interpretations of verbal acoustic signals in terms of motor programs and facilitate continuous control of speech output by comparing the signal produced with that intended. Most likely, information on motor aspects of suprasegmental signal characteristics contributes to the evaluation of affective components of spoken language. In consequence, the right hemisphere (RH) holds a merely relative dominance, both for processing of F0 and for evaluation of emotional significance of sensory input. Psychophysically, an important determinant on expression of lateralization patterns seems to be given by the degree of communicative demands such as solely perceptive (RH) or perceptive and verbal-expressive (RH and LH). emotion, prosody, subvocal rehearsal system, event-related potential, hemispheric specialization AP = anterior, posterior, DC = direct current, EMO = emotional category, F0 = fundamental frequency, LH = left hemisphere, MANIP = acoustic parameter manipulation, RH = right hemisphere Introduction Human behaviour in general, and speech communication in particular, is essentially stimulated by motivational and emotional states. Information about these psychophysical preconditions can be perceived from the speech signal, either explicitly from a word's meaning `I am excited (happy, angry …)' or implicitly from the speaker's tone of voice. Usually, the latter speech component is referred to as `affective prosody' (Ackermann et al., 1993). The fact that we easily succeed in decoding the semantic message, i.e. the meaning of words and sentences as well as the intentional or emotional, which is delivered by prosodic cues, points at two basic conditions constituting the conceptual frame of this paper. First, a perceived emotional state must have its correlate in the acoustic of the speech signal. However, since our impression is sometimes uncertain or wrong, the acoustic cues may fail exhaustive specification of affective states. As meaningful percepts result from neural activity that goes beyond the mere processing of physical stimulus characteristics, acoustic parameters are apt not to define, but rather to describe prosodic contents in terms of an approximation. Secondly, two distinct messages, e.g. the semantic and the emotional, are presumably represented by spatially separated neuronal networks. According to lesion studies and functional imaging experiments, circumscribed brain areas [predominantly in left hemisphere (LH) regions] could be determined as prerequisites for perceptual and expressive language functions, especially of phonological, semantic and syntactic aspects (Posner and Raichle, 1994; Springer and Deutsch, 1995). However, no consensus has been achieved so far in localizing the processing of intonational features subserving affective and sociolinguistic impressions in the right hemisphere (RH) or LH (for review, see Pell and Baum, 1997a). In correspondence to vocal linguistic communication, the perception of a speaker's emotional tone of voice is based on the decoding of various acoustic parameters. Fundamental frequency (F0) represents the closing frequency of the vocal cords during phonation. It determines largely its main psychophysical correlate, i.e. the speaker's pitch, which presumably holds transcultural pertinence to express emotion (Ohala, 1984) and which is also of linguistic relevance, e.g. to realize a stress pattern or to indicate a question. The extent of F0 variation (F0 variability, F0 range) constitutes a variability measure of F0. Its expression in extreme either reflects a melodic speech with vivid changes of pitch or a monotonous way of speaking that lacks melodic elements. Simple temporal parameters like syllable duration are difficult to interpret, since vocal emotional expressions inconsistently vary not only voiced but also unvoiced elements and are also influenced by speech pauses (Bergmann et al., 1988). The variation of vowel length seems to hold a higher degree of explanatory value. In parallel with F0-range, it indicates dynamic properties of a voice either typifying happiness (high variation) or amplifying the impression of melancholy and sadness (low variation) (Tischer, 1993). The acoustic parameters selected for systematic variation in this study, i.e. F0-range and duration of stressed vowels, were chosen because they yielded consistent effects on speaker state attribution. Bergmann and colleagues varied different acoustic parameters in short speech utterances and had them judged according to their affective and attitudinal expressiveness (Bergmann et al., 1988). In agreement with Ladd and colleagues (Ladd et al., 1985), they found that F0-range and temporal parameters, especially duration of stressed segments, independently modulate expressiveness as continuous variables. Strong attribution effects were found for F0-range (broad = high arousal; narrow = sadness) and temporally modulated stressed vowels (short = joyful; long = sadness). The neurobiological basis subserving the decoding of prosodic cues is far from being identified. Patients with damage to the RH have been reported to be more impaired than subjects with left-sided lesions on tasks requiring the discrimination and/or identification of emotional tone (Tucker et al., 1977; Weintraub et al., 1981; Heilman et al., 1984). Other studies, however, found similar performance of patients with unilateral damage to either hemisphere (Schlanger et al., 1976; Van Lancker and Sidtis, 1992; Pell and Baum, 1997a). Thus, lesion studies have not unequivocally clarified the locus of neuronal networks primarily involved in affective speech processing (for a comprehensive review, see Gainotti, 1991; Heilman et al., 1993). Possibly, cerebral mechanisms underlying prosodic perception are bilaterally located, as concluded by Pell and Baum (Pell and Baum, 1997b). Aside from the problem of localizing specified areas subserving the construction of emotional percepts, it is unclear to what degree the representation as such is affected when perceptual deficits are detectable. Blonder and colleagues suggested a high level disruption of affective representations (Blonder et al., 1991); also, impairments of low level mechanisms such as the processing of one of the above-mentioned acoustic parameters remain conceivable. More psychophysically oriented studies yielded some evidence that the RH has an advantage over the LH in extracting pitch information from complex auditory stimuli. Patients with RH lesions were found to make no use of F0-variability, but rather relied on duration cues to assess affective prosody (Van Lancker and Sidtis, 1992). Robin and colleagues observed that right temporoparietal lesions disrupted the discrimination of tones, but not the perception of time patterns, while lesions in the homologous regions of the LH had the opposite effects (Robin et al., 1990). Possibly, the expected contribution of the RH to the processing of affective speech prosody reflects the discrimination of pitch contours rather than the evaluation of emotional significance of verbal utterances. Studies in healthy subjects represent a further approach to investigate emotion processing. For example, a PET study reported by George and colleagues examined the topography of cerebral blood flow change while subjects identified either the affective tone, the emotional propositional content (sentence meaning) or the second word in acoustically presented sentence utterances (George et al., 1996). The judgement on the emotional propositional content correlated with a bilateral prefrontal activation, while the discrimination of affective intonation evoked a lateralized right frontal response. Considering the aforementioned lesion studies, one would have expected, in addition, significant activation of temporoparietal fields. Their absence may partly be due to limitations inherent to the PET approach. Its low temporal resolution amounting to 1 min or longer leads to integration of multiple cognitive operations which presumably correspond to widespread cortical activation. With respect to intersubject averaging, effects of distributed neural networks tend to be blurred (Chertkow and Murtha, 1997). The recording of evoked potentials provides an alternative in this regard as it allows assessment of cortical activation with a high temporal resolution. The present investigation recorded direct current (DC) components of the EEG signal at the scalp. This method has been proven valuable for the identification of the language-dominant hemisphere, e.g. the search for synonyms yielded a localized maximum of cortical activation over left dorsolateral prefrontal areas (Altenmüller et al., 1993). Investigations on melody perception have shown a predominant involvement of right frontal and right central fields, especially in musically untrained subjects (Altenmüller, 1986). In our preceding study on affective speech processing, the same areas were found to be strongly activated. Left/right comparison of hemisphere activation revealed a highly significant lateralization towards the RH. This effect appeared largely independent of emotional stimulus category (Pihan et al., 1997). Post hoc evaluation of subjects' strategies to resolve the perception task revealed that one male test subject repeated the stimuli in parallel with the ongoing presentation using inner speech. His cortical activation pattern was balanced between the LH and RH, interrupting the clear RH effects observed in the others. Although it is known that lateralization effects can be masked if linguistic operations are concurrently performed with non-linguistic tasks (e.g. after raise of numbers of response categories, demonstrated by Tompkins and Flowers, 1985), the neural mechanism underlying this effect remains to be clarified. Possibly, the conflict of lesion studies on the issue of hemispheric specialization in affective speech processing is a problem related to variable LH and RH involvement due to acoustic, biological or cognitive determinants that are not sufficiently controllable or even yet considered. Also, understanding cerebral processing of linguistic and affective components of speech perception might depend critically on the degree to which we succeed in evaluating and separating linguistic task demands and strategies from non-linguistic ones. By measuring cortical activation during discrimination of emotional expressiveness that is digitally altered by temporal and spectral manipulation of natural speech, the present study aims to elaborate further LH and RH mechanisms of non-verbal acoustic communication. Material and methods Experimental procedure A non-invasive EEG technique was applied to investigate cortical activation during discrimination of affective intonations of spoken language. DC (slow) components of the EEG signal can be recorded from the surface of the scalp provided that mental activity extends over a time interval of several seconds. They presumably represent sustained excitatory input to apical dendrites of cortical pyramidal cells (Rockstroh et al., 1989). Local distributions of these surface-negative low frequency DC potentials constitute cortical activation patterns. Since DC potentials are lower in voltage than the ongoing background EEG, the signal to noise ratio has to be enhanced by averaging task-related EEG activity over several trials. Activation patterns obtained with this method are highly task-specific and intra-individually reproducible (for neurophysiological details of the method, see Altenmüller and Gerloff, 1999). Mental activity is maintained over seconds when changes of a speaker's emotional state are perceived from his or her voice. This takes place during verbal communication, e.g. while the listener perceives spoken language. In this study, stimuli were presented as sequences of two successive sentences, each of them with identical wording. The paired stimuli represented variants of the same emotional category differing in expressiveness. Perceptual effects were created by systematic variation of single acoustic parameters. Subjects were asked, first, to recognize the emotional category, i.e. neutral or sad or happy, and secondly, to indicate whether the first or the second stimulus of the pair sounded happier or sadder or more excited (in instances of neutral sentences). In a second task, subjects were asked to repeat simultaneously the wording of the presented utterances using inner speech. Subjects were requested to avoid any phonation or articulation during this task as electric fields from muscular activity and movement effects represent major sources of artefacts in DC recordings. Four stimulus pairs were chosen to help subjects to become acquainted with the task demands prior to the start of recordings. All subjects gave informed consent to participate in the study which was approved by the ethics committee of the University of Tübingen. Stimulus generation Four different declarative sentences were digitally recorded by a professional actress, each with neutral, happy and sad intonation. The actress was instructed to stress four predetermined syllables and to avoid any inconsistencies with respect to sentence focus and contrastive stress patterns (Table 1). Propositional content of the resulting 12 utterances was compatible with each of the three prosodic modulations considered. All sentences were digitally adapted to equal levels of perceived loudness prior to the following manipulations. By means of commercially available software (LPC Parameter Manipulation/Synthesis Program of the Computerized Speech Lab CSL 4300, Kay Elemetrics, New York, USA), three stimuli differing in F0-range were resynthesized from each of the 12 spoken sentences. In line with psychoacoustic standards, the pitch contours of the original utterances were extended or reduced in relation to the sentence-final F0 level, which was kept unchanged (Ladd et al., 1985). Figure 1A illustrates the procedure and presents the average of each maximum F0 range and its standard deviation for all resynthesized stimuli. A copy of the original utterances was used to create a second set of test sentences in which emotional expressiveness was altered by varying vowel duration of stressed syllables. From each original sentence, three variants with either short, middle or long duration of stressed syllables were obtained by either cutting out or doubling single pitch periods of the acoustic signal. On average, the shortened and lengthened variants had durations of 80, 110 or 125% of the original vowels, respectively. The rhythm of each sentence was largely kept constant as manipulation of vowel length was performed in proportion to its absolute duration. Figure 1B exemplifies the time-manipulation and its effect on total stimulus duration. Altogether, a corpus of 72 sentences (12 utterances produced by an actress × 3 F0 variants plus 3 × 12 durational variants) was created. The naturalness of all the sound was verified by two certified speech pathologists as well as the authors. At perceptual evaluation, the pitch- and time-manipulated variants of happy and sad utterances differed in the degree of perceived intensity of the respective emotion. In contrast, sentences with neutral intonation sounded more or less excited. Forty-eight pairs of test sentences were assembled with the following characteristics. The two paired utterances were always derived from the same original recording and, therefore, had identical wording and belonged to the same emotional category. At the acoustic level they either differed in F0 range or in duration of stressed syllables. Half of the stimulus pairs were characterized by a maximal difference in the respective acoustic parameter, i.e. smallest versus largest F0 range or shortest versus longest vowel duration, the other half by the contrast, minimal versus medium F0 range or minimal versus medium vowel duration. Participants and procedure Altogether, 24 right-handed healthy students and staff (10 males, 14 females; age 19–34 years) from the University of Tübingen participated in the present study. In the first experiment, 16 subjects (8 males, 8 females) discriminated the stimuli without additional demands. In order to evaluate the role of inner speech on affective prosody perception, we conducted a second experiment in which eight test persons (2 males, 6 females) performed inner speech in addition to the discrimination task. Handedness was assessed by means of the Edinburgh Inventory (Oldfield, 1971). None of the participants was familiar with the aims of the research work. Forty-eight stimulus pairs were presented in a randomized order and repeated with the sequence reversed. A second run was performed in the same way (one individual time course is displayed in Fig. 2). At the end of each trial the subjects' forced choice was to specify the emotional category of each pair (happy, sad or neutral) and indicate the sentence (first or second) which showed stronger expression of the perceived emotion (happier, sadder) or speaker's arousal (more excited). An answer was considered `correct' if the sentence with broader F0 range or shorter vowel duration was recognized as `more expressive'. In sadly intonated pairs the utterance with longer syllable duration was expected to be labelled as `sadder'. Recording procedure DC potentials were recorded from the scalp by 26 non-polarizable AgCl electrodes (impedance below 10 kΩ). Stabilization of the electrode potential and reduction of the skin impedance was ascertained by applying the method for high quality DC recordings described by Bauer (Bauer et al., 1989). Subjects were instructed and trained not to perform any muscular movement during the recording period and especially to avoid articulatory gestures in the second experiment (inner speech condition). Signals were amplified and digitized using Neuroscan soft- and hardware (NeuroScan Inc., Herndon, Va., USA). Electrode positioning was conducted according to the Jasper 10/20 System (Jasper, 1958) using unipolar leads with linked mastoid electrodes as a reference. Simultaneously, electro-oculogram was assessed through an additional diagonal bipolar recording between nasion and the anterior zygomatic arch. For further data processing, only trials without artefacts were accepted. Generally, between 10 and 20 trials per condition and subject were averaged and analysed relating the DC amplitudes during the presentation periods to a baseline taken from a 1.5 s prestimulus period. As shown in Fig. 2, activation of the second analysis interval was considerably higher compared with the first. We specifically tried to elaborate the interaction between the inner speech condition and comparative processing of sentence pairs. Therefore, mean amplitudes within analysis period 2 provided the basis for data normalization and statistical evaluation. Analysis period 1 was not considered as we could not exclude influences from a varying onset of inner speech performance and of phonological encoding as wording was not known before presentation start. Evoked potentials within the first second of stimulus presentation do partly reflect unspecific activation and orientation responses and, therefore, were not analysed either. Prior to statistical analysis the data were normalized. All values of a given subject were divided by his/her most negative mean value of a single analysis period and scaled such that the minimum value was `–1'. A repeated measures ANOVA (analysis of variance) was performed, in which the factor group (Experiment 1, no inner speech; Experiment 2, with inner speech) was entered as a between-subjects factor since different subjects participated in the two experiments. Emotional category (EMO: happy, sad, neutral) and acoustic parameter manipulation (MANIP: F0, time) were taken as within-subject factors. Recorded DC potentials at 26 electrode positions constituted the dependent variables and were used to create the maps shown in Fig. 4. In order to evaluate topographic differences in cortical activation, mean values of the following electrodes were grouped as follows: F7, F7F3, F3, FC3: anterior left (AL) F8, F8F4, F4, FC4: anterior right (AR) T3, T5, PT3, P3: posterior left (PL) T4, T6, PT4, P4: posterior right (PR) The grouping categories were considered in the ANOVA by introducing two additional within-subject factors: side (left, right) and AP (anterior, posterior). Results Evaluation of response behaviour showed that subjects of the first experiment correctly identified the intended affective prosody of the original utterances (neutral, sad, happy) (Fig. 3, left panel, dark grey boxes). Few sentences with happy or sad tone were categorized as instances of neutral intonation. Discrimination of expressiveness within a given emotional category yielded less consistent results, with the highest percentage of expected answers in the group of pitch-varied happy and time-varied neutral sentences (Fig. 3, right panel, dark grey boxes). Few subjects rated expressiveness at random or contrary to the predictions made. In the second experiment, additional linguistic demands imposed on subjects by the inner speech task did not disrupt their ability to discriminate the sentence pairs (Fig. 3, light grey boxes). Rating results of emotional categorization and dimensional evaluation were ranked and subjected to an ANOVA taking emotions and acoustic parameter manipulations as repeated measures. No significant differences between the two experiments were observed with respect to discrimination of expressiveness. However, a tendency emerged towards reduced performance in denoting the correct emotion (Table 2A, Group, upper panel). In addition, evaluation of expressiveness yielded a significant main effect for emotion and significant interaction of emotion and acoustic parameter manipulations (EMO × MANIP). This indicated, first, a relatively low performance in discriminating expressiveness of sentence pairs with sad intonation and, secondly, more consistent attributions made to pitch-varied happy and duration-manipulated neutral stimuli as opposed to their time- and pitch-varied counterparts (Fig. 3, right panel, and Fig. 6, left panel). Results of the statistics on behavioural data are summarized in Table 2A. Comparing the two grand average plots in Fig. 4, a clear effect of inner speech can be assessed in terms of dissolving the predominant right frontocentral activation. Without inner speech, discrimination of affective intonations evoked brain potentials with clearly higher amplitudes over the RH compared with the LH, whereas additional performance of inner speech resulted in a bilateral activation with left frontal preponderance. Several major findings emerged from the statistical analysis of DC potentials by repeated measures ANOVA (all significant effects and interactions are listed in Table 2B). As indicated by a significant main effect of AP, activation was more pronounced in anterior compared with posterior regions. The effects of inner speech can be characterized as follows. First, significant interactions with localization (side × group and AP × side × group; see Fig. 4C, upper panel, and Fig. 5) demonstrate a shift of activation pattern, from predominant RH and frontal activation without inner speech to pronounced left frontal activation in Experiment 2. Since data were normalized by subject and since different subjects participated in the two experiments, no final conclusion can be drawn as to what extent inner speech resulted in RH deactivation and/or additional LH involvement. However, visual inspection of the brain maps derived from non-normalized data (Fig. 4) gives the impression that the left shift of stimulus processing during inner speech might, in part, be due to a reduction of RH activation. Secondly, inner speech showed significant interaction with the acoustic parameters manipulated to create differences in expressiveness (MANIP × group; Table 2B). During inner speech, activation was more pronounced in response to time-varied compared with pitch-manipulated sentences, whereas the reverse pattern was observed during spontaneous discrimination in Experiment 1 (Fig. 4C, lower panel). No significant interaction could be detected between the inner speech condition and the emotional category (P > 0.1). However, the factor EMO showed a significant interaction with the kind of acoustic parameter manipulations (MANIP × EMO; Table 2B). Pitch manipulation in neutrally intonated sentences and duration variation in happy stimuli evoked higher activation compared with their pitch- and time-varied emotional counterparts, whereas overall activation was similar in sadly intonated sentence pairs (Fig. 6, right panel). The two intrasubject factors EMO and MANIP did not yield significant main effects, indicating that overall activation was independent both of emotional content and of acoustic parameters used for manipulation of expressiveness. Discussion With respect to the behavioural data, subjects' responses demonstrated that expressiveness of emotional intonations can be predictably varied by changing F0 range or duration of stressed syllables. Inner speech performed concurrently with stimulus discrimination elicited a tendency to attribute more often a neutral intonation to sentence pairs with happy or sad tone of voice, whereas no effect on discrimination of expressiveness became apparent. The main results of the brain activation study showed that recognizing emotional intonations and discriminating their expressiveness leads to a predominant activation of the RH with right frontal preponderance only in the absence of linguistic task demands. Inner speech performed in addition and concurrently with the identification/discrimination task gave rise to a balanced RH/LH activation pattern with left frontal preponderance. This lateralization shift was linked to an inverse response of cortical activity to F0- or time-varied stimuli. Without inner speech, pitch manipulations resulted in higher activation compared with durational manipulations, whereas the reverse effect, that is higher cortical activity in case of durational variations, was observable in subjects instructed to use inner speech. Significant interactions of acoustic parameter manipulations with emotions emerged both in the behavioural analysis and, with reverse features, in cortical activation strength (Fig. 6). They presumably result from varying cognitive efforts required to derive an impression. In other words, pitch might represent a stronger cue to expressiveness in happy intonations, whereas durational differences seem to be more usable in neutral utterances. Similar effects of varying cognitive demands on corresponding cerebral activation have been described in a linguistically oriented functional MRI study on comprehension of sentences with different structural complexity (Just et al., 1996). Acoustic structure and perception of affective prosody As in other psychophysically oriented studies, the pertinence of the observed effects depends on clarifying the interdependence of physical stimulus characteristics and the construction of a mental representation. Different theories of emotion perception approach this still unresolved issue. The category conception, which considers emotional perception and expression to manifest in separate, unrelated stimulus classes, faces a major problem which inheres in studies on acoustic affective communication. Few emotional categories present unique acoustic features and few acoustic features correspond uniquely to given emotional categories (Pakosz, 1983). Observations that misclassifications of affective expressions are based on certain similarities between emotions gave rise to dimensional models which conceptualize emotional perception as a result of multidimensional evaluation. Typical dimensions introduced are activation (e.g. excited versus calm), valence (e.g. pleasant versus unpleasant) and control (e.g. intentional versus unintentional). Binary terms were used to express contrasting poles and to permit further investigations in perceptual–acoustic interrelations (for a detailed discussion of this topic, see Frijda, 1969). Recognition of speakers' affective states does change during the course of their utterances. Tischer presented affectively intonated, naturally spoken phrases with an increasing number of words and asked subjects to rate them dimensionally (Tischer, 1993). He demonstrated that differentiation of the valence dimension occurs only in the course of the phrase and continues to develop throughout the whole utterance. In contrast, differentiation of the activation dimension was already completed in the shortest phrase, which comprised the first two words of a short sentence. This effect resulted in a typical change of affective state attribution: while the shortest phrase with happy intonation was often mistaken for an angry one, increasing stimulus length reduced false negative and produced positive (happy) impressions. It can be concluded from his study that evaluation and discrimination of emotional intonations presumably require the integration of different dimensional aspects in a continuously updated impression. The parameters considered for stimulus manipulation in this study, i.e. F0 range and duration of stressed syllables, presumably influence both the activation and the valence dimension, depending on whether dynamic or absolute aspects of stimulus perception are considered. Variations of dynamic characteristics (pitch range, modulation of vowel duration) were found to point at differences within the valence dimension. They gain importance with increasing utterance length (Tischer, 1993). At the same time these manipulations also varied aspects of perception like mean pitch height and speech rate which predominantly link with the activation dimension (Frick, 1985). Subjects in this study correctly identified the intended affective prosody of the original utterances. In contrast, evaluation of expressiveness yielded less consistent results with a broader range of performance (Fig. 3). As stimuli with happy and sad intonation showed marked differences in constant and dynamic aspects (i.e. activity and valence dimension) of emotion perception, correct identification of emotional category was expected. Accordingly, difficulties in evaluating expressiveness might at first be thought of as resulting from only small variations of the activity dimension, introduced by the acoustic manipulations. However, regarding the large variation of performance in Fig. 3 (right panel) another explanation becomes evident that refers to the inherent ambiguity of terms like `happier', `sadder' or `more excited' with respect to dimensional specifications. For example, an utterance can be perceived as sadder if it expresses passivity and less speaker activation (depression) or if it expresses despair when high speaker arousal and activity is signalled. Even happier might equally denote a more extrovert, highly excited state (e.g. enthusiasm) as well as a rather relaxed, self-content attitude like placidity. Consequently, it cannot be excluded that some subjects changed their attitude during the experiment alternately labelling stimuli with higher or lower activation as more expressive. In order to avoid direction of the subjects towards a pure evaluation of the activity dimension and, possibly, towards a decision strategy that relies on physical stimulus characteristics, response terms were intentionally kept general. As controlling of this effect was not looked upon as being essential with respect to the activation study, it was not further investigated. Lateralization of affective speech processing As lesion studies could not unequivocally outline specific cortical areas in which emotional meaning derived from affective intonation is represented (for a comprehensive review, see Gainotti, 1991; Heilman et al., 1993; Pell and Baum, 1997b), the current theories of emotional processing still await biological confirmation. While the valence hypothesis suggests RH dominance for negative and LH dominance for positive emotions, the RH hypothesis proposes a RH superiority, regardless of valence. A third hypothesis states that bilateral mechanisms underlie the perception of affective intonation. They might be based on a relative RH dominance in processing pitch-related parameters and a predominant LH processing of temporal cues (Robin et al., 1990; Van Lancker and Sidtis, 1992). In our study, a predominant RH activation with right frontal preponderance was observed when subjects discriminated affective speech without any additional demands. Within limitations, this result suggests a dominating RH function for emotion processing according to the RH hypothesis. However, specific effects of inner speech on affective intonation processing question a simple LH/RH dichotomy projected on to language and emotion perception. Under additional inner speech demands, a balanced RH/LH activation pattern arose with left frontal preponderance and reverse responses to pitch- and time-related information (Fig. 4C). Although our intersubject design does not allow clear distinctions in terms of RH deactivation and/or additional LH involvement, further evaluation of lateralization effects can be performed by considering the different task demands. First of all, effects of inner speech might be attributed to an inclusion of LH resources by a supervening linguistic performance. The cognitive function of repeating words `in our head' has been conceptualized in terms of a phonological loop (Baddeley, 1995). It involves two components, a memory store retaining phonological information and an articulatory control process. Paulesu and colleagues were able to demonstrate the underlying anatomy and localized the phonological store to the left supramarginal gyrus (Paulesu et al., 1993). The subvocal rehearsal system could be attached to Broca's area, whereas the left superior temporal gyrus seems to contribute to memory-independent phonological processing. These findings corroborate our results which revealed a preponderance of activation recorded from the left frontal electrode group (Figs 4B and 5). The number of electrodes used and the restrictions of spatial resolution inherent in the DC approach prevented reliable discrimination of temporal and parietal activation. However, the selectivity of temporoparietal brain responses towards frequency- and time-related information in this area (Fig. 5) can hardly be attributed to a general phonological storage, supporting our view that no significant phonological memory component was involved. Besides, there was no need to make use of a phonological store while listening to the second sentence and performing inner speech. This part of the task was not directed to any further linguistic stimulus analysis. It can be characterized as a process that quickly became automated in the absence of considerable interference (subjective and behavioural) with prosodic task demands. According to the RH hypothesis of emotion processing one would expect a rather independent activation of corresponding left- and right-sided neural networks and would have to interpret the activation pattern in Experiment 2 as resulting from an increase of LH activity due to pure inner speech. Regarding the comparatively little effort to perform this additional task, this suggestion seems not to be justified. Comparing the brain maps derived from non-normalized data (Fig. 4) and the behavioural results (Fig. 3, left panel, and Table 2A), indications of a reduction of RH activation under inner speech might be seen to correspond with the behavioural tendency to falsely denote an increased number of stimulus pairs as neutral. However, this effect represents only a tendency, leaving recognition of emotions basically intact. We presume that the lateralization effects observed are neither attributable to a repeated low level linguistic performance like `inner speech' nor to some kind of linguistically provoked distraction from discrimination of emotional intonations, for which we hold no behavioural evidence. As a consequence, results from the second experiment do not support the RH hypothesis. LH processing of affective prosody? Cortical activation patterns during affective speech processing have been demonstrated to be dependent on concurrent recruitment of the subvocal rehearsal system. As no significant emotion main effect emerged, the results do not support the valence hypothesis, which suggests RH dominance for negative and LH dominance for positive emotional contents. In addition, interaction between emotional category and inner speech remained clearly non-significant. Our data rather show that concurrent demands on the subvocal rehearsal system result in a bilateral activation of the two hemispheres, irrespective of emotional category. The striking finding is that this effect resulted from a reversal of preference towards the use of duration- or frequency-related acoustic features compared with spontaneous discrimination (Fig. 4C, lower panel). Although not significant in the omnibus statistics, a differential use of left and right temporoparietal regions towards the processing of either parameter is indicated in Fig. 5, which demonstrates left temporoparietal effects of temporal cue activation and right temporoparietal effects when pitch-related information is processed. Evidence from the literature indicates that bilateral mechanisms may underlie decoding of affective intonation. RH- and LH-damaged patients were found to show no difference in performance when asked to identify four different emotional intonations of short sentences (Van Lancker and Sidtis, 1992). The authors demonstrated in a meta-analysis on subjects' errors that RH-damaged patients preferentially relied on duration cues to make affective judgements, whereas LH-damaged subjects appeared to make use of F0 information. However, both groups performed poorly compared with normal controls, indicating that preferential use of either parameter was not sufficient for a good performance. Further evidence for different LH and RH specialization in analysing acoustic signals was obtained by Robin and colleagues (Robin et al., 1990). They showed left temporoparietal lesions to impair the ability of gap detection and pattern perception when sequences of tones were presented, while frequency perception was completely preserved. Lesions of homologous regions of the RH had opposite effects with normal processing of temporal information. The authors suggested that RH-damaged patients might be impaired in making prosodic judgements by a deficit in processing frequency-related information. Temporal processing capabilities of the LH were looked upon as referring to language and (linguistic-) prosody perception. This seems evident, as important linguistic information of the speech signal such as voice onset time or fast formant transitions depends highly on temporal signal properties. In this study, inner speech resulted in a balanced RH/LH activation pattern with left frontal preponderance. Compared with Experiment 1, this shift in lateralization effects was paralleled by higher activation during discrimination of duration-varied stimuli and less activity elicited by pitch-varied sentences. As duration of stressed vowels was changed relative to their absolute length, the manipulations failed to yield any difference in rhythm or stress pattern. It can be assumed that duration variations did not introduce any linguistically relevant difference into the speech signal. Presumably, the only pertinence of perceived temporal information resided within the prosodic task demand, which was the discrimination of emotional expressiveness. We assume that the shift of lateralization during inner speech reflects a neural involvement in the LH that goes beyond the activity of a subvocal rehearsal system. Conceivably, inner speech results in a bilateral hemisphere involvement together with new characteristics of acoustic signal processing. Weighting of acoustic parameters relevant for evaluation of emotional content changes, resulting in an increase of LH activation by temporal cues and reduced RH processing of frequency-related information. This change in preferential use of either parameter proved to be compatible with a good performance, at least in non-brain-damaged subjects. Coupling of acoustic input and verbal output channel: suggestion for a new approach towards lateralization of affective speech processing Cortical activation during discrimination of affective speech has been shown to be dependent on concurrent demands on the subvocal rehearsal system. The resulting bilateral processing as such is not surprising as the two hemispheres are closely connected through the corpus callosum and other commissures. Rather striking is the observation that a predominant RH activation during spontaneous discrimination changed into a bilateral pattern together with an inverse RH and LH preference towards temporal- and spectral-related information. We assume that bilateral involvement during inner speech reflects inherent LH functional coupling of acoustic input and verbal output channels. This mechanism has been shown to provide motor control information in the early phase of speech analysis, enabling subjects to shadow (i.e. to repeat as fast as possible) speech segments with latencies at the lower limit for execution of motor gestures (Porter and Lubker, 1980). These observations indicate parallel processes of auditory analysis and generation of an articulatory gesture. The purpose of this mechanism has been assumed to be to facilitate continuous control of speech output by comparing the signal produced with that intended. In more general terms, shadowing studies suggest that perceived speech is rapidly represented by neural networks, which provide an interpretation of the acoustic structure in terms of a motor program. Other input channels like the visual do not seem to have the same coupling strength to verbal output. Shaffer, for example, showed that auditory-vocal shadowing of continuous speech can be successfully combined with visually controlled copy-typing (Shaffer, 1975). In contrast, copy-typing of continuous speech was not compatible with a vocal reproduction of a written text, even for highly skilled audio-typists. To our knowledge, there are no studies investigating to what degree the construction of an articulatory representation of perceived speech also refers to suprasegmental intonational aspects. These processes must be expected close to neural structures in the frontal lobes, which are functionally connected to articulophonatory programs. They could provide feedback and control of prosody production, for example, if someone tries to speak intentionally in a happy or angry voice. Unfortunately, the neural structures underlying prosody production have not, been identified yet as recently concluded by Baum and Pell (Baum and Pell, 1997a). Conceivably, the high activation over left frontal regions observed in this study indicates the localization of the proposed mechanism. Early representation of suprasegmental information may also be relevant for evaluation of linguistic information like syntactic phrase structure. In this context, an early left anterior negativity was described by Friederici and colleagues in an event-related potential study (Friederici et al., 1999). LH-damaged patients have been shown to improve in repetition and discrimination of affective prosody under reduced verbal-articulatory load (Ross et al., 1997). We suggest that the underlying mechanism is a decoupling of LH acoustic input and verbal output channel resulting in a facilitated RH stimulus processing and performance improvement. Table 1 Test sentences of the present study Vowels of accented syllables are in bold-italic letters (English translation in parenthesis). Sie wollte seh'n, was das Leben im Süden bieten kann. (She wanted to see what life in the South could give her.) Sie gab nach, und verflogen war der ganze Unmut. (She gave in and all annoyance vanished.) Er kam spät am Abend und ging früh am Morgen. (He came late at night and left early in the morning.) Er sprach mit langen Sätzen, und niemand hörte zu. (He spoke long winded sentences and nobody listened.) Vowels of accented syllables are in bold-italic letters (English translation in parenthesis). Sie wollte seh'n, was das Leben im Süden bieten kann. (She wanted to see what life in the South could give her.) Sie gab nach, und verflogen war der ganze Unmut. (She gave in and all annoyance vanished.) Er kam spät am Abend und ging früh am Morgen. (He came late at night and left early in the morning.) Er sprach mit langen Sätzen, und niemand hörte zu. (He spoke long winded sentences and nobody listened.) Open in new tab Table 1 Test sentences of the present study Vowels of accented syllables are in bold-italic letters (English translation in parenthesis). Sie wollte seh'n, was das Leben im Süden bieten kann. (She wanted to see what life in the South could give her.) Sie gab nach, und verflogen war der ganze Unmut. (She gave in and all annoyance vanished.) Er kam spät am Abend und ging früh am Morgen. (He came late at night and left early in the morning.) Er sprach mit langen Sätzen, und niemand hörte zu. (He spoke long winded sentences and nobody listened.) Vowels of accented syllables are in bold-italic letters (English translation in parenthesis). Sie wollte seh'n, was das Leben im Süden bieten kann. (She wanted to see what life in the South could give her.) Sie gab nach, und verflogen war der ganze Unmut. (She gave in and all annoyance vanished.) Er kam spät am Abend und ging früh am Morgen. (He came late at night and left early in the morning.) Er sprach mit langen Sätzen, und niemand hörte zu. (He spoke long winded sentences and nobody listened.) Open in new tab Table 2A Significant effects and interactions in the repeated measures ANOVAs performed on ranked behavioural data Effect . d.f. . F . P . Emotional category Group 1.22 3.63 0.0700 Dimensional evaluation Group 1.22 0.30 0.5931 EMO 2.44 15.37 0.0001 EMO × MANIP 2.44 11.76 0.0001 Effect . d.f. . F . P . Emotional category Group 1.22 3.63 0.0700 Dimensional evaluation Group 1.22 0.30 0.5931 EMO 2.44 15.37 0.0001 EMO × MANIP 2.44 11.76 0.0001 Open in new tab Table 2A Significant effects and interactions in the repeated measures ANOVAs performed on ranked behavioural data Effect . d.f. . F . P . Emotional category Group 1.22 3.63 0.0700 Dimensional evaluation Group 1.22 0.30 0.5931 EMO 2.44 15.37 0.0001 EMO × MANIP 2.44 11.76 0.0001 Effect . d.f. . F . P . Emotional category Group 1.22 3.63 0.0700 Dimensional evaluation Group 1.22 0.30 0.5931 EMO 2.44 15.37 0.0001 EMO × MANIP 2.44 11.76 0.0001 Open in new tab Table 2B Significant effects and interactions in the repeated measures ANOVAs performed on normalized cortical activation values Effect . d.f. . F . P . The between-subjects factor group (with, without inner speech) and the repeated factors AP (anterior, posterior), side (left, right), MANIP (pitch, time) and EMO (sad, neutral, happy) were tested. All effects not included here were non-significant (P > 0.1; cortical activation analyses only). AP 1.22 44.37 0.0001 GRP × SIDE – 9.74 0.0050 GRP × SIDE × AP – 5.01 0.0356 GRP × MANIP – 5.92 0.0236 EMO × MANIP 2.44 6.23 0.0041 Effect . d.f. . F . P . The between-subjects factor group (with, without inner speech) and the repeated factors AP (anterior, posterior), side (left, right), MANIP (pitch, time) and EMO (sad, neutral, happy) were tested. All effects not included here were non-significant (P > 0.1; cortical activation analyses only). AP 1.22 44.37 0.0001 GRP × SIDE – 9.74 0.0050 GRP × SIDE × AP – 5.01 0.0356 GRP × MANIP – 5.92 0.0236 EMO × MANIP 2.44 6.23 0.0041 Open in new tab Table 2B Significant effects and interactions in the repeated measures ANOVAs performed on normalized cortical activation values Effect . d.f. . F . P . The between-subjects factor group (with, without inner speech) and the repeated factors AP (anterior, posterior), side (left, right), MANIP (pitch, time) and EMO (sad, neutral, happy) were tested. All effects not included here were non-significant (P > 0.1; cortical activation analyses only). AP 1.22 44.37 0.0001 GRP × SIDE – 9.74 0.0050 GRP × SIDE × AP – 5.01 0.0356 GRP × MANIP – 5.92 0.0236 EMO × MANIP 2.44 6.23 0.0041 Effect . d.f. . F . P . The between-subjects factor group (with, without inner speech) and the repeated factors AP (anterior, posterior), side (left, right), MANIP (pitch, time) and EMO (sad, neutral, happy) were tested. All effects not included here were non-significant (P > 0.1; cortical activation analyses only). AP 1.22 44.37 0.0001 GRP × SIDE – 9.74 0.0050 GRP × SIDE × AP – 5.01 0.0356 GRP × MANIP – 5.92 0.0236 EMO × MANIP 2.44 6.23 0.0041 Open in new tab Fig. 1 Open in new tabDownload slide (A) Acoustic signal (upper panel) and three synthetic pitch contours (lower panel) of the test sentence `Sie gab nach, und verflogen war der ganze Unmut', produced with neutral intonation. Voiced signal portions are indicated by bars below the acoustic signal, numbers indicate peak and end-point frequencies in Hz. In the lower panel, F0 range of the medial pitch contour is slightly increased compared with the original utterance. All test sentences in Table 1, each spoken with happy, sad and neutral intonation, were subjected to F0 range manipulation in the same manner. Averages of F0 range (highest minus lowest value) over four test sentences of each emotional and synthetic category are listed below (standard deviation in parenthesis). (B) Durational variants of the vowel /o/ of the stressed syllable in `verflogen'. In the upper acoustic signal, vowel duration has been reduced to 143 ms by cutting out single pitch periods; the middle and lower signals were lengthened to 195 ms and 232 ms, respectively, by doubling single pitch periods. Other accented vowels of each test sentence in Table 1 were shortened/lengthened accordingly giving rise to 4 × 3 durational variants (short, medial or long vowel duration) within each emotional category. Averages of total stimulus duration are listed below (standard deviation in parenthesis). Fig. 1 Open in new tabDownload slide (A) Acoustic signal (upper panel) and three synthetic pitch contours (lower panel) of the test sentence `Sie gab nach, und verflogen war der ganze Unmut', produced with neutral intonation. Voiced signal portions are indicated by bars below the acoustic signal, numbers indicate peak and end-point frequencies in Hz. In the lower panel, F0 range of the medial pitch contour is slightly increased compared with the original utterance. All test sentences in Table 1, each spoken with happy, sad and neutral intonation, were subjected to F0 range manipulation in the same manner. Averages of F0 range (highest minus lowest value) over four test sentences of each emotional and synthetic category are listed below (standard deviation in parenthesis). (B) Durational variants of the vowel /o/ of the stressed syllable in `verflogen'. In the upper acoustic signal, vowel duration has been reduced to 143 ms by cutting out single pitch periods; the middle and lower signals were lengthened to 195 ms and 232 ms, respectively, by doubling single pitch periods. Other accented vowels of each test sentence in Table 1 were shortened/lengthened accordingly giving rise to 4 × 3 durational variants (short, medial or long vowel duration) within each emotional category. Averages of total stimulus duration are listed below (standard deviation in parenthesis). Fig. 2 Open in new tabDownload slide Time course of a single discrimination trial displaying presentation periods and time-correlated DC-potentials at electrode position FC4 (grand average across all trials of a single test person). Sentence durations are indicated by horizontal bars; the light parts at the right ends indicate the range of durational variability between stimuli. Fig. 2 Open in new tabDownload slide Time course of a single discrimination trial displaying presentation periods and time-correlated DC-potentials at electrode position FC4 (grand average across all trials of a single test person). Sentence durations are indicated by horizontal bars; the light parts at the right ends indicate the range of durational variability between stimuli. Fig. 3 Open in new tabDownload slide Percentage of `correct' answers across subjects obtained during Experiment 1 (spontaneous discrimination, n = 16, dark grey boxes) and Experiment 2 (discrimination plus inner speech, n = 8, light grey boxes). The left panel refers to identification of `emotional category', the right panel to discrimination of `expressiveness'. F0-varied stimulus conditions are indicated by the suffix `-P', duration-manipulated conditions are marked by `-T'. Fig. 3 Open in new tabDownload slide Percentage of `correct' answers across subjects obtained during Experiment 1 (spontaneous discrimination, n = 16, dark grey boxes) and Experiment 2 (discrimination plus inner speech, n = 8, light grey boxes). The left panel refers to identification of `emotional category', the right panel to discrimination of `expressiveness'. F0-varied stimulus conditions are indicated by the suffix `-P', duration-manipulated conditions are marked by `-T'. Fig. 4 Open in new tabDownload slide (A and B) Grand average data of activation amplitudes. Mean amplitude values corresponding to grey levels and their units are indicated by the reference bar of map (A) and (B). Interelectrode values were computed by a linear smoothing algorithm taking into account the nearest four electrode values (Buchsbaum et al., 1982). (C) Plot of normalized amplitudes averaged across all subjects including values of the second presentation/analysis periods at frontal and temporoparietal electrode positions (listed above). The upper panel displays mean activation of LH and RH in Experiment 1 (white rhombi) and Experiment 2 (inner speech, black rhombi). The lower panel shows mean activation during discrimination of F0 and duration-varied stimuli in Experiment 1 (inner speech: no) and Experiment 2 (inner speech: yes). Fig. 4 Open in new tabDownload slide (A and B) Grand average data of activation amplitudes. Mean amplitude values corresponding to grey levels and their units are indicated by the reference bar of map (A) and (B). Interelectrode values were computed by a linear smoothing algorithm taking into account the nearest four electrode values (Buchsbaum et al., 1982). (C) Plot of normalized amplitudes averaged across all subjects including values of the second presentation/analysis periods at frontal and temporoparietal electrode positions (listed above). The upper panel displays mean activation of LH and RH in Experiment 1 (white rhombi) and Experiment 2 (inner speech, black rhombi). The lower panel shows mean activation during discrimination of F0 and duration-varied stimuli in Experiment 1 (inner speech: no) and Experiment 2 (inner speech: yes). Fig. 5 Open in new tabDownload slide Plot of normalized amplitudes averaged across all subjects including values of the second presentation/analysis periods at frontal (white background) and temporoparietal electrode positions (grey background). Mean values are plotted separately for the LH and RH, and for pitch- and duration-varied stimuli in Experiment 1 (white rhombi) and Experiment 2 (inner speech, black rhombi). Fig. 5 Open in new tabDownload slide Plot of normalized amplitudes averaged across all subjects including values of the second presentation/analysis periods at frontal (white background) and temporoparietal electrode positions (grey background). Mean values are plotted separately for the LH and RH, and for pitch- and duration-varied stimuli in Experiment 1 (white rhombi) and Experiment 2 (inner speech, black rhombi). Fig. 6 Open in new tabDownload slide Interaction of emotional intonation and acoustic parameter manipulations. Left panel: means of correct answers (ranked values), averaged over all subjects of Experiments 1 and 2. Right panel: plot of normalized amplitudes averaged over all subjects (Experiments 1 and 2) including values of the second presentation/analysis periods at all electrode positions considered for topographical analyses. Fig. 6 Open in new tabDownload slide Interaction of emotional intonation and acoustic parameter manipulations. 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TI - Cortical activation patterns of affective speech processing depend on concurrent demands on the subvocal rehearsal system A DC-potential study
JF - Brain
DO - 10.1093/brain/123.11.2338
DA - 2000-11-01
UR - https://www.deepdyve.com/lp/oxford-university-press/cortical-activation-patterns-of-affective-speech-processing-depend-on-gNv421BQwx
SP - 2338
EP - 2349
VL - 123
IS - 11
DP - DeepDyve
ER -