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Vocal emotion processing
Vocal emotion perception is the understanding of an emotion through auditory expression. An expression may either be inclusive of speech or be a non-verbal vocalisation e.g. a scream or a laugh. When research into vocal emotion processing is carried out there are several parameters in the voice that must be considered. These include changes in respiration, phonation and articulation, which would not be considered in the examination of non-vocal emotional events such as facial expressions or body language. Additionally, vocal emotion processing refers to the neural networks and pathways involved in the process of perception and there are differences in such networks depending on whether an utterance is verbal or non-verbal. Processing verbal utterances requires semantic meaning to be understood from the sentence, whereas a non-verbal expression of emotion only requires the emotion of the sound to be classified. In order to carry out research on these networks functional magnetic resonance imaging (fMRI) and other techniques are implemented. These allow the regions of interest within the brain to be identified with reasonably high spatial resolution compared to techniques such as electroencephalography (EEG) that analyse event-related potentials (ERP). This said, the temporal time line of vocal emotion processing has been loosely defined, see Model below (Section 2.2).

Verbal emotion
Verbal emotion is the emotion portrayed through prosody and semantics in a persons spoken language. Prosody is an important variable in emotional signalling. The level, range, and contour of the fundamental frequency (f0) of a person’s voice are extremely influential factors in the communication of emotions, along with the vocal energy and distribution of energy within the frequency spectrum. With regard to semantics, words that relate to specific emotions usually have a similar semantic structure in place that related directly to the emotion being expressed. Also, in terms of cultural differences in semantics, basic emotions including happiness, fear and anger that are words in the English language may equate to a universal 'natural semantic metalanguage' that explains the universality of understanding emotion from speech.

fMRI research
One piece of research involved an emotion categorization task using pseudo words spoken in neutral or angry tones. The functional imaging data was used for diffusion tensor imaging (DTI). This revealed a functional connectivity between subthalamic nucleus (STN) and other structures. Results below. Structures relating to emotion with functional connectivity: Structures relating to emotion with structural connectivity: The results of this research reinforced the findings relating to the STN and its role in the processing of affective stimuli, allowing conclusions to be drawn about the role of the STN within the processing of emotional prosody in a vocal modality.
 * Orbitofrontal cortex (OFC)
 * Bilateral amygdala
 * Inferior frontal gyrus (IFG)
 * Pallidum
 * Orbitofrontal cortex (OFC)
 * Bilateral amygdala

Another investigation also utilised fMRI and DTI to obtain data from participants during the presentation of pseudo sentences of different emotional tones (angry, sad, joyful, relieved and neutral) .Results below. Emotional voice regions:
 * Posterior two thirds of bilateral superior temporal gyrus (STG)
 * Right primary auditory cortex
 * Medial geniculate body
 * IFG
 * Inferior parietal lobe

Model
There is one main theoretical model that has been created to describe the processing of vocal emotion processing, based on results from spoken language. In order to explain the perception of vocal emotion, the effect of amplitude, timings and fundamental frequency of a speakers voice are considered. For example, there is a noticeable difference when comparing the emotional prosody of happiness and sadness. Happiness is characterised by fast speech and high energy, whereas sadness shows the opposite trend being slower and having less energy, demonstrating how these factors contribute to our experience and perception. In order to convey certain emotions and shape the way voices come across, physiological alterations and changes occur depending on the communicative intention.

The model discusses brain mechanisms involved in vocal emotional processing in terms of paralinguistic objects and consists of 3 steps, described as:


 * 1) Sensory Processing
 * 2) *After the presentation of an utterance sensory processing happens mainly in the primary auditory cortex and Heschl's Gyrus.


 * 1) *The primary auditory cortex region is tonotopically organised to react to certain frequencies.


 * 1) *The reaction of the cortex to certain frequencies creates a profile of the sound based on these acoustic factors.
 * 2) *N1 negativity (see Event-related potential) peaks at roughly 100ms after the presentation of auditory stimuli that is attended to by the participant.
 * 3) *The time scale for this stage spans from 0-150ms.
 * 4) Integration
 * 5) *Here, emotionally significant acoustic cues within the sound are identified.


 * 1) *This stage follows the ‘what’ pathway from auditory cortex to the lateral STG and the superior temporal sulcus (STS).


 * 1) *STG and STS correspond to the belt and parabelt of primary auditory cortex.


 * 1) *Processing gets progressively more complicated as it heads towards the anterior STS. The chief role of this stage is to label auditory objects, selecting those with emotional content.


 * 1) *Processing in the ‘what’ pathway is said to be especially important when processing both verbal and non-verbal vocalisations.


 * 1) *Differences between emotional stimuli and neutral stimuli peak in ERP data at around 200ms.
 * 2) *The time scale for this stage spans from 150ms-300ms.
 * 3) *The event related potential (ERP) reveals mismatch negativity (MMN) which is measured and interpreted as peaking during extraction of emotionally significant acoustic cues.
 * 4) Cognition
 * 5) *In this stage emotional information selected from the previous stage is used in higher level cognitive processing.


 * 1) *The right IFG and OFC are responsible for evaluative judgements. This is where a sound is given its emotional label.


 * 1) *The left IFG is responsible for integration of emotional prosody into language understanding. This is where the overall meaning of the sentence is construed.


 * 1) *The N400 on the ERP trace relates to the semantic processing of the words, therefore it is assumed this process is occurring later at around 400ms.
 * 2) *The time scale for this stage spans from 300ms-500ms.

PET research
Research based on the processing of non-vocal stimuli is relatively scarce, though one investigation was one of the first to study emotional vocalisations using positron emission tomography (PET) scanning. This investigation used several different emotions, these being sad, happy, fearful, and neutral non-verbal vocalisations. The sounds were based from vocalisations made by male and female speakers with voiced nasals used as neutral sounds. The two speakers were native english speakers, though the sounds made were non-verbal. Regions when all emotion conditions were analysed against non-emotional conditions displayed activation in:
 * Left middle temporal gyrus
 * Left superior frontal gyrus
 * Right caudate nucleus
 * Bilateral anterior insula
 * Bilateral ventral prefrontal cortex (PFC)

These results suggest that perceiving emotional vocalisations is a more complicated process that originally suggested by lesions studies. The processing of non-vocal emotions requires communication between hemispheres and the involvement of both cortical and subcortical structures.

fMRI research
In terms of fMRI, research has investigated the nonlinguistic signals that influence emotion perception by using non-speech affective vocalisations presented in a scanning setting. . Analysis looked at both physical differences and perceptual differences between a morphed stimuli continuum between the emotions anger and fear. Looking at the change across morph steps not only allows direct comparisons but also comparisons between morphs in a continuous carry over sequence. To assess physical and perceptual differences regressors are entered into a general linear model.

The results of the study are as follows: Physical difference between non-vocal stimuli elicit activation in:
 * Bilateral voice-sensitive auditory regions
 * Right amygdala

Perceptual difference between morphs elicited activation in:
 * Bilateral anterior insulae
 * Medial superior frontal cortex
 * Precuneus
 * Subcortical regions
 * Bilateral hippocampi
 * Bilateral amygdalae
 * Bilateral thalami

These results suggest a multistep process in the cognition of vocal emotion processing similar to the multistep model for verbal processing explained by the model detailed in section 2.2. However, the results from this research add detail to the cognition stage of the verbal emotion processing model by detailing the difference between physical and perceptual differences.

The Role of the Amygdala
The true role of the amygdala in emotional processing has been questioned since the turn of the century. Whilst it is tightly associated with the experience of emotions in terms of the fight or flight response the amygdala does not seem to have a strong direct role in the perception of vocal emotion, as demonstrated by the research provided in previous sections (See 2.1, 3.1 & 3.2). Moreover, the amygdala has a more integrated role along with other structures such as the STN and frontal cortex regions compared to a more central role, as previously assumed.

Further research discusses how in general, the amygdala works to detect and evaluate stimuli, assessing a multitude of emotions and other types of non-emotional objects. For example, an fMRI investigation has demonstrated the amygdala's involvement of in the presentation of novel stimuli. In summary, there are mixed results about the involvement of the amygdala in vocal emotion processing, which leads focus to be drawn to the additional structures found to be active during this neural process. Overall, our knowledge of the true role of the amygdala has developed from improvements in neuroimaging and led to progression in brain research.