Which Bit of the Brain Detects the Emotions in Speech?

BMC Neurosci. 2010. 11. 93. ‘Figure 1. Region of Interest. The Inferior Frontal Gyrus, drawn in MRIcro’*

 

There is an interesting paper by Hoekart and colleagues. It’s got a very long title ‘Results of a Pilot Study on the Involvement of Bilateral Inferior Frontal Gyri in Emotional Prosody Perception: An rTMS study’ and freely available here. Communicating emotions is an essential part of everday life. In speech, the emotions are conveyed using subtle changes in characteristics  including pitch, tone, rhythm and volume. A term used to summarise some of these characteristics is prosody and this is described by the authors thus

‘Emotional prosody, a paralinguistic feature of language, is characterized by intonation, loudness and stress placement in speech‘

So to understand emotions in speech, you have to take a step back and look at some fairly abstract properties of speech. Without going into too much detail about these properties, various research groups have looked at these properties while a person’s brain is being imaged using paradigms such as functional Magnetic Resonance Imaging (fMRI). They have found some evidence from these studies that emotional prosody is processed in a region of the brain known as the frontal-parietal operculum. The operculum is the outer surface of the brain.

In this study, the authors wanted to examine this further. They used a technique known as Transcranial Magnetic Stimulation. This approach generates a Magnetic field in a localised part of the brain which crudely speaking stuns that part of the brain into inactivity effectively taking it offline for a short period of time. Although it sounds a little alarming the evidence suggests that the brain returns to normal functioning after a while. In this way researchers are able to investigate the function of different parts of the brain. Compared to intaroperative electrode stimulation experiments such as that developed by Wilder Penfield, TMS is relatively crude as it involves the application of the magnetic field through the scalp, skull and meninges to the surface of the brain. Nevertheless it has provided some useful evidence in building up a picture of how certain parts of the brain work and has even been investigated for illnesses such as depression.

In this study, the researchers investigated 6 ladies and 4 men aged 18-26 from the University of Groningen. There were various exclusion criteria for MRI and TMS. They had undergone a structural MRI scan to obtain an image of their brain and this was then used to guide the TMS device to the right part of the person’s brain. This was achieved by using stereotactic apparatus which essentially is a device which calculates the position of different brain regions from the brain images in relation to the surface features of the head. So there’s no need for any invasive procedures. The researchers detail the characteristics of the TMS stimulus that is applied.

The subjects then needed to perform some tasks related to emotions in speech. Subjects would hear either neutral or emotion content read with either neutral or emotional intentional.  The TMS was applied to the left or right inferior frontal gyrus and also sham TMS was applied. The researchers detail the statistical analysis used. The ANOVA is useful for teasing out the effects of one amongst many variables.

Predictably it took longer for subjects to respond to the emotional content than the emotional intonation. Presumably this is because they have to think about the content of the word rather than judge the emotional presentation of the speech. Interestingly with the TMS there was no difference between the stimulation and the sham stimulation of these brain regions (right and left inferior frontal cortex) on response to the emotional content of the words spoken. There was a difference however for the emotional prosody. In this case, subjects took longer to process the emotional prosody of the speech when these brain regions were stimulated. Since they are effectively inactivating these areas, this infers that these brain regions are involved in processing the emotional prosody of speech.

So from this study at least, it looks like the left and right interior frontal cortex are involved in processing the emotional content of speech. However there are a few complications. Thus these parts of the brain might connect to other areas that are needed to process the information in which case these areas can be thought of as conduits for the information rather than the processing centres. This is a common criticism of localising studies. An additional interpretation which is a slight variation of the above is that these areas operate as part of a wider circuit and knocking one part out impairs the functioning of the wider circuit. Nevertheless regardless of whether they are considered a part of a circuit or the sole location of the processing, this study does provide tantalising evidence that these areas are important for the processing of emotions in speech.

* The diagram is presented under a Creative Commons Attribution License 2.0 and the diagram is located here.

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