Psycholinguistics/Neural Components of Speech Production

MRI scan of the human head showing the brain and other important organs in speech production

Introduction

The ability to verbally communicate with the world is a precise and precious ability. Humans are the only specie on earth that produces such a dynamic form language to communicate. One important feature in the verbal communication of humans is speech production. Speech production is the conversion of individual thoughts into structured sentences for the purpose of communication. The ability to produce speech comes from the integration of physiological, neurological, and cognitive efforts. Humans use these efforts to translate their emotions and thoughts into strings of meaningfully plotted words that are relevant to the topic at hand. It is interesting to see the dynamics of the human brain when producing speech and noting that there is not necessarily one neural area that is activated. When a human utters a word, the lexical, emotional, motoric, semantic, phonological, and syntactic neural areas are all activated. Speech production is a fascinating ability that humans use, and an interesting area of research. This paper will attempt to summarize physiological speech production, anatomical neural areas associated with speech, and further make links to cognitive theories in speech production

The Physiological Production of Sound

The machinery involved in the physiological production of sound can be divided into two active parts; the source and the modulators. The exhaled air, being pushed out by the diaphragm, from the lungs is the source of sound in humans. As the air leaves the lungs and moves up the trachea it enters the larynx. The larynx houses the vocal folds, which are the most important organs in speech production. As the vocal folds, or more commonly called cords, vibrate the air that is channeled around them is converted into sound. Those cord oscillations are what determines the pitch of the sound being produced. A human can produce a frequency of anywhere between 200 and 7000 Hz. That sound then travels through the pharynx, and both the oral and nasal cavities where is it expelled. In order to articulate the sound the lips, tongue, and soft palate quickly modify the shape of the vocal tract, allowing for speech to be modulated and made.

Phonetics is the study of speech sounds including their production and perception. In regards to the tongues phonetics there is a traditional and modern view of how to conceptualize the articulation of the tongue. In the traditional view there is discrimination between high, low, mid, front, and back vowels. An example of a low vowel would be the “a” in daughter and how the tongue makes a low flat position in the mouth. This view however, does not take into account the dynamic movements and changes that the tongue makes in articulation. A modern view was then suggested. The modern view takes into account the changing position of the tongue in relation to height vs. how forward or backward it is. The vertical and horizontal abilities of our tongue within our vocal tract are what provide humans with the precision needed to produce speech.It is interesting to note that the only difference between the chimpanzee and human vocal tract is the location of the larynx. Human evolution has lowered the larynx, which allows us to produce a greater variety of formants.

Anatomical Areas Associated with Speech

Illustration of Broca's and Wernicke's area

The localization of speech production in the human brain varies greatly. In order to visualize and better understand the neural components of speech production studies mainly use lesion studies or neuroimaging. This chapter will outline some important lesion studies as well as giving some insight using modern imaging techniques like fMRI and PET. The areas that are most frequently identified as playing major roles in speech production are Broca’s area and Wernicke’s area. Broca’s area lies in the left inferior frontal gyrus, just above the lateral fissure. It role is believed to be in the production of speech. Broca’s area stores the motor programs needed to produce words. Wernicke’s area is located in the left superior temporal gyrus closer to the end of the lateral fissure. Wernicke’s area has been suggested to be important in the comprehension of words, as it holds the meanings, or semantics, of words. Damage to these areas results in Broca’s aphasia and Wernicke’s aphasia, which will be discussed in the aphasias section. Although these dominant areas are usually located in the left hemisphere the right hemisphere also has important roles in higher order speech tasks. The right hemisphere has been shown to be involved in speech perception, grammar, and sentence level semantics (Shuster, 2010). Also, interesting studies have shown that patients with right hemisphere localized strokes or brain damage have a disrupted ability or inability to maintain or initiate normal speech prosody (Brådvik et al, 1991). As there are lateralized functions in the brain there are also important bilaterally activated regions. These regions include the motor cortex, thalamus, dorsolateral caudate nucleus, and the cerebellum.

The Wernicke-Geschwind Model

The Wernicke-Geschwind model is a three-section model that evolved from lesion studies. It outlines the proposed mechanisms that control comprehension, speech, and reading in the brain. The Wernicke-Geschwind model initiates the production of speech with cognition, which is the mental representation or plot of what the individual wants to express. It then moves to Wernicke’s area, which would outline the meanings of the words needed, that information is then sent along the arcuate fasciculus to Broca’s area. Broca’s area is where the motor plans of speech are assembled. Broca’s area then sends these instructions to the adjacent areas in the motor cortex, which communicates with the brainstem to activate the facial motor neurons, and further activate the over 100 correct facial muscles needed to produce speech (Geschwind, 1972). Studies were done to support this suggested role of Broca’s area. It was found that stimulating Broca’s area, in awake patients, gave them the inability to make the voluntary facial movements needed in speech production. Interestingly, stimulation of this area also disrupted the patient’s ability to discriminate phonemes and also affected the patient’s ability to produce gestures associated with speech (Ojemann, 2003). This inability to produce gestures is because the areas used for gestural language and vocal language are mapped in adjacent areas, and it has even been proposed that they depend on the same brain structures. Using fMRI it was found that native signers (those who learned sign language early in life) have the same localized left areas activated during signing that a person who was communicating audibly would have (Newman, 2002).

Other major brain areas have just recently been suggested to aid speech production. Having so many distinct brain areas activated in speech production has provided evidence that rather than focusing on localization, that speech production is more about the network connections. Studies have shown that the articulo-phonological brain network is much more dynamic that once proposed. Here is a small summary of the important secondary brain regions involved in speech production as studied using fMRI technology:

Motor & Pre-Motor Areas: quite obviously the motor areas are involved in speech production, as they are the centers of control for all the muscles and cranial nerves involved. Results from an fMRI study on speaking shows that these areas a bilaterally activated. Thalamus & Basal Ganglia: there are distinct parallel circuits between the basal ganglia, thalamus, and cerebellum. Initiation and control of voluntary movement has been seen to activate the basal ganglia. It has been suggested that the thalamus is involved in the preparation of the movements of speech. Cerebellum: studies have shown that in order to carryout speech production the cerebellum is one of the most important regions. The cerebellum also shows activation with vertical movements of the tongue and lip modulations. Insula: it has been proposed that the involvement of the posterior insula in speech production is to contribute to the conscious awareness of your own speaking. The left anterior insula, comparatively, has been shown to be involved in the formulation of an articulatory plan.

Newer studies have shown the involvement of many other aspects of the brain in speech production. In a meta-analysis it was found that in basic speech production the posterior, middle, and superior temporal gyrus are activated, compared to in more complex speaking tasks the left inferior frontal gyrus, cerebellum, and left caudate nucleus (Soros et al., 2006). This means that with varying intensity of cognitive effort toward speech production, the neural activation map changes. To conclude many areas of the brain are activated in speech production and more research needs to be done to specify the role of each of these areas. The next part of this chapter takes the neural areas mentioned and structures them into a map cross-listed with a classic cognitive model of speech production.

Interesting Findings from PET Imaging Studies

An analysis of PET scans revealed that when speaking there is bilateral activation the facial neurons, both motor and sensory. Interestingly when speaking verbs the areas of activation are; the frontal lobe, posterior temporal cortex, anterior cingulate cortex, and the cerebellum (Petersen, 1988). This suggests that when speaking an action word aloud the brain is not only considering it verbally but also activating the motor regions involved in partaking that action.

Weaver Model Linked with the Neural Regions in the Brain

The Weaver model was designed to show, at a conceptual level, the processes and the outputs from each cognitive level within the brain. This section will attempt to link the classic Weaver model with newer research to propose locations and regions that these processes and outputs are taking place neutrally. This linkage was summarized nicely in a article by Gregory Hickok in 2010.

1. The first process in Weavers model is conceptualization. At a neural level it is hard to definitively outline the area that this process activates. Studies show that conceptualization is widely distributed in the human brain. The output from this process is the lexical concept.
2. The second process is lexical selection. The output of this level is lemma, which is the mental wordedness. The lemma is found by using abstract conceptualizations to find a word that you aspire to use by only taking into account the meaning you are attempting to suggest and not the sound. This lexical interface has been shown to have weak bias in left hemisphere and activation in the posterior medial temporal gyrus and the posterior inferior temporal sulcus.
3. Morphological encoding is the third process in the Weaver model. Its output is the morphemes needed. The specific region of the brain responsible for morphemes has not been well defined.
4. Phonological encoding, more specifically syllabification, is the fourth process involved, and its output is the phonological word. The main brain area involved in this process is Wernicke’s area (in the superior temporal gyrus in the left hemisphere).
5. The fifth process is phonetic coding (syllabary). The output from the process is phonetic gestural scores. It has been determined that the area involved in phonological network is the middle posterior superior temporal sulcus. It has been proposed that the dorsal pathway maps phonological representations onto articulatory motor maps and a ventral stream take the phonological representations into lexical conceptual representations.
6. The final process in Weaver’s model of speech production is articulation. The output of this process is sound waves. The main outlined areas for articulatory networks are the posterior inferior frontal gyrus and the anterior insula, both being left lateralized.

Disorders of Speech Production

Speech production aphasia is a defect or loss of the ability to express or produce speech. There are many different symptoms that can accompany aphasia's of this type. The main symptoms that are present in speech disorders are; poor articulation, anomia (word-finding defect), paraphrasia (unintentional phrases), loss of grammar or syntax, inability to repeat, defects in verbal fluency, and aprosodia (loss of tone in voice). Aphasic Syndromes regarding speech production can be classified in two categories; fluent and non-fluent.

Fluent Aphasia

This type of aphasia means that the individual’s speech is still fluent and they do not make articulatory mistakes. They do however; have poor comprehension, poor ability to repeat, and frequent anomias or paraphrasias. Aphasias of this nature include:

1. Wernicke’s Aphasia: Also called sensory aphasia. This is the inability to arrange speech sounds or to comprehend speech. Develops due to damage to Wernicke’s area.
2. Transcortical Aphasia: Also called isolation syndrome. This is the inability to speak spontaneously or to comprehend words, although their speech is still defined as fluent and their ability to repeat words is intact.
3. Conduction Aphasia: This is an interesting aphasia in that individuals with this would be able to speak fluently, understand, and name but are not able to repeat words. Develops due to damage or disconnection between the word image and the motor system to produce the word.
4. Anomic Aphasia: this type of aphasia, as the name suggests, is the inability to name objects with all other speech properties intact. Interestingly people with this disorder are only unable to name nouns. If the word they are trying to express is also a verb they can usually mentally find and dictate it. Develops due to damage to the temporal cortex.

Non-Fluent Aphasia

This type of aphasia means that the individual’s comprehension is correct but they have trouble initiating, producing, and articulating speech. It is interesting to note that most individuals with these types of aphasias do know what they want to express and can structure it in their minds correctly; they just are challenged by their inability to convert those thoughts into words. Aphasias of this nature include:

1. Severe Broca’s Aphasia: This is the complete inability to actively produce speech. Aphasias of this nature also show problems with repetition.
2. Mild Broca’s Aphasia: A less severe form of Broca’s aphasia. This syndrome shows noticeable articulation problems, anomia, agrammatism, dysprosody.
3. Transcortical Motor Aphasia: this includes the intact ability to repeat and name, but difficulties with spontaneous speech production.
4. Global Aphasia: this is the most severe type of aphasia it symptoms include speechlessness, poor comprehension, and laborious articulation.

Conclusion

The human brains ability to produce speech is a well researched area of psycholinguistics. Although there is so definitive map that outlines the regions clearly it appears that science is moving in the right direction by use of fMRI and lesion studies. The neural precision needed to produce even basic speech activates many regions in the brain. Broca’s area and Wernicke’s area are the most mentioned but newer studies have placed emphasis on other regions to aid in speech production. Without the combined efforts of the physiological, neural, and cognitive processes humans would not be able to communicate in such effective ways. It is also important to understand the mechanisms of action by recording how damages or lesions affect the brain. At the completion of this chapter the reader should have a better understanding of the neural components of speech production.

Learning Exercise

Instructions Part 1

For this exercise you need to correctly identify the certain type of aphasia to each passage presented. Taking what you have learned about Broca’s and Wernicke’s aphasia in the chapter, read aloud the below passages to a partner and:

• mm tommmato.. fry-paan. Tuessday….ahh Anne Jacob Tom… mmm lettuce an’ cheese. table Ahhhh six Tuesday family. Ahhhh meat spice. Ya ahhh

• Sharon high school was Pennsylvania. Penguins and poppies. Mothers flower was a lily she likes read they eat apples. Kids go on in directions with cars. Traveling not comforting never talking to each other. Talk sing and they’re balancing. I whale and water jumps.

1. State the type of aphasia. Be specific and correctly identify the brain region that is associated with the disorder (which hemisphere, anatomical location including adjacent major sulcus’ and gyrus’). (4 points each)

2.Attempt to decipher the meaning of the passages or what is trying to be communicated. Is this harder to deceiver for the first passage or the second? (2 points)

3.Now that you can understand fluent and non-fluent aphasia try to communicate with each to your partner (or just aloud) in a similar way. For non -fluent Broca’s aphasia you could try reading an excerpt from a book but only saying the nouns. To generate a thought pattern like a fluent Wernicke’s aphasic write one random word on a piece of paper and think about its associates with other words and write down the first that comes to (for example fire truck, red, apples, trees, smells good, rain…). (1 point)

Instructions Part 2

Talking what you know about anatomical areas of speech production and the physiology of speech attempt to diagnose these patients. Be sure to clearly outline why you made each diagnoses. (3 points each).

1..John is a 47-year-old lawyer. Two months ago and MRI showed a large tumor that was encroaches on his left middle cerebral artery. John has trouble initiating speech and sometimes says the wrong words. Most of the time he accurately comprehends what others are trying to communicate but reciprocal speech is challenging and takes a lot of effort. John can actively name objects presented and repeat them. John has also noticed some changes in his motor control.

2.Susan suffered a temporal lobe stroke last year. She has had a lot of speech therapy but is now speaking fluently although with some notable problems. She has a hard time finding the names of objects. For example when she is showed a picture of a pear she responds by saying a delicious green fruit, but if you asked what do shoes come in? She would respond pairs.

3.Timmy took a direct blow to his head at practice on Monday. He was hit on his left side in the fontal-parietal junction. Timmy’s speech is very confusing to understand and it sounds like gibberish. What are some possible reasons for this?

Instructions Part 3

Taking what you have learned in this chapter about the Weaver model and its associated brain regions, challenge yourself to a matching task! Print off the below exercise and see if you can match the Weaver model with its brain area! (6 marks)

References

Brådvik, B., Dravins, C., Holtås, S., Rosén, I., Ryding, E., Ingvar, I.D. (1991). Disturbances of speech prosody following right hemisphere infarcts. Acta Neurologica Scandinavica. Volume 84, Issue 2, pages 114–126,

Geschwind, N. Language and the brain. Scientific American 226:76-83, 1972.

Hickok, G. The functional anatomy of speech processing: From auditory cortex to speech recognition and speech production. DOI: 10.1007/978-3-540-68132-8_8

Newman, A.J., Bavelier, D., Corina, D., Jezzard, P., Neville, H.J. A critical period for the right hemisphere recruitment in American Sign Language processing. Nature Neuroscience 5:76-80, 2002.

Ojemann, G.A. The neurobiology of language and verbal memory: Observations from awake neurosurgery. International Journal of Psychophysiology 48:141-146, 2003.

Petersen, S.E., Fox, P.T., Posner, M.I., Mintun, M., Raichle, M.E. Positron emission tomographic studies of the processing of single words. Journal of Cognitive Neuroscience. 1:153-170, 1988.

Shuster, L.I. The effect of sublexical and lexical frequency on speech production: An fMRI investigation. Brain and Language, 111(1), pp.66-72, 2009.

Soros, Sokoloff L.G., Bose, A., McIntosh, A.R., Graham S.J., Stuss, D.T. Clustered functional MRI of overt speech production. Neuroimage 32,pp.376-37 8, 2006