Exploring the Relationship Between Mouth Asymmetry and Speech-Language Disorders

Exploring the Relationship Between Mouth

Asymmetry and Speech-Language Disorders

Harry W Stephen

English Language and Linguistics

University of Kent

Note: Formatting may be strange as this is a plain text copy, especially for mobile devices.

Abstract:

    This paper develops an extensive review of existing literature to examine how right-sided

oromotor (mouth and facial motor) asymmetry correlates with speech and language processing in

both neurotypical individuals and those with speech-language impairments. Based on studies of

babies babbling (Holowka and Petitto, 2002), adults who stutter (Choo et al., 2010), a seven-

year-old dysphasic brain dissection (Cohen et al., 1989), and neuroimaging of adults with

specific language impairment (SLI) (Badcock et al., 2012), analysis suggests that abnormal

lateralization of brain function and brain structure variation may manifest as atypical oromotor

asymmetry during speech. While substantial research supports the left hemisphere's role in

speech and language production (Berker, 1986; Manns, 2019), more extensive research is

required to establish a stronger link between irregular manifestation of oromotor asymmetry

being a result of abnormal brain structure. This paper suggests that research integrating

neuroimaging with detailed oromotor assessments during speech, of both neurotypical and SLI-

affected individuals would provide much more conclusive evidence regarding oromotor

asymmetry variation indicating potential speech-language disorders.

Methodology

    Sources were found using the Google Scholar and APA PsycNet databases and PubMed

for medical terms (e.g. Dysphasia definition), as well as cited sources within other sources. Key

search terms included some of the following: "oromotor", "asymmetry", "hemisphere

specialization", "SLI", "mouth asymmetry", "right asymmetry", "babbling", "babies". Studies

were limited to articles published in accredited peer-reviewed journals such as

Neuropsychologia. Some sources were originally written in a different language and in those

circumstances, an official translation copy was cited (e.g. Berker (1986) is an official translation

of Broca (1865)). Findings were synthesized based on evidence provided within the source and

the conclusion/discussion sections provided as well as the methodology used and how that could

affect results found, and legitimacy of the source. Within this paper, findings from across several

different sources were compared based on the same criteria. Caution was used when cross-

comparing various sources as majority of the evidence regarding the relationship of brain

structure variation and mouth asymmetry is largely preliminary. Studies were also grouped by

topic rather than methodology, though there are references to all major sources throughout the

paper.

1. Hemisphere Specialization and Asymmetry

    Hemisphere specialization (HS) refers to the asymmetric structure of the brain that causes

certain cerebral processes to become lateralized across each hemisphere: speech processing

in the left hemisphere (Berker, 1986), and emotional processing in the right (Gainotti, 2012)

to note some prominent examples. For the case of speech production which is lateralised in

the left-hemisphere, right side asymmetry of the mouth can be a direct result due to a higher

frequency of nerve impulses from the left hemisphere than the right. (Berker, 1986; Manns,

2019). Furthermore, right asymmetry manifests during word list generation and rhyming

which are also known as propositional tasks, which consists of spontaneous utterances that

conveys specific meaning (Hamilton et al., 2010). This is a result of propositional tasks being

processed predominantly through the Broca’s area, located within the left frontal lobe

(Berker, 1986; Graves, 1985; Graves and Landis, 1990). Why the asymmetry of the mouth

appears on the right side of the mouth is because each hemisphere controls the opposite side

of the face as well as other major body parts (Gardner, 1968). Further examples of the

physical manifestation of function lateralization: Hand asymmetry appears to manifest during

speech based on auditory input as both speech and non-speech sounds processed by the left

and right hemispheres, respectively (Kimura, 1973a, 1973b). Direction of eye gaze during

cognitive activity (Ehrlichman and Weinberger, 1978), and facial movement during

emotional expression (Borod et al., 1981; Ekman et al., 1981; Sackeim and Gur, 1978). These

examples are more clear indicators that hemisphere specialization can manifest physically

based on other cognitive tasks.

    The mouth within the study is the most promising indicator of cerebral specialization

because it appears to provide detail of hemisphere specialization as seen in studies by Graves

and Nicholls. Furthermore, the mouth specifically is what one could expect to articulate the

most during speech given its role in speech production.

1.1. Visual Expression of Right-Side Asymmetry

    It is possible that the right side of the mouth plays more of a role in day-to-day

communication than the left: Two out of the three main categories of mouth asymmetry in

Nicholls and Searle (2006) paper demonstrate that phenomenon: (1) Asymmetries in the

speakers mouth, and (2) asymmetries in the visual expression of the speakers mouth

(Nicholls and Searle, 2006).

    (1) The right side of the mouth opens further and faster than the left side during both

verbal and non-verbal speech because the left hemisphere is responsible for both

motor control of that area (oromotor control: Mouth motor function), and language

processing. However, verbal speech is considered to be more reliable in terms of right

oromotor asymmetry than non-verbal (Graves and Landis, 1990) especially during

speech activities like spontaneous speech production, and reciting song lyrics. Despite

this, non-verbal speech can demonstrate right oromotor asymmetry during non-verbal

singing. Singing songs without lyrics still uses external articulators (Nicholls and

Searle, 2006).

    (2) Right oromotor asymmetry is believed to play a key role in the visual and auditory

expressivity of a speaker’s speech. In a study by Graves and Potter (1988),

participants were told to listen to people saying tongue twisters articulated with either

the left or right side of the mouth. Many participants found the tongue twisters

articulated by the right side of the mouth to be clearer, which could be because the

right side of the mouth is able to articulate more than the left (Graves and Potter,

1988). However, Nicholls and Searle did not explicitly state neurological reasons for

this conclusion. Nevertheless, one could connect that the greater articulation of the

right side of the mouth could be cause by the increased neuromotor impulses from the

left hemisphere as seen on Graves and Landis (1990) (Berker, 1986; Graves and

Landis, 1990; Manns, 2019).

    Lip reading is often a seen as a tool for deaf people but is also a valuable tool for

speech between hearing people when considering the McGurk effect (McGurk and

Macdonald, 1976). For example, if what you heard does not line up with what you

expected to hear from the shape of one’s lips, you could potentially merge what you

heard and the sound you expected (McGurk and Macdonald, 1976). If you heard the

word /bɒg/, but it looks as if they said /dɒg/, you may misinterpret the sound in your

head. An experiment using the McGurk effect was done by playing a video which

consisted of two sounds being played at the same time but showing only the

articulation of the left side of the mouth for one sound, and the right side for the other

in a video. Participants were then asked to choose which sound was most clear, and

like (Graves and Potter, 1988), majority picked the sound depicted to come from the

right side of the mouth (Campbell, 1986). This supports the idea that not only is the

right side of the mouth more auditorily expressive as seen with tongue twisters

(Graves and Potter, 1988), but visually expressive too by manner of greater

articulation(Campbell, 1986).

    What these two categories of mouth asymmetry could be interpreted as is that the left

hemisphere plays a key role in speech production and perception. Left hemisphere

specialization during speech is what causes the right side of the mouth to articulate further

and faster which is what causes the right side of the mouth to be more visually and auditorily

comprehensible than the left. Replicating this study on those with atypical oromotor

asymmetry, as we will discuss further could show how the visual expression of speech could

change with the variations of brain structure.

2. Oromotor Asymmetry by Demographic

    Oromotor asymmetry can manifest differently depending on the type of speech task. For

propositional tasks such as word-list generation and rhyming words, a lateralization index

(LI) of +0.93 was calculated (Graves and Landis, 1990). (L - R)/ (L + R) = LI is the formula

of the lateralization index, but the number used to calculate that figure are not found which

does take away from its reliability. Furthermore, there is a discrepancy of LI between sexes.

Men tend to score higher LI where women would score lower. For example, when describing

images, women saw on average less mouth asymmetry than men because women’s responses

were more “amused” than men’s (Graves et al., 1982). However, this discrepancy is not

noticeable in babies, where a blanket LI for all 10 participants were calculated regardless of

baby's sex (Holowka and Petitto, 2002). Holowka and Petitto's study were only on a small

sample of 10 babies so though their results are important, they are limited. The discrepancy

of sex related differences in Graves and Holowkas study could indicate a psychological

development later in later brain development and not a result of underlying brain structure,

but no reason is given.

2.1. Mouth Asymmetry in Adults

    The following analysis is of neurotypical adults, and though they are referred to as

“normal” in several studies cited below, they will be referred to as “neurotypical” in this

paper. Furthermore, analysis on Graves et al. (1982) experiment one is limited due to a

missing page from original source.

    Of 196 subjects, 76% (150 out of 196) displayed right-side mouth asymmetry during

bilabial production (Graves et al., 1982). What is more, linking back to Gardner (1968) and

the idea of contralateral hemisphere control, it would explain both why right asymmetry of

the mouth occurs in Graves study based on the evidence supporting language processing in

the left hemisphere. No conclusive evidence within Graves et al. (1982) definitively confirms

this idea, but its validity appears likely. However, Graves et al. (1982) was not able to state

that the left hemisphere is speech dominant, only motor dominant. Further research could

demonstrate how variation oromotor asymmetry during both propositional and non-

propositional speech tasks could indicate a potential shift in hemispheric dominance.

Damage to the Wernicke’s area (exactly opposite of the Broca’s area), would cause

sentence content to become illegible but sentence structure would remain completely intact,

and there would be minimal effect on articulation. Damage to the Broca's area would result in

a notable change in speech. Weakness in right side mouth muscles, dysarthria (Neuromotor

disorder affecting speech articulation (Jayaraman and Das, 2025)), and inability to form

coherent sentences. The reason more dramatic affects occur from a damaged Broca's area is

because the left hemisphere processes not only speech, but language too. The right

hemisphere mainly processes just speech during propositional tasks (Berker, 1986; Graves

and Landis, 1990). Additionally, 90% of right-handed and 70% of left-handed people affected

by either lesion/disease (Berker, 1986), or injection of anesthetic (Rasmussen and Milner,

1977) was seen to develop symptoms of a damaged Broca's area just described. This

observation, in relation to provided evidence of language localization provides a compelling

argument for language processing in the left hemisphere and more specifically, the Broca's

area.

    Overall, based on Graves et al. and Berker, connection between left hemisphere

specialization and speech-language production is not an unlikely theory. Though there is no

conclusive evidence, examples of right-side asymmetry during word propositional tasks or

brain structure variation and its effect on speech could link towards the theory of left

hemisphere specialization of language, even without that conclusive evidence.

2.2. Mouth Asymmetry in Babies

    In comparison to adults, there is less research published regarding the mouth asymmetry

of babies, Holowka and Petitto (2002) study takes 10 babies and uses a simple laterality

index to calculate which hemisphere was perceived to be more dominant in any given

linguistic motor-based task.

Babies as young as five-twelve months old can show signs of right oromotor asymmetry

whilst babbling (Holowka and Petitto, 2002), which based on previous evidence can be

linked to left hemisphere specialization. Babbles in this context were defined as single

    syllable vocalisations that were syllabically organized (consonant – vowel, etc…) and

reduplicated. Any other vocalisation that did not fit these criteria were considered Non

babbles. Laterization indexes (LI) were calculated from 150 randomly selected sections of

babbles, non-babbles, and smiles. LI = (R-L)/(R+L+E), where R = Right Opening, L = Left

Opening, E = Equal Opening. The following scores were calculated from that formula (no

individual values for each variable were provided in the source). +0.88 during babbling, -

0.82 during spontaneous smiles, and -0.08 during non-babbles. (Holowka and Petitto, 2002).

There is a noticeable difference in hemisphere specialisation when observing babbling and

spontaneous smiles. Right mouth asymmetry was found for babbling, like adults in Graves

and Landis (1990). Left side asymmetry was found during spontaneous smiles, and though

there is not a direct link, it could be comparable to how women were observed to have less

right asymmetry than men when describing pictures (Graves et al., 1982). However, LI's

calculated demonstrated that though babbles were seen as left-hemisphere dominant, since

the LI for babbles was +0.88 and not higher, that could indicate bilateral speech processing,

though this is only speculation.

    Based on Holowka and Petitto (2002) some strong references can be made to similar

studies of adults by Graves, Nicholls, etc which could indicate that babies as young as 5

months demonstrate cerebral hemisphere specialization. Despite that, it is important to note

that the pool of subjects in this study was 10 (5 English and 5 French babies) babies, unlike

Graves et al (1982) 196 adults. Further research on oromotor asymmetry in babies of a larger

sample size would improve the validity of the results found in Holowka and Petitto (2002).

Theory could suggest that atypical mouth asymmetry in babies could potentially predict

speech and language disorders that are not traditionally diagnosed until later stages in life.

This topic is expanded on in sections (3) and (4).

2.3. A Variation of Mouth Asymmetry – Adults Who Stutter

    Unfortunately there is very limited research on the potential of mouth asymmetry

manifestation in people with SLI, so comparisons of different types of lip asymmetry found

in adults who stutter (Choo et al., 2010) can be compared to brain scan results of adults with

SLI (Badcock et al., 2012) to compare EMG results with white and grey matter of adults with

SLI to make a prediction of hour oromotor asymmetry could manifest in adults with SLI.

Choo et al. (2010) hypothesized that EMG results for adults who stutter (AS) would show

right hemisphere dominance of speech processing and oromotor asymmetry that manifests on

the left side of the mouth. Adults who do not stutter (ANS), in line with (Graves and Landis,

1990) were predicted to demonstrate expected left hemisphere dominance of speech

processing and oromotor asymmetry of the right side of the mouth. Choo et al. (2010)

focused on the bottom lips as they are the most active during speech. Adult stutters (AS)

(ranging from mild – severe stuttering) and 5 Adult non-stutters (ANS) were used for

comparison. All participants were self-reported to not have any neurological or health

condition and all 10 participants are right-handed. Asymmetry was based on lip muscle

activity shown by EMG results which were used to measure oromotor activity. Results found

demonstrated what was predicted in the hypothesis. AS demonstrated greater right-

hemisphere involvement than ANS, as well as left-sided lower lip asymmetry. It was

discussed that several factors could have caused the variation of lip asymmetry. AS could

reflect a poor connection between the left hemisphere and the right side of the mouth.

Speculation was also shown that timing differences in cerebral activation could be a cause as

in ANS, the left hemisphere is often seen activating before the right (Palolahti et al., 2005;

Saarinen et al., 2006). There was also the discussion of variation in muscle strength of the

mouth, though there does not seem to be as much strength in this argument. Overall, there

does seem to be a neurological difference between both AS and ANS that causes hemisphere

dominance to shift.

    Choo et al. (2010)'s results are significant but limited because they show a high tendency

that oromotor asymmetry will change/vary based on brain structure variation without using

neuroimaging software, which would show more conclusive evidence. Studies from the likes

of Graves, Nicholls, etc use neurotypical participants, so results are consistent and

comparable. Furthermore, stuttering is not a language disorder, but a neuromotor disorder, so

making connections to language is complicated because they are seen as two separate

systems. However, with sources on brain dissections of a dysphasic brain and neuroimaging

of adults with SLI, a case can begin being built connecting language and speech.

3. Brain Structure Variation and its Effects

    Looking previous studies of hemisphere specialization and their manifestation in

oromotor asymmetry, only so much evidence can be provided to support the initial theory of

the relationship between oromotor asymmetry and language disorders. However, there

appears to be a lack of research regarding the idea of predicting speech and language

disorders based of mouth asymmetry. Using a dysphasic brain dissection and study of SLI

brain structure, the aim of this paper is to make a supportive argument of the further research.

Further research regarding brain structure, nerve impulses, and oromotor asymmetry with

speech and language disorders in mind would contribute to much more conclusive evidence.

The following sections bring the established idea of hemisphere specialization and mouth

asymmetry one step further connecting it to real life examples brain structure variation and

its effect on oromotor asymmetry, as well as applications in a real language disorder, SLI.

3.1. Case Study of Dysphasia Patient

    Although the primary focus of this paper is oromotor asymmetry, it is important to note

that manifestation of asymmetry in motor function is not confined solely to the mouth, as

mentioned in (1.2). Below is a case study regarding the dissection of a 7-year-old white girls'

brain, who suffered from dysphasia (Cohen et al., 1989).

    
An initial neurological examination (at 3 years and 2 months of age) revealed asymmetric

muscle stretch reflex 2+ on the right, within normal limits, and 3-4+ on the left side,

demonstrating a stronger reflex on the left side of her body. Subsequent neurological

examinations did not see a replication of this finding. Despite that, even a single instance of

neuropathological symptoms such as asymmetric muscle reflex is offers valuable insight into

cognitive functions. Even temporary deviations of motor asymmetry could be reflective of

underlying developmental variations in brain structure. Although this evidence is not

conclusive of speech and language disorders manifesting in oromotor or other neuromotor

functions of the body, it provides a potential indicator for further research into whether

similar temporary manifestations could be linked to brain structure variation.

Figure 1: Detail of the dysplastic gyrus. (Luxol fast blue-periodic

acid-Schiff stain, x 80 before 52% reduction.) – (Cohen et al.,

1989)

Figure (1) shows glial scarring caused by

dysplastic microgyrus in the left insular

cortex. Cohen et al. (1989) argues that this

was a result of disturbance in neuronal

migration assembly. In simpler terms,

when neurons were migrating into cortical

layers, something did not go right. and

similar cases have been observed in people

with dyslexia. Cohen et al., (1989) goes on

to suggest that this abnormal development

may be related to oromotor apraxia,

anomia, and reliance on gesture. However,

though Cohen et al., (1989) did imply this

connection, the study does not establish a direct link between the glial scarring and

impairments as mentioned before. However, this could demonstrate the correlation between

cortical structure variation and oromotor function as we can see a possible direct relation

between structure and oromotor variation.

    Findings from Cohen et al. (1989) are thought to show results of a strong connection

between brain structure and manifestation in the mouth with oromotor apraxia. Emphasis

should still be made that this was a single individual with other health related issues. Though

findings from Cohen et al. (1989) does contribute to previous ideas of oromotor asymmetry

variation in speech and language disorders, there is not enough data to act as conclusive

evidence; there is only data from one individual. However, connections made between brain

structure variation and oromotor apraxia are strong indicators that based on atypical oromotor

asymmetry, could indicate underlying abnormal brain structure, speech, or language disorders.

Further research should be done, potentially on living patients to see a real-life correlation

between brain structure variation and its effect on oromotor asymmetry. Overall, evidence from

Cohen et al. (1989) does seem to suggest that variation in brain structure manifest in oromotor

asymmetry, glial scarring within the left insular cortex could potential be linked to motor

functions such as oromotor apraxia, anomia, and reliance on gesture.

3.2. Brain Structure of Adults with SLI

    Due to abnormal brain structure of adults with specific language impairment (SLI),

irregular hemisphere specialization in cognitive tasks could be expected (Badcock et al.,

2012). Badcock et al. (2012) used voxel-bases morphometry (VBM) protocol, a statistical

device used to measure differences in brain concentration, on fMRI scans of participants

brains. It was found that adults with SLI have increased grey matter in the left inferior frontal

region but reduced activation (Badcock et al., 2012). Furthermore, altered patterns of grey

matter in the temporal areas and the caudate nucleus. These areas are essential for the

planning and execution of speech. Mapping out grey matter is essential to one day

associating grey matter variation to oromotor asymmetry variation. Making this connection

could mean being able to predict SLI in younger individuals (assuming grey matter is

consistent across age groups). However, that is only speculation and needs more conclusive

evidence to support its validity. The findings in Badcock et al., (2012) are consistent with the

observed speech production difficulties and oromotor control issues that individuals with SLI

have. Based on the notion that individuals with SLI have reduced activation in the frontal left

hemisphere of the brain and the idea that individuals with SLI exhibit speech difficulties,

there seems to be a strong connection between variation of brain structure and how that

manifests through mouth asymmetry.

    Though Badcock et al. (2012)'s research was robust, their paper was theoretical in that it

did not seem to apply the findings from neuroimaging to real life applications of people with

SLI. Due to this, like Cohen et al (1989), evidence is not conclusive, and it merely acts as a

support validating the research potential of the theory one could predict speech and language

disorders based on oromotor asymmetry.

Further research is using similar techniques of neuroimaging used in Badcock et al.

(2012) in correlation to oromotor asymmetry assessments of participants could create a much

stronger connection between brain structure variation and oromotor asymmetry. This more

conclusive evidence could act as a foundation to studying how accurately one’s speech-

language disorder could be predicted based on oromotor asymmetry. With these in mind,

Badcock et al (2012) is still critical in supporting the idea that variation of brain structure

could potentially affect mouth asymmetry during speech production.

4. Conclusion: How Brain Structure Variations Influences Mouth Asymmetry

    In summary, the evidence presented in this paper suggest a critical role of hemispheric

specialization and its manifestation in oromotor asymmetry, during speech and language

production. The left-hemisphere, which has been well for processing speech and language

faster than the right (Berker, 1986; Palolahti et al., 2005; Saarinen et al., 2006) is what could

be the cause of observable right-side asymmetry of the mouth. This phenomenon is what

could have caused the right-side asymmetry during propositional tasks in Graves and Landis

(1990) where linguistic function of said tasks is widely believed to be left hemisphere

dominant, through the Broca’s area. Asymmetry is further supported by non-verbal motor

tasks, where similar asymmetry was found in hand, eye, and facial movement.

Moreover, the oromotor asymmetry across demographics such as similarity of

lateralization between adults and babies further supports the existence of cerebral

specialization of speech and language in early development. Research into adults who stutter,

a dysphasic brain dissection, and SLI brain structure further suggest that variation in brain

structure may indeed influence oromotor asymmetry, potentially serving as an early indicator

for speech and language disorders.

    While these findings establish a solid foundation for justifying further research regarding

brain structure variation and its effect on oromotor asymmetry, the current body of evidence

remains preliminary. Future studies with neuroimaging techniques alongside oromotor

assessments are essential to conclusively establishing a relationship between brain structure

variations and mouth symmetry. Research within this field is critical because if being able to

predict speech and language disorders based on oromotor asymmetry is established, this

could potentially be extrapolated to infants, like those in Holowka and Petitto (2002). Their

study already suggest that infants aged five to twelve months develop hemisphere

specialization of speech and language based on lip asymmetry. With that connection in mind,

speech and language disorders could be predicted earlier than ages they are currently

diagnosed at now.

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