Abstract caused by corpus callosum could affect speech

Abstract

 

Previous studies
show evidence for the left hemisphere dominating the right one. This is due to
the fact that the two language areas – Broca’s and Wernicke’s – both located on
the left side of the brain and also the left hemisphere controls movements of
the preferred hand of most people. Nevertheless, it is unclear whether
left-handed and non-sided people have the same dominant hemisphere. An experiment
was conducted through a lexical decision task to answer this question. The
research involves participants completing a sidedness questionnaire to
determine their true sidedness. Participants then were exposed to a list of
five-letter words or non-words, which appeared on the partakers’ left or right
visual field, with a colour-changing dot as a fixation point after each letter
string. Partakers were asked to decide if the letter string they had seen was a
word or non-word as quick as possible. It was expected to find that the letter
strings exposed on the left visual field were answered more correctly and
quickly by left-sided people while non-sided people’s success rate and response
time were the same for letter strings on both visual fields.

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Cerebral Asymmetries: The correlation
between sidedness and cerebral dominance

 

It is known that
the human brain contains two separate hemispheres, which control the movements
and reception of the opposite side of the body, for example, left hemisphere
controlling the movements and reception of the right side. These two
hemispheres are connected by corpus callosum consisting of millions of nerve
fibres. Different experiments on split-brain operations were conducted in which
researchers compared the behaviour of split-brain patients and normal people in
order to determine the function of the corpus callosum on language. It has been
concluded that corpus callosum acts as the information transfer between the two
hemispheres. Some researchers argued that the stimulation caused by corpus
callosum could affect speech ability. Nevertheless, it has been agreed that
language is controlled mainly by the dominant hemisphere, which is often
considered to be the left hemisphere (Selnes, 1974).

The reason to
why the belief of the left hemisphere dominating the right one can be explained.
In terms of movement, most people are right-handed, which implies that the left
hemisphere controlling the movement of the right side of the body dominates the
right hemisphere. In terms of cognitive functions, as the two language areas
are both situated in the left hemisphere, it can be seen that the right
hemisphere acts as the non-dominant one. According to Havas et al. (2015), Paul
Broca was known for his case of a patient with speech problems. This patient’s
damaged area, which is now known as Broca’s area, was located in the inferior
frontal gyrus of the frontal lobe on the left hemisphere. The patient, though
had severe problems producing speech, could perceive and understand
information. This phenomenon is now called Broca’s aphasia. On the other hand,
Ardinal, Bernal and Rosselli (2016) note the symptoms of Wernicke’s aphasia.
Patients with Wernicke’s aphasia, who are damaged in Wernicke’s area – an area
in the temporal lobe of the left hemisphere, can speak fluently but cannot interpret
what has been spoken to them. Nonetheless, Selnes (1974) justifies that the
minor hemisphere (often thought of as the right hemisphere) can perform
language process well enough. By contrast, it is also stated in the article
that split-brain people can perform better than people with aphasia in terms of
processing language either verbally or cognitively. This indicates the
importance of language processing areas in the left hemisphere.

Although it is
believed that left hemisphere is the dominant one, it is only based on the
preferred hand of most individuals. It is also questioned whether the same
dominant hemisphere applies to everyone or if left-handed people have dominant
right hemispheres. Several researchers have conducted different studies to find
the answer to this question. Bradshaw, Gates and Nettleton (1977) carried out a
lexical decision task involving showing participants strings of three or four
letters of words or non-words on their left or right visual fields. It was
found that left-handed people (presumably having dominant right hemispheres)
could perform well, but not as well as right-handed people. It should be
acknowledged from this finding that left hemisphere works more efficiently than
right hemisphere, at least in terms of perception. However, it should be
remembered that this research’s sample size was low (48 participants for the
first experiment and 8 for the second one), which means that the results may
not be true. Carmon and Gombos (1970) also claim that it is impossible to
predict the sidedness of one person due to his handedness as he may have other
body parts preferred on the opposite side like ear, along with the fact that
practice can affect sidedness. Hence, the results of the study conducted by
Bradshaw et al. (1977) cannot be taken into consideration if the sidedness and
handedness were not the same for every participant. It is clear that there has
been no clear evidence about the correlation between sidedness and cerebral
dominance.

This study aims
to find out if there is a difference in the dominant hemisphere among
right-sided, left-sided and non-sided people. In order to do this, the lexical
decision task was used to measure partakers’ success rate and response time. It
was expected that left-sided people responded faster and more accurately to the
letter strings appearing on their left visual field while non-sided people had
the same accuracy and response time in both visual fields.

 

Method

 

Participants

There was one
participant group engaging in the research. Participants were chosen through
opportunity sampling method who are students majoring in Psychology at
Lancaster University. Partakers include 90 Psychology students with 21 males
and 69 females from the age of 17 to 20 (male mean = 18.6, male standard
deviation = 0.87; female mean = 18.5, female standard deviation = 0.69). After
a questionnaire to determine sidedness, it was found that whereas the majority
of participants were right-sided, there were 12 left-sided participants and 4
non-sided participants.

 

Design

The study was
designed to determine how the correlation between sidedness and visual fields. A
questionnaire was used to ask people which side of hand, foot and eye they use
to do particular actions, which helped decide whether the participant is
right-sided, left-sided or non-sided.

In order to
determine the stronger visual field of a participant, a task was designed on a
program called Psychopy2. Participants were exposed to 48 letter strings, which
consist of both real words and non-words appearing on either their left or
right visual field randomly. The real words used in the experiment are common
while the non-words are simple words altered by one letter in the middle. This
allows the equality for partakers to complete the task. All strings contain
five letters so that participants spend roughly the same amount of time to look
at the word or non-word appearing on the screen. Each letter string is visible
for a fifth of a second so as to ensure that the letter strings cannot be
looked directly at and be processed properly. Participants were to decide
whether the letter string appeared was a word or a non-word and press the right
key to express their answer (‘h’ for a word and ‘b’ for a non-word) as quick
and accurate as they could. After each letter string, a colour-changing (first
black then red) dot is presented in the middle of the screen to act as the
fixation point for a few seconds before the next letter string appears. The
program recorded the results, which are the accuracy and response time of each
participant. Overall, the task lasted for approximately five minutes for each
participant.

 

Stimuli and Materials

In this experiment, materials include a
set of sidedness questionnaires and computers with the app called Psychopy2,
which contains the lexical decision task. The task involves exposure to 48 strings
of five letters, which are words or non-words, presented to the participants on
either left or right side of the screen. Partakers are to look at the equal
number of words and non-words on each side in a random order. The results
collected were accuracy and response time.

 

Procedure

There was only one participant group
partake in the study as all participants were in the same experiment condition.
Participants were asked to complete a questionnaire which refers to handedness,
footedness and eyedness to determine their sidedness. The purpose of this is to
predict the dominant hemisphere for each participant. After that, partakers were
instructed to start the lexical decision task in the app Psychopy2 on a
computer. Partakers were to sit in the correct position and use the first two
of their right fingers to press either ‘h’ or ‘b’ key (with ‘h’ for a word and
‘b’ for a non-word). Participants were to decide whether the word they had seen
was real or not and press the correct key in the quickest time possible without
moving their eyes or guessing. The results recorded are how accurate the
participants performed and their response time.

 

 

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