Language abilities and word processing are superior functions of the human brain. In this regards, the article “Choosing words: left hemisphere, right hemisphere, or both? Perspective on the lateralization of word retrieval” by Riès et al. published in Annals of the New York Academy of Sciences in April 2016 is an excellent review of literature regarding the lateralization of language functions of the brain. This journal review article aims to review and reflect on the role of the brain in language ability and word processing in humans. The article starts with an introduction describing the topic and a summary of the above mentioned journal article. This is followed by an analysis of the key findings of the article and concludes with a critical conclusion about the strengths and weaknesses of the article. ?
The human brain, alongwith having a lot of sensory, motor and cognitive functions has a great role in language abilities and word processing in the human beings. A lot of research has been done on the language functions of the brain and on the areas of the brain which are involved in the development of language skills. An interesting research title “Choosing words: left hemisphere, right hemisphere, or both? Perspective on the lateralization of word retrieval” was done to access the functions of brain with respect to language and vocabulary, especially in the context of the lateralization of word retrieval from either left or right hemisphere of the brain (Riès et al., 2016)
The central purpose of the above cited article is to review language functioning of the brain while focusing on a core process of the human brain functions to synthesize language which is defined as conceptually-driven word retrieval, allowing the humans to retrieve vocabulary from long-term memory while speech. The article seeks to make a research into the belief of language being a brain function which is highly lateralized. As per available literature, the article acknowledges left-hemisphere dominance for language to be consistently proven in various research as well as clinical contexts, and that the core principles of neurology are based on this finding. It further moves on to challenge this finding with recent studies especially functional neuro-imaging studies which have demonstrated the role of right hemisphere of brain in various language abilities. The article specifically notes the finding that in case of incidents leading to a deficit in language abilities of the left hemisphere like stroke, the right hemisphere takes up some of this ability as a compensation.
In this regard, the article presents a literature review of the researches which target the human ability of choosing words to build the language. The choice of words, although, is easily performed by the brain in subjects having an intact-language, it gets quite compromised in those having aphasia. The article employs division of the process of word retrieval into two sub processes namely lexical activation and lexical selection in order to understand the lateralization of word retrieval more deeply, by evaluating the sub-process which is liable to be compensated for after a left-sided stroke.
The method applied by the research is a literature review and analysis. It concludes the result that in line with previous clinical observations and studies, the left-hemisphere of the brain possesses a greater potential for word retrieval in comparison to the right-hemisphere. Nevertheless, right-hemisphere areas, particularly the right inferior-frontal areas, can potentially be activated as a compensation consequent to a stroke affecting left-hemisphere areas carrying out the function of word-retrieval. Furthermore, the potential of right-hemisphere areas to aid in compensation of word retrieval abnormalities consequent to aphasia due to left-hemisphere stroke seems to be dependent on the peculiar sub process which is affected. Therefore, right-hemisphere areas particularly the right-frontal areas seem to be superior in making a compensation for lexical-selection compared to the abnormalities in lexical-activation. This observation positively tests the hypothesis that domain-general networks play an important role in recovery from aphasia instead of linguistic ones.
The article has an excellent and engaging title. Being a scientific research article, it expresses the overall goal of the study quite well. It also points to a key function of the human brain i.e. choosing words which is an essential component of the language formation. The article has a clear statement of purpose and objective which is clearly stated at the end of the introduction as well as in the abstract, the abstract one being somewhat not as precise as the one described at the end of the introduction. The introduction very well describes the scope of the study by stating that although language/communication functions like prosody, pitch and some forms of discourse level processing have been significantly researched to have a right lateralization, the research for the article is merely centered upon the single-word retrieval function. This very well defines the scope of the study and its limitation. The researchers also give a clear picture of the order in which the research was carried out, firstly by focusing on the left-hemisphere areas accounting for retrieval of words & the impact of abnormalities in these areas secondary to a stroke. Secondly, the researchers bring into discussion the right-hemisphere areas of brain involved in retrieval of words and their possible roles in compensation subsequent to a break in the retrieval of words word retrieval secondary to lesions in the left-hemisphere.
The study very well reviews the results obtained by evaluating two core types of literature. One is the functional-imaging literature in normal subjects as well as those who had suffered a stroke. The other one pertains to lesion symptoms mapping studies in those who have suffered a stroke. The study concludes by proposing hypotheses regarding various sub-processes of retrieval of words and why such sub-processes are or are not compensate for the brain’s right-hemisphere.
The article starts off with a description of the role of left hemisphere of the brain in the retrieval of words. It first describes the regions of the brain’s left-hemisphere which are associated with the retrieval of words in a normal human brain. It puts forward a review by Price that posterior areas of the left middle/inferior temporal gyri, the superior-temporal gyrus & left hippocampus, the left superior/middle/inferior frontal gyri, the medial-frontal areas like pre supplementary motor areas as well as the anterior cingulate cortex are involved in retrieval of words, which thus has a very broad spectrum of participating brain areas (Price, 2012).
The authors then move on to describe the insights from stroke induced aphasia on establishing the causal-role of left-hemisphere cortical regions involved in the retrieval of words. Thus is done by first reviewing researches employing various methodologies for identification of brain regions critical in retrieving words in subjects with aphasia and then reviewing functional-imaging studies in subjects suffering chronically from stroke having the aim of identification of the brain areas concerned with retrieval of words during recovery phase. The authors describe the critical role of left-temporal lobe in retrieval of words in those with aphasia. They mention the newly described importance of left-posterior-middle temporal gyrus in deficits of word retrieval (Turken & Dronkers, 2011). They also review the association of difficulty in finding words with lesions in the left-frontal lobes especially in the areas of inferior-frontal gyrus (Ries et al., 2014).
The authors note that various researches including those done by the author themselves suggest that the left inferior frontal gyrus participate in the anticipatory-control of action (Jonides. 2006). Furthermore, newer studies point out that the left inferior-frontal/occipital fasciculus, linking the left inferior frontal gyrus to the posterior-temporal regions, is involved in the resolution of semantic-interference in picture-naming as well as in the working-memory (Harvey & Schnur, 2015). They also refer to the studies which demonstrate the positive correlation of recruitment of per-lesional tissue to getting recovered well from post-stroke aphasia and to retrieval of words (Szaflarski et al., 2013). The authors summarize there discussion on the left brain hemisphere by concluding that substantial evidence establishes its role in making possible the retrieval of words. They furthermore conclude functional-neuroimaging studies to demonstrate that left-temporal, left-lateral/frontal, and medial-frontal regions of having association with various functions of retrieval of words in usual disease-free subjects. Regarding the left brain hemisphere, the authors conclude that the left-temporal regions are associated with lexical activation while the left frontal regions are associated with lexical selection. They reach the consensus that upon functional neuro-imaging, the patients recovering from left-brain aphasia demonstrate significant peri-lesional activation alongwith lateral/medial prefrontal cortex activation.
After concluding the role of left brain hemisphere, the authors discuss the role of right brain hemisphere in retrieval of words. The research on role of right brain hemisphere starts with a discussion of the brain areas belonging to the right brain hemisphere involved in retrieval of words in the normal/healthy human brain. The authors describe that although right brain hemisphere hasn’t normally been the subject of neuroimaging studies with respect to retrieval, of words, a lot of f-MRI investigations have reported of the activation of right brain hemisphere (Hocking et al., 2009). The authors note that upon employing perfusion-fMRI, left as well as the right hippocampus was observed to have sensitivity to the difficulty of retrieval of words (Whitney et al., 2009).
The authors then move on to describe the insights from stroke induced aphasia on establishing the role of right-hemisphere cortical regions involved in the retrieval of words. The authors begin by not knowing any prior studies which proposed deficits in retrieval of words consequent to a right brain hemisphere stroke, with the exception of crossed-aphasia (Henderson, 1983). The authors, however, observe a potential role in compensation by right brain hemisphere areas, specifically the right-frontal areas, in patients recovering from left brain hemisphere deficits subsequent to stroke (Turkeltaub et al., 2011). The authors note that right frontal-activation in retrieval of words subsequent to a stroke affecting left brain hemisphere reflects higher domain-general attentional-recruitment, which also occurs in recovery from aphasia (Geranmayeh, 2014). The authors conclude the insights on role of right brain hemisphere by declaring that the activation of right brain hemisphere, though quite weak, is established while retrieval of words in the normal human brain. In those with an injured one, the right frontal-cortex seems to have some role in a process of compensation subsequent to a deficit in retrieval of words, especially for deficits of lexical-selection (Winhuisen et al., 2005). However, generally, right-frontal recruitment subsequent to lesions of the left brain hemisphere is normally sub-optimal in comparison to peri-lesional recruitment (Vitali et al., 2007).
The article concludes by a discussion on the hemispheric asymmetries in the retrieval of words, throwing light on why there is a left brain hemisphere bias for word retrieval. The authors review various past studies to answer the question. However, they remain non-conclusive here, observing finally that it still remains unclear that the anatomic-asymmetries reported in the left and right brain hemispheres (Fernández-Miranda et al., 2014) are a reason or a result of the peculiarity of the left brain hemisphere regarding language function. Finally, at the end of the article, the authors hypothesize that portions of the brain areas dealing with normal language-formation like the left inferior frontal gyrus ; pre-supplementary motor area /anterior cingulate cortex, can also have a part in the domain-general cognitive control processes. If these brain areas are disrupted, affecting the processes they support, it might be easy for further brain areas linked to domain-general cognitive-control processes, like the ones in the right-frontal lobe or some areas of the medial-frontal cortex, to help in recovering the language functions.
The article under review is based upon extensive research by the authors on the available literature, studies and researches on the subject matter. It is based upon and reviews as much as on hundred and sixty nine researches already done in the area to develop a discussion and arrive at some conclusions. It adds much to the body of knowledge on the language functions of the human brain and their lateralization, especially in context of word retrieval, and sums up much of the knowledge which already exists in this regard.
The weaknesses of the article include it being finally not much conclusive about the question it needs to answer as per its title, though it does suggest some hypotheses. The language used is very technical and needs a strong background in the anatomy, physiology and pathology of human brain in order to read, interpret or apply it. The research is evaluated to be quite significant in clinical cases of aphasia consequent to stroke in determining the outcomes of such a stroke. With more research to follow, based on the hypotheses put forward by this one, it may contribute to better understand the retrieval of words by human brain and be a source for the development of a better cure and treatment for aphasia compared to those which are currently in vogue.