Handedness, Laterality, and Mirror Neurons

By Alexis Mingos and Beau Perkins

Image of the Tin Man wishing for empathy, published by Yohan John March 13, 2013. Accessed May 11, 2020. Original image from Wikimedia. Licensed under CC-NC-SA 3.0.

The question of handedness and its relationship to the brain is a key case study for the history of science.  To date, scholars still disagree about the origins of handedness—some emphasize genetic models while others support a multifactorial approach, for example. Left-handedness has been a point of stigma since at least ancient Mesopotamia,  up through ancient Roman myths, tales of the Middle Ages, and into the modern day where, for example, in Japan early conversion to right-handedness is prevalent.  Despite the significant history of social discrimination against left-handers, the scientific enterprise continues to uncritically investigate questions such as the relationship between schizophrenia and handedness, claiming conclusive results. 

A hot theory of handedness origins relating hand gestures and language has come into the spotlight as a result of the discovery of mirror neurons (Corballis, 2003). In this post we’ll talk more about that theory and mirror neurons, as well as the history of handedness. We will also give a brief explanation of digital methods, presenting an initial text mining project on the scientific literature regarding handedness and mirror neurons. Finally, we argue for a critical approach to handedness research that takes into account historical precedent.

The Mystery and Mythology of “Lefty Brains”

Language in the brain is lateralized, meaning that it is dominantly processed by a single hemisphere, most often the left. Handedness had previously been viewed as a ‘proxy’ for cerebral lateralization. However, it is a misconception that all left-handers are right-hemisphere-dominant, as both left- and right-handers have similar relatively low instances of right-hemisphere language lateralization.  

Misconceptions about left handedness are not sparse in the world of scientific literature. In his book On the Other Hand, Howard I. Kushner discusses how “lefties” have been historically associated with autism, schizophrenia, dyslexia, and learning disorders. He continues to explain the context of journal articles supporting these connections and their inability to be replicated. 

Mirror Neurons and Handedness

Mirror neurons are brain cells that fire while not only performing  an action, but also when observing an action. While one is completing a physical movement, brain cells called motor neurons fire in order to execute and process actions. For cells in the motor neuron system (MNS), these same neurons will also fire when watching another individual complete the same action.

This was discovered in the 1990s by researchers studying macaque monkeys. In the study scientists recorded activation in single neurons while having the monkey subjects grasp or observe the grasping of objects. Activation of mirror neurons was originally discovered in the premotor area f5, a brain area in monkey species. This region is a homolog, or similar structure, to Broca’s area, an often left-lateralized speech production area of the human brain.

Image of Broca’s area from Wikimedia. Licensed under CC-BY-SA-2.1-jp.

Because of the lateralization of Broca’s area, researchers are interested in the links between language, laterality, and mirror neurons. Psychologist Michael C. Corballis  hypothesized that mirror neurons were an essential component to the development of human language. He suggested in a journal article that the MNS led to advanced ability for gesturing communication.  Corballis argues that this advanced gesturing ability then led to further development of speech and language. Within this hypothesis, mirror neurons and handedness are related through developmental lines. Handedness and mirror neurons also intersect at their hypothesized implications for language and laterality. 

With digital methods, we can further elucidate the current understanding of handedness and the MNS in scientific literature.

Why Digital Methods?

The advent of powerful computing techniques for visualizing data and analyzing natural language has birthed a new approach to humanities, the Digital Humanities (DH). While there is decidedly no one definition for DH, an important motivation is the pursuit for new questions. 

Digital methods are often discussed in opposition to “close-reading,” the careful, contextual, concentrated work of a historian. Digital techniques, on the other hand, are efficient, devoid of context, and broad. But there’s no need for conflict; the two methods must work together. While there are plenty of problems with digital methods from OCR limitations to environmental and sociocultural impacts, their ability to sort through massive databases of literature is unprecedented. Although they are not immune to reinforcing existing social structures, they prove to be useful for discovering perspectives that couldn’t be found otherwise.

It’s clear that text mining, the automated extraction of data, finds practicality in the meta-analysis of scientific literature. There’s a lot of research out there, too much for anyone to sort through by hand. DH tackles this problem by indexing chunks of words over all retrievable works, allowing for a closer look with organization and visualization tools.

Our Approach

The questions we sought to investigate in undertaking this project were twofold.

  • What can digital methods reveal about the literature accessible through online databases on mirror neurons, handedness, and laterality?
  • What is the relationship between the use of the words “handedness” and “laterality” in this corpus of text?

We used the NCBI PubMed search engine to obtain sources for analysis. The site has a search builder that allows for queries that follow the rules of Boolean logic. Our query is shown below. We used Citavi to organize and download all results.

((mirror neuron) OR (mirror neurons)) AND ((laterality) OR (handedness) OR (hand preference) OR (non-dominant hand) OR (hand dominance) OR (asymmetry) OR (left hand) OR (right hand))

After sieving accessible, workable files and removing duplicates from the ~300 results, there were 124 papers which we converted to .txt files with ABBYY FineReader. This plaintext format allowed for us to use R code to extract and manipulate the data. The code can be found on the RStudio Cloud for free (under CC BY-NC-SA 4.0) here.

Screenshot of RStudio Cloud taken by the authors May 5, 2020. Licensed under CC BY-NC-SA 4.0.

The following digital methods were employed:

  • Extraction and conversion to Tidy format. This loads all of the documents and converts them to a neat bag of words.
  • Latent Dirichlet Allocation (LDA) topic modeling. This predicts what the key topics of the documents are given the bag of words. It does so by assigning every word to a potential topic with a certain probability. The mathematics is complicated, but there’s no need for one (ahem, us) to understand the inner workings, as the topicmodels package does that for us.
  • N-Gram analysis. This converts the bag of words into groups of n words. For example, a set of bigrams (2-grams) from the false sentence, “Handedness is laterality and laterality handedness,” is as follows: {“Handedness is”, “is laterality”, “laterality and”, “and laterality”, “laterality handedness”}.  This method is helpful for analyzing the context in which words are found, among other things.

Text Mining and Analysis

Image of LDA topic modeling results, generated by the authors in RStudio Cloud May 5, 2020. Licensed under CC BY-NC-SA 4.0.

Having extracted the data, a two-topic LDA topic modeling analysis was performed. The graphic above is a visualization of the topics and the key words associated with them. It’s not clear that the two are significantly different, although topic 1 (in red) seems to be more “sciency” and related to the literature on neural structures, while topic 2 (in blue) is more “embodied,” covering words that pertain to actions and body parts. It seems that the literature in question is influenced by the trend of embodied cognition, the idea that body and brain are inseparable. It’s also worth noting that “imitation,” an important concept for the mirror neuron/gestural theory of handedness, is a top word for topic 2.

While the LDA is not particularly revealing here, we include it as a taste of what digital methods are capable of. The reader should feel free to modify the project in RStudio Cloud to do topic modeling on their own corpus.

Image of ‘identical’ bigram frequency chart for the words “laterality” and “handedness”, generated by the authors in RStudio Cloud May 5, 2020. Licensed under CC BY-NC-SA 4.0.

We then performed the bigram context analysis. First, we collected the entire document into bigrams. We identified 455 bigrams that contained either “handedness” or “laterality”. 

We wanted to see if the words “handedness” and “laterality” were used in similar contexts, so we searched for ‘interchangeable’ bigrams. For example, since the bigram “handedness index” was found, we searched for all instances of “laterality index”. There were six such instances of either. 

Out of the 455, 56 (~12%!) of these bigrams had an identical pair for the other word. While this doesn’t say anything definite without a close reading, it suggests that the words “handedness” and “laterality” are used in similar contexts in the literature.

We concluded our analysis with a frequency analysis of the bigrams. We wanted to see how many unique bigrams contained a given word per source. Worth mentioning from our results is that the word “autism” was found in 305 unique-to-source bigrams over about half of the sources. In a similar vein, the word “schizophrenia” had 132 unique-to-source bigrams from a handful of sources.

What This Means

While digital methods almost never offer anything conclusive, they invite us to ask further questions. 

There is strong evidence that the words “handedness” and “laterality” are used interchangeably throughout our corpus. That could suggest that this shorthand in the literature is reifying the idea that all right-handers are left-brained, or that all left-handers are right-brained. We must conduct further context analysis by identifying the sources the relevant bigrams come from.

On another note, the results show that the literature discusses the possibility of a link between schizophrenia and mirror neurons. Researchers ought to be wary of this pursuit, as, if mirror neurons are assumed implicated in the origins of handedness, it could be another way of reinforcing stereotypes about left-handers. Similarly, the research proposing a link between autism and mirror neurons, based on the assumption that mirror neurons are a mechanism for empathy, should also be assessed critically.

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