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Sex Differences Are No Afterthought

Authors:  Jolande Fooken, Xiuyun Wu, & Doris Chow

In recent years more scientists have advocated an increased focus on the role of sex and gender differences in neuroscience research. Specifically, it is important to study sex-related mechanisms in the brain and how they affect sensory processing and motor behaviour. 

Considering that historically most research included only male participants, research standards are trending in the right direction. Today, researchers are strongly encouraged to take sex and gender into consideration for their research. However, is it enough to simply collect the same number of male and female participants or should we think about sex as a scientifically relevant and meaningful variable?

Missed Opportunities in Present Research

A recent study by Mathew and colleagues investigated the effect of sex as a biological variable on hand-eye coordination and processing by the brain, led us to reconsider how sensory and motor neuroscience currently take sex differences into account.[1] It’s true that researchers pay greater attention to sex and gender balance in their research than in the past. However, we still know very little about how visuomotor function might vary between people of different sex and gender. 

Driven by that very question, Mathew and colleagues decided to analyze how the ability to manually track a moving target varies between self-reported male and females.[1] Whereas the general research question could fill a large gap in our current literature, the authors missed their chance to make a significant contribution and advance our knowledge about the importance of sex as a biological variable.

The study, entitled “Sex Differences in Visuomotor Tracking”, was published in July 2020 in Scientific Reports and could potentially reach a high impact in the field. Yet, this study has several gaps, which we will later discuss,  that are not sufficiently addressed in the paper. Therefore, the conclusions drawn should raise red flags as they may mislead some readers and future research. In the following, we will comment on three major points that scientists—interested in sex differences in behavioural neuroscience—should take into account

1. Sex differences are no afterthought

We are living in a time where there are vast amounts of data being used for scientific research. Often scientists publish a subset of the full dataset that was collected, reporting only those measures that address the current research question. At the same time, there may be additional information about the participants, such as age, handedness, or biological sex not included in the analysis. Therefore, it is very tempting to later re-analyze the data to look for differences across various variables. Such differences are exactly what Mathew and colleagues reported.

The authors analyzed data from a manual-tracking baseline task that usually preceded the main experiment in their lab. Manual tracking is a common task in sensorimotor research, in which participants move a joystick to align a visual cursor with a moving target. Typically, experiments aim to investigate how participants adapt their visuomotor control to changes in the visual scene. In the study by Mathew et al., the baseline task required participants to track the moving target as closely and accurately as possible. The authors observed differences between female and male participants: females tracked the unpredictably moving target with a larger time lag than their male counterparts.

These findings should spark interest in any curious scientist. However, there is also a problem: the authors did not have an a priori hypothesis (reasoning based on inference before the study, rather than evidence) about the role of biological sex with respect to visuomotor tracking. Instead, they performed their analyses after the fact. Accordingly, several factors that may influence the role of sex and gender on the results were not controlled for. 

One factor that may have skewed the results of this study is video-gaming, an activity more common among males.[2, 3] Video game experience most likely influences participants’ ability to accurately use a joystick to track a moving target. Additionally, general personality traits could explain the observed individual differences in manual tracking behaviour. For example, risk-averse individuals may track the target with a higher time lag to be able to adjust their hand movements more precisely to sudden changes of target motion. All of this is of course speculative. The study was not designed to study sex differences and therefore cannot answer if visuomotor tracking truly differs between males and females.

2. The battle of the sexes: what is a male advantage?

Some readers may cringe when they read that Mathew and colleagues wrote that they found, “a clear male advantage in hand tracking accuracy”. The study shows differences, yes, but an advantage? Tracking a target as fast as possible does not necessarily translate to advantages in everyday life in which timing is often relevant. 

Imagine getting into a car with someone who is trying to follow the car in front as closely as possible—keeping a larger distance may in fact be the safer option. Driving and many other naturalistic scenarios may require us to choose a tracking behaviour that differs from a pure distance minimization. To study optimal visuomotor behaviour we need to first define normative models that allow us to evaluate a decision or judgement.[4] What is the goal of the actor? What does it mean to be successful in a task? What is the error we should aim to minimize?

In motor control, there may be some tasks in which action accuracy is easy to judge. For example, how well a person can throw a ball can be measured in the distance the ball has travelled. However, when we change the definition of success, we also change our measure of accuracy. For daily activities, it may be relatively easy to judge success, but in basic sensorimotor research, it is less clear. 

Thus, optimal performance during manual tracking can be defined in different ways: it may be optimal to track a moving target as closely as possible or to track it as smoothly as possible and thereby reduce overall movement cost. Defining optimal task performance is key to understanding individual strategies when solving any given task. Only then can we draw conclusions about so-called advantages.

3. Males are different from females, now what?

For a moment let’s assume that, despite all the weaknesses in the study by Mathew et al., that we have pointed out, that there is a difference between male and female participants during visuomotor tracking. What does that mean? 

First, we should aim to investigate where such a difference originates. Mathew and colleagues conclude that males may rely on “faster decisional processes dynamically linking visual information of the target with forthcoming hand actions”.[1] However, some of these proposed processes, such as visual processing and early visual brain regions have relatively fixed time scales.

So the question that arises is at what stage of the decision process do differences occur? Does the transformation from visual information into motor commands take longer in female participants? Why? Does this difference change with experience or training? Or are there other factors, such as knowledge about motor uncertainty, that influence visuomotor tracking? Again, we simply don’t know yet and we need more carefully designed research to find these answers.

Another important question that we should ask ourselves is what the implications of the observed differences are. For example, visuomotor control has become very important in the medical field, where robotic devices enable complicated brain surgeries. Are such devices tailored to an average operator? Should we design different devices for female neurosurgeons as compared to males? Is there an actual physiological difference between males and females or are we just measuring differences along a spectrum of individual performance variability? At this time we do not have concrete answers to these questions. So, until we have better answers to the questions of how these differences arise, it may be harmful to make sweeping statements of sex-based advantages. 

We have the opportunity to study sex differences in sensory and motor neuroscience in a meaningful and thorough way. Let’s not report sex differences as an afterthought, but instead study biological sex as an interesting and important factor at the centre of our research agenda.   

REFERENCES

1. Mathew, J., Masson, G. S., & Danion, F. R. (2020). Sex differences in visuomotor tracking. Scientific reports, 10(1), 1-12. https://doi.org/10.1038/s41598-020-68069-0

2. Terlecki, M.S., Newcombe, N.S. (2005). How Important Is the Digital Divide? The Relation of Computer and Videogame Usage to Gender Differences in Mental Rotation Ability. Sex Roles, 53, 433–441. https://doi.org/10.1007/s11199-005-6765-0

3. Quaiser-Pohl, C., Geiser, C., & Lehmann, W. (2006). The relationship between computer-game preference, gender, and mental-rotation ability. Personality and Individual Differences, 40(3), 609-619. https://doi.org/10.1016/j.paid.2005.07.015

4. Körding, K. (2007). Decision theory: what" should" the nervous system do?. Science, 318(5850), 606-610. https://doi.org/10.1126/science.1142998

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