Data extraction and analysis of the systematic search on gender differences on brain MRI structures and connectivity

Magnetic resonance imaging (MRI) is an invaluable imaging technique used in medicine, generating images of structures inside the body. The images produced are used to examine almost any part of the body and are essential in diagnosing conditions, planning treatments and assessing treatment effectivity. The development of MRI has allowed for great advancements in health and biomedical research. In particular, it has granted us better opportunities to expand our knowledge of the intricate workings of the human brain, relating to its structure and function. Of particular interest both in neuroscience and society, is how structures and connectivity within the brain might differ between men and women, and whether these differences contribute to the differences demonstrated by men and women in their personality traits, behaviors and emotions. Previous MRI-based studies consistently report sex differences in overall brain size. Indeed, it is widely acknowledged that male brains are approximately 9-12% larger compared to female brains. However, published literature addressing whether differences exist in more defined sub-regions of the brain is highly inconclusive and recent research efforts by Joel et al. (2015) have challenged the broadly conceived idea that human brains are distinctly ‘male’ or ‘female’. Many factors can contribute to the reporting of inconsistent findings across studies, including variations in study design, duplicate reporting of results in more than one publication and selective reporting of favorable findings. The goal of a systematic review is to address a specific research question and provide a thorough summary of the existing literature, through the collection and integration of multiple research studies. The selection of these studies for the review is based upon a structured methodological search, thereby preventing any bias in study selection. This review aimed to identify whether structural and connectivity differences exist between the brains of men and women based upon magnetic resonance imaging (MRI) data from existing literature, and in which regions these differences exist. After collating the data from 171 publications that were selected for inclusion, this review found that men and women demonstrate significant structural differences in the regions of the Broca area, cerebellum and in features related with disease throughout the brain. However, several factors including the number and age of subjects in each individual study as well as the MRI-imaging techniques and processing carried out heavily influence these findings. These results highlight the need for future neuroscientific research to account for the differences between men and women in regional brain areas. In addition, studies comprising large numbers of individuals at different stages across the lifespan and in which imaging methods are consistent and reproducible are required to allow more precise conclusions regarding specific sex differences that exist in the brain, where they exist and how they develop over time. Grasping a better understanding of how sex influences the brain will aid in efforts to identify the underlying mechanisms of the differences in the behavior of men and women, as well as in the differences between men and women in vulnerability to certain brain-related disorders.

磁共振成像（Magnetic Resonance Imaging, MRI）是医学领域极具价值的成像技术，可生成人体内部结构的影像。其所生成的图像可用于检查人体几乎所有部位，在疾病诊断、治疗方案制定以及疗效评估中发挥着不可或缺的作用。磁共振成像技术的发展推动了健康与生物医学研究的重大进步，尤其为我们深入了解人类大脑复杂的结构与功能运作机制提供了绝佳契机。
在神经科学与社会大众的共同关注下，一个关键议题愈发受到重视：大脑内部的结构与连接模式在男性与女性之间是否存在差异，以及这些差异是否会影响男女在人格特质、行为表现与情绪层面的差异。既往基于MRI开展的研究始终报告大脑整体尺寸存在性别差异：学界已普遍公认，男性大脑体积较女性大9%-12%。然而，针对大脑更细分亚区是否存在此类差异的相关研究结论却极不统一，且Joel等人2015年的最新研究对“人类大脑可明确划分为‘男性型’或‘女性型’”这一主流观点提出了挑战。
导致不同研究结论不一致的影响因素众多，包括研究设计差异、多篇文献重复报道同一结果，以及仅选择性报告阳性发现等。系统综述的目标在于明确特定研究问题，并通过收集整合多项相关研究，对现有文献进行全面系统的总结。该综述的文献筛选过程依托结构化的方法学检索完成，从而避免了文献遴选环节的偏倚。
本综述旨在基于现有文献中的磁共振成像数据，探究男性与女性大脑在结构与连接模式上是否存在差异，以及差异具体存在于哪些脑区。在纳入171篇已筛选的文献并整合其数据后，本综述发现：男性与女性在布罗卡区（Broca area）、小脑以及全脑与疾病相关的特征层面，均存在显著的结构差异。然而，各项研究的样本量、受试者年龄、MRI成像技术以及图像处理流程等诸多因素，均会对研究结果产生显著影响。
上述研究结果提示，未来的神经科学研究需充分考虑脑区层面的性别差异。此外，还需开展纳入大量不同生命阶段受试者、成像方法统一且可重复的研究，才能针对大脑中存在的特定性别差异、差异的具体位置，以及其随时间的演变规律得出更精准的结论。深入理解性别对大脑的影响机制，将有助于我们进一步探明男女行为差异的潜在根源，以及二者在特定脑部疾病易感性方面存在差异的原因。