Motivation and Motor Control: Hemispheric Specialization for Approach Motivation Reverses with Handedness

BackgroundAccording to decades of research on affective motivation in the human brain, approach motivational states are supported primarily by the left hemisphere and avoidance states by the right hemisphere. The underlying cause of this specialization, however, has remained unknown. Here we conducted a first test of the Sword and Shield Hypothesis (SSH), according to which the hemispheric laterality of affective motivation depends on the laterality of motor control for the dominant hand (i.e., the “sword hand," used preferentially to perform approach actions) and the nondominant hand (i.e., the “shield hand," used preferentially to perform avoidance actions).
Methodology/Principal FindingsTo determine whether the laterality of approach motivation varies with handedness, we measured alpha-band power (an inverse index of neural activity) in right- and left-handers during resting-state electroencephalography and analyzed hemispheric alpha-power asymmetries as a function of the participants' trait approach motivational tendencies. Stronger approach motivation was associated with more left-hemisphere activity in right-handers, but with more right-hemisphere activity in left-handers.
ConclusionsThe hemispheric correlates of approach motivation reversed between right- and left-handers, consistent with the way they typically use their dominant and nondominant hands to perform approach and avoidance actions. In both right- and left-handers, approach motivation was lateralized to the same hemisphere that controls the dominant hand. This covariation between neural systems for action and emotion provides initial support for the SSH.

研究背景：数十年间针对人脑情感动机的研究已证实，趋近动机状态主要由大脑左半球支撑，回避动机状态则主要由右半球介导。但该半球特化现象的潜在成因迄今仍未明晰。本文首次对"剑盾假说（Sword and Shield Hypothesis, SSH）"开展检验。该假说指出，情感动机的半球偏侧性，取决于优势手（即"剑手"，优先用于执行趋近动作）与非优势手（即"盾手"，优先用于执行回避动作）的运动控制偏侧性。

方法与主要发现：为明确趋近动机的偏侧性是否随利手性发生改变，我们在静息态脑电图（electroencephalography, EEG）记录期间，测量了右利手与左利手被试的α波功率（alpha-band power，其与神经活动水平呈负相关），并基于被试的特质性趋近动机倾向，分析了半球间α功率不对称性的变化规律。结果显示，趋近动机越强，右利手被试的左半球神经活动越显著，而左利手被试则表现为右半球神经活动越显著。

研究结论：趋近动机的半球神经关联模式在右利手与左利手人群中呈现反转，这与两类人群分别使用优势手与非优势手执行趋近、回避动作的典型方式一致。无论右利手还是左利手，趋近动机的偏侧化半球均与控制优势手的大脑半球相匹配。这种动作与情绪的神经系统间的共变关系，为剑盾假说提供了初步的实验支持。