The left map/right max hypothesis of the role of IFG in probabilistic learning

Human brains are great at detecting and making use of reliable patterns in their surroundings. The inferior frontal gyrus (IFG) is a brain structure that is crucial for this probabilistic learning. Previous studies also suggest that the IFG is uniquely divided in function between the left and right hemisphere. While the left IFG is greatly associated with language processing (Hagoort, 2013; Uddén et al., 2017), the right has been more linked to the ability to stop motor responses (Hampshire et al., 2010; Hartwigsen et al., 2019). However, we do not believe the function of the left and right IFG to be specific to language and motor function, respectively. Instead, we hypothesize that the difference is actually in the nature of the probabilistic information the areas are sensitive to in probabilistic learning tasks. By probabilistic learning, we mean learning a system of various possible transitions between sound-cued key presses with a set of allowed transitions with different probabilistic properties. In other words, the rules that determines which sounds, and associated key press, can follow each other and with what probability the allowed transitions occur. We thus propose a novel left map/right max hypothesis of the role of the IFG in probabilistic learning. It states that the right IFG maximizes detection of the most probable option while the left IFG instead maps the full probability distribution in a probabilistic learning task. This study will use a speech sound cued key press sequence learning task, an auditory version of the one used in (Sidenius et al., 2025), that combines the probabilistic sequence from language research, reaction-time (RT) based continuous measure of learning from motor research and novel generation tasks to test the left map/right max hypothesis. The functional importance of the left and right IFG will be tested by applying continuous theta burst stimulation (cTBS) targeting either hemisphere in separate groups, with a sham stimulation control group, a stimulation technique that can temporarily affect decrease the excitability of the cerebral cortex for up to 60 minutes (Chung et al., 2016). This study will provide causal evidence for the functional role of the left and right IFG in probabilistic learning. This will elucidate common learning mechanisms relevant for language, music, and complex motor skills.