word count: 250 Introduction word count: 1,309 Discussion word count: 2,372ABSTRACT Memory formation involves the synchronous firing of neurons in task-relevant networks, with recent models postulating that a decrease in low frequency oscillatory activity underlies successful memory encoding and retrieval. To date, this relationship has predominantly been investigated using objects (e.g., faces, natural scenes); however, considerably less is known about the oscillatory correlates of complex rule learning (e.g., language acquisition). Further, recent work has shown that aperiodic (non-oscillatory) 1/ƒ activity is functionally and behaviourally relevant, yet its interaction with oscillatory activity during complex rule learning remains virtually unknown. Using spectral decomposition and power-law exponent estimation of human EEG data, we show for the first time that 1/ƒ and oscillatory activity jointly influence the learning of different word order rules of a miniature language system. Fixed word order rules were associated with an increased power-law exponent (i.e. steeper 1/ƒ slope) compared to flexible word order rules. We also show that stronger anterior beta synchronisation predicts fixed word order rule learning and subsequent behavioural performance, while stronger theta/alpha synchronisation is associated with the learning of flexible word order rules. These results also revealed nonlinear differences between word order rules as a function of time and sensor space. Moreover, we demonstrated that inter-individual variations in spectral power across the learning task interacted with aperiodic activity to influence subsequent behavioural performance. Together, these results suggest that 1/ƒ activity plays an important role in higherorder cognition, including language processing, and that grammar learning is modulated by different word order permutations, which manifest in distinct oscillatory profiles.