Stephen J. Gotts ^1* Adrian W. Gilmore ² Alex Martin ¹

• ¹ Laboratory of Brain and Cognition, National Institute of Mental Health (NIH), Bethesda, MD, United States
• ² Department of Psychological and Brain Sciences, College of Arts and Sciences, University of Delaware, Newark, Delaware, United States

Understanding brain-behavior relationships is the core goal of cognitive neuroscience.However, these relationships -especially those related to complex cognitive and psychopathological behaviors -have recently been shown to suffer from very small effect sizes (0.1 or less), requiring potentially thousands of participants to yield robust findings. Here, we focus on a much more optimistic case utilizing task-based fMRI and a multi-echo acquisition with trial-level brain-behavior associations measured within participant. In a visual object identification task for which the behavioral measure is response time (RT), we show that while trial-level associations between BOLD and RT can similarly suffer from weak effect sizes, converting these associations to their corresponding group-level effects can yield robust peak effect sizes (Cohen's d = 1.0 or larger). Multi-echo denoising (Multi-Echo ICA or ME-ICA) yields larger effects than optimally combined multi-echo with no denoising, which is in turn an improvement over standard single-echo acquisition. While estimating these brain-behavior relationships benefits from the inclusion of a large number of trials per participant, even a modest number of trials (20-30 or more) yields robust group-level effect sizes, with replicable effects obtainable with relatively standard sample sizes (N = 20-30 participants per sample).