Localizing the Frequency x Regularity word reading interaction in the cerebral cortex

The aim of this work is to combine behavioural and functional magnetic resonance imaging (fMRI) data to advance our knowledge of where the Frequency x Regularity interaction on word naming is located in the cerebral cortex. Participants named high and low frequency, regular and exception words in a behavioural lab (Experiment 1) and during an fMRI study (Experiment 2). We used the Additive Factors Method (AFM) to localize the expected overadditive Frequency x Regularity interaction both temporally, through word naming reaction times (whereby low frequency exceptions produce the longest reaction times), and spatially on the cortex, through hemodynamic response measures from fMRI (whereby low frequency exceptions produce the highest activation intensities). Activation maps revealed significant activation for low frequency exception words in the supplementary motor association cortex (SMA). We interpret the SMA activation as increased articulatory preparation, given previous demonstrations of the SMA's involvement in motor programming. Hemodynamic time courses were extracted from four regions of interest: the middle temporal gyri, SMA, insula and the inferior frontal gyri. Importantly, hemodynamic intensities within the SMA displayed an overadditive interaction pattern parallel to that found with naming reaction times. Thus, we provide an application of the AFM to fMRI intensity measures and evidence that the SMA is a potential cortical source of the Frequency x Regularity interaction during a basic naming paradigm. While the AFM has traditionally been used to localize factors in time we provide evidence that the AFM is useful in understanding how variables influence one another in the brain.