Sulcal Depth in Lateral Prefrontal Cortex Predicts Working Memory in Childhood
Authors: Yao Jewelia , Voorhies Willa , Miller Jacob , Bunge Silvia , Weiner Kevin 
Published: April 2020

Sulci, the indentations in the brain, develop over childhood (Zilles et al., 1988; Alemán‐Gómez et al., 2013), and demonstrate individual differences linked to cognition (Cachia et al., 2018; Im et al., 2011). Previous neuroimaging work implicates prefrontal cortex (PFC) in working memory, a key facet of cognition. In particular, maintaining items in working memory (e.g., keeping a series of digits in mind) engages ventrolateral prefrontal cortex (VLPFC), whereas maintaining items in working memory (e.g., reversing a series of digits) engages both VLPFC and dorsolateral prefrontal cortex (DLPFC; Crone et. al, 2006). We propose to examine the relationship between sulcal depth in these regions and working memory for two tasks: Digit Span Forwards, which taxes maintenance, and Digit Span Backwards, which taxes manipulation and maintenance. Sulcal morphology of the PFC has been classically proposed to be functionally relevant to cognition that engages this region (Sanides, 1964). Thus, we hypothesized that there would be a relationship between sulcal depth and behavioral performance, whereby greater sulcal depth would predict better working memory. Cortical morphometric analyses were performed on high resolution T1‐weighted MPRAGE anatomical scans in 60 participants ages 6–18, from a previously published dataset (Wendelken et al., 2017). Cortical surface reconstructions were generated using Freesurfer. Sulci in lateral PFC were manually defined based on recently proposed definitions (Petrides & Pandya, 2012; Petrides, 2019), resulting in 2,157 labels (~18 labels per hemisphere, per subject). Mean sulcal depth was extracted for each sulcus. Relevant sulci for task performance were determined using a least absolute shrinkage and selection operator (LASSO) regression, fit separately for each hemisphere with each task condition using leave‐one‐out cross‐validation. Our analysis approach revealed four main findings. First, there is a relationship between PFC sulcal depth and Digit Span Backwards, but not Digit Span Forwards scores. Second, the depth of eight sulci (five in the left hemisphere and three in the right hemisphere) predict working memory manipulation. Third, while greater sulcal depth predicted better performance for six of these sulci, shallower sulcal depth predicted better performance for the other two sulci, which were restricted to the right hemisphere. Finally, these sulci are located around the contours of functionally‐defined DLPFC and VLPFC as proposed by Amiez and Petrides (2007). Taken together, these findings reveal that the depth of specific sulci in the lateral PFC can predict behavioral performance on a working memory manipulation task. Importantly, using a data‐driven approach we show that these sulci are also boundaries for functional regions known to be engaged in working memory manipulation. These results begin to shed light on the complex relationship between sulcal morphology in lateral PFC, functional parcellations of lateral PFC, and the development of working memory skills in children.