Cortical thickness gradients in structural hierarchies

Abstract

MRI, enabling in vivo analysis of cortical morphology, offers a powerful tool in the assessment of brain development and pathology. One of the most ubiquitous measures used—the thickness of the cortex—shows abnormalities in a number of diseases and conditions, but the functional and biological correlates of such alterations are unclear.If the functional connotations of structural MRI measures are to be understood, we must strive to clarify the relationship between measures such as cortical thickness and their cytoarchitectural determinants. We therefore sought to determine whether patterns of cortical thickness mirror a key motif of the cortex, specifically its structural hierarchical organisation. We delineated three sensory hierarchies (visual, somatosensory and auditory) in two species—macaque and human—and explored whether cortical thickness was correlated with specific cytoarchitectural characteristics. Importantly, we controlled for cortical folding which impacts upon thickness and may obscure regional differences.Our results suggest that an easily measurable macroscopic brain parameter, namely, cortical thickness, is systematically related to cytoarchitecture and to the structural hierarchical organisation of the cortex. We argue that the measurement of cortical thickness gradients may become an important way to develop our understanding of brain structure–function relationships. The identification of alterations in such gradients may complement the observation of regionally localised cortical thickness changes in our understanding of normal development and neuropsychiatric illnesses.

Large-Scale Gradients in Human Cortical Organization

“…For example, MRI measures that are sensitive to intracortical myelin have revealed a pattern of high myelin content in primary sensorimotor regions, which systematically decreases toward transmodal areas in parietal, temporal, and particularly prefrontal cortex [3,[25][26][27][28] (Figure 1A). A similar spatial distribution has been described for MRI-based measures of cortical thickness [12] and myelinated thickness [29]. Cortical thickness coarsely tracks changes in cytoarchitecture [30] and myelin content [25], and can be viewed as a pragmatic surrogate for cortical microstructure [12].…”

“…A similar spatial distribution has been described for MRI-based measures of cortical thickness [12] and myelinated thickness [29]. Cortical thickness coarsely tracks changes in cytoarchitecture [30] and myelin content [25], and can be viewed as a pragmatic surrogate for cortical microstructure [12].…”

“…Traditionally, areas are ordered based on their degree of microstructural differentiation, and the classification of their connections as feedforward or feedback [11]. The framework we advocate emphasizes that an area's position in the visual processing hierarchy -and thus many of its microstructural and connectional features -is strongly related to its distance from the primary visual area [12,13].…”

“…Traditionally, areas are ordered based on their degree of microstructural differentiation, and the classification of their connections as feedforward or feedback [11]. The framework we advocate emphasizes that an area's position in the visual processing hierarchy -and thus many of its microstructural and connectional features -is strongly related to its distance from the primary visual area [12,13].More generally, we propose that the spatial arrangement of areas along a global gradient between sensorimotor and transmodal regions is a key feature of human cortical organization. A gradient is an axis of variance in cortical features, along which areas fall in a spatially continuous order.…”

Language experience and the brain: variability, neuroplasticity, and bilingualism

“…thicker cortex) is somehow better functionally. To be sure, the neurobiological bases of current structural measures are being actively investigated (Vandekar et al, 2015;Wagstyl, Ronan, Goodyer, & Fletcher, 2015) and a strong consensus is absent. Nevertheless, it behoves us as (neuro)language researchers to think more deeply about how bilingualism could affect brain structure or brain activity (e.g.…”