Mapping an intrinsic MR property of gray matter in auditory cortex of living humans: A possible marker for primary cortex and hemispheric differences

Abstract

Recently, magnetic resonance properties of cerebral gray matter have been spatially mapped--in vivo--over the cortical surface. In one of the first neuroscientific applications of this approach, this study explores what can be learned about auditory cortex in living humans by mapping longitudinal relaxation rate (R1), a property related to myelin content. Gray matter R1 (and thickness) showed repeatable trends, including the following: (1) Regions of high R1 were always found overlapping posteromedial Heschl's gyrus. They also sometimes occurred in planum temporale and never in other parts of the superior temporal lobe. We hypothesize that the high R1 overlapping Heschl's gyrus (which likely indicates dense gray matter myelination) reflects auditory koniocortex (i.e., primary cortex), a heavily myelinated area that shows comparable overlap with the gyrus. High R1 overlapping Heschl's gyrus was identified in every instance suggesting that R1 may ultimately provide a marker for koniocortex in individuals. Such a marker would be significant for auditory neuroimaging, which has no standard means (anatomic or physiologic) for localizing cortical areas in individual subjects. (2) Inter-hemispheric comparisons revealed greater R1 on the left on Heschl's gyrus, planum temporale, superior temporal gyrus and superior temporal sulcus. This asymmetry suggests greater gray matter myelination in left auditory cortex, which may be a substrate for the left hemisphere's specialized processing of speech, language, and rapid acoustic changes. These results indicate that in vivo R1 mapping can provide new insights into the structure of human cortical gray matter and its relation to function.

Effective and Structural Connectivity in the Human Auditory Cortex

“…3). This similarity in functional connectivity across the two variations in auditory cortex is not surprising given that the more rostral gyrus is considered to be the PAC (Sigalovsky et al, 2006), whereas the caudal gyrus can be considered an extension of planum temporale. DTPM showed that regions depicting effective connectivity were structurally connected (Figs.…”

“…Two of the eight subjects possessed a rostral and caudal Heschl's gyrus. In these cases, two VOI were made, one consisting of both gyri and one VOI consisting of only the rostral gyrus, which is considered to correspond to the true PAC (Sigalovsky et al, 2006). The caudal Heschl's gyrus is thought to be an extension of the planum temporale.…”

Early Maturation of the Linguistic Dorsal Pathway in Human Infants

“…In adults, Sigalovsky et al (2006) using the longitudinal relaxation time (R1) and Yoshiura et al (2000) using T2 images observed subtle signal differences between Heschl's gyrus and other superior temporal regions related to a higher myelin content and cell density in the auditory koniocortex. The well known heterogeneous darkening of T2 images along the first year of life and the decrease of the difference between the STS and the frontal areas noted here during the studied time period (see also Fig.…”

Studying cyto and myeloarchitecture of the human cortex at ultra-high field with quantitative imaging: R1, R2* and magnetic susceptibility

“…Various studies have shown a direct relationship between R1 and myelin content (Bot et al, 2004;Mottershead et al, 2003;Schmierer et al, 2004). Particularly in the human cortex, R 1 variations reflect water mobility (Bock et al, 2009;Geyer et al, 2011;Sigalovsky et al, 2006), yet the impact of iron content on R 1 tissue contrast should not be fully neglected (Rooney et al, 2007). This has now been clearly demonstrated in a recent ex-vivo study showing that it is beneficial to think of R 1 in grey and white matter to have a multivariate dependence on both myelin and iron concentration (Stüber et al, 2014).…”