A morphometric analysis of auditory brain regions in congenitally deaf adults

Abstract

We investigated whether variation in auditory experience in humans during development alters the macroscopic neuroanatomy of primary or auditory association cortices. Volumetric analyses were based on MRI data from 25 congenitally deaf subjects and 25 hearing subjects, all right-handed. The groups were matched for gender and age. Gray and white matter volumes were determined for the temporal lobe, superior temporal gyrus, Heschl's gyrus (HG), and the planum temporale. Deaf and hearing subjects did not differ in the total volume or the gray matter volume of HG, which suggests that auditory deafferentation does not lead to cell loss within primary auditory cortex in humans. However, deaf subjects had significantly larger gray matter-white matter ratios than hearing subjects in HG, with deaf subjects exhibiting significantly less white matter in both left and right HG. Deaf subjects also had higher gray matter-white matter ratios in the rest of the superior temporal gyrus, but this pattern was not observed for the temporal lobe as a whole. These findings suggest that auditory deprivation from birth results in less myelination and͞or fewer fibers projecting to and from auditory cortices. Finally, the volumes of planum temporale and HG were significantly larger in the left hemisphere for both groups, suggesting that leftward asymmetries within ''auditory'' cortices do not arise from experience with auditory processing.T he study of congenitally deaf adults provides a unique opportunity to investigate potential changes in neural organization and structure resulting from sensory deprivation. Animal studies have shown that congenital deafness produces degenerative changes in the central auditory pathway (1, 2). Degeneration in the central auditory system subsequent to profound hearing loss has also been reported in humans. For example, Moore and colleagues (3) observed cell size reductions in the cochlear nucleus of profoundly deaf adults. However, it is unclear whether auditory deprivation from birth results in degeneration of primary auditory cortex in either animals or humans. The pattern of subcortical projections to primary auditory cortex in congenitally deaf cats is similar to that of normally hearing cats (4, 5), suggesting that cortical auditory regions may continue to receive input from subcortical regions and might not exhibit degeneration. However, functional deficits are observed in synaptic activity and organization within auditory cortex (6), suggesting the possibility of variation in the structure of auditory cortex as a consequence of congenital deafness.We investigated whether congenital and profound hearing loss in humans results in reduced volume and͞or altered morphology of cortical brain regions involved in auditory processing. Specifically, we investigated whether lack of auditory input from birth affects gray matter (GM) and white matter (WM) volumes within primary auditory cortex [defined as the transverse gyrus of Heschl (7)], within auditory association cortex in the planum temporale (PT), or ...

Human Primary Auditory Cortex Follows the Shape of Heschl's Gyrus

“…This distinction affects a broad literature that uses anatomical criteria to estimate the size of human PAC (and the adjacent planum temporale) in relation to brain laterality, language and music abilities, and auditory-related pathologies, including dyslexia, autism, and schizophrenia (for review, see Abdul-Kareem and Sluming, 2008). By current convention, only the anterior division of duplicated HGs is included in PAC measurements (Rademacher et al et al, 1993;Penhune et al, 1996;Leonard et al, 2001;Schneider et al, 2002;Emmorey et al, 2003;Wong et al, 2008;Gage et al, 2009;Schneider et al, 2009;Warrier et al, 2009;Hubl et al, 2010) with the posterior division assigned instead to the planum temporale (Dorsaint-Pierre et al, 2006). The criteria proposed by Penhune et al (1996) (to consider only the anterior division as part of PAC if there is an SI extending half the length of HG) would wrongly exclude the posterior division of PAC (the entire hA1 subfield) in 7 out of 20 of our cases.…”

Do deaf individuals see better?

“…The case of the primary auditory cortex in the Deaf remains puzzling. The primary auditory cortex sustains its size in Deaf individuals -and thus remains available for functional recruitment [46,47]. Several studies indicate a peak of reorganization in the area caudal to the primary auditory cortex, with activation from that peak spreading, in some studies, to the posterior part of the primary auditory cortex [24,37,41,48].…”

Differential activity in Heschl's gyrus between deaf and hearing individuals is due to auditory deprivation rather than language modality

“…Morphometric differences in the white matter/grey matter ratio between deaf and hearing individuals in HG (Emmorey et al, 2003; but see Lyness et al, 2014), provide an anatomical correlate for potential visual crossmodal plasticity.…”