Anatomical tracing in animal models has shown that the thalamus projects to regions in the temporal lobe around primary auditory cortex, and many regions anterior to this. During development, post-mortem studies in humans have shown that myelination in the temporal lobes follows a posterior-to-anterior progression. Our aim was to investigate auditory development, and specifically the thalamo-temporal white matter connexions, in infants during the first postnatal year. We hypothesised that the connectivity will strengthen from birth through the first year, particularly in anterior temporal areas.
We recruited 4 healthy controls (1 month, 3 months, 9 months and 11 months). We assessed white matter tracts using diffusion-weighted MRI with probabilistic tractography. A highly accelerated multiband EPI sequence (monopolar acquisition, acceleration factor=4, iPAT=0) with 128 non-collinear diffusion weighting directions was acquired (TR/TE=1980/71, voxel size 2 × 2 × 2 mm3, b=1500 s/mm2).
We found an increment in the strength of connectivity from 1 to 11 months between the thalamus and cortex (thalamo-temporal 60% and temporo-thalamic 37%). Moreover, the pathway’s fractional anisotropy increased by 26% and its mean diffusivity dropped by 90%. Furthermore, the pattern of development followed a posterior-to-anterior progression, with an extension of connections to more anterior temporal regions at 9–11 months.
In conclusion, thalamic connections to the temporal lobe were found to strengthen in the first postnatal year, around primary auditory cortex, and especially in anterior temporal regions. The enhanced anterior temporal connectivity may reflect the development of the posterior-to-anterior cortical processing stream that in adults processes complex sounds such as language.