Intrinsic connectivity of human auditory areas: a tracing study with DiI.

The human supratemporal plane contains the primary as well as several other auditory areas. We have investigated the intrinsic connectivity of these areas by means of antero- and retrograde labelling with the carbocyanin dye DiI (1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate). A total of 30 injections was placed in both hemispheres of four freshly fixed postmortem brains. Labelled neurons and axons were found in cortex around the injection. The retrograde labelling varied from faint to Golgi-like; most of the retrograde labelled neurons were layers II-III pyramids and only a few were nonpyramidal neurons. Labelled axons were dense in all layers near the injection site, while they became relatively rare in layer IV further away. The tangential spread of labelling differed among auditory areas. On Heschl's gyrus (corresponding to the primary auditory cortex and cytoarchitectonic areas TD and part of TB) intrinsic connectivity involved a relatively narrow part of cortex. They spread over larger parts of cortex in plana polare and temporale (areas TG, TA and the remaining part of TB). A number of injections also produced anisotropic labelling patterns. These results reveal differences in intrinsic connectivity between auditory areas. They suggest that intrinsic connections within the primary auditory area, area TD and part of TB that is on Heschl's gyrus, involve mainly nearby units or modules, probably with similar coding properties, whereas in surrounding areas, connections spread over more distant units and may play an important role in the integration of different auditory features.