AUDITORY CONNECTIONS AND NEUROCHEMISTRY OF THE SAGULUM

We studied the cytoarchitecture, neurochemical organization, and conne ctions of the sagulum. The goal was to clarify its role in midbrain, l ateral tegmental, and thalamic auditory processing. On cytoarchitecton ic grounds, ventrolateral (parvocellular) and dorsomedial (magnocellul ar) subdivisions were recognized. The patterns of immunostaining for g amma-aminobutyric acid (GABA) and glycine were distinct. Approximately 5-10% of the neurons were GABAergic, and more than one type was ident ified; GABAergic axon terminals were abundant in number and varied in form. Glycinergic neurons were much rarer, <1% of the population, and glycinergic axon terminals were correspondingly sparse. Wheat germ agg lutinin conjugated to horseradish peroxidase was used for purposes of connectional mapping, and biotinylated dextran amines revealed the str ucture of corticosagular axons. All nine cortical areas injected proje ct to the ipsilateral sagulum. Five (areas AI, AII, SF, EPD, and Te) h ad heavier projections than the others. Areas AI and AII projected thr oughout the rostrocaudal sagulum. Labeling from AI was moderate in den sity and concentrated in the central sagulum, whereas the input from A TI was heavier and ended more laterally. Suprasylvian fringe input was light, especially caudally, and was chiefly in the central sagulum. T he projection from the dorsal region of the posterior ectosylvian gyru s was comparatively stronger and was in the dorsolateral sagulum. Fina lly, the temporal cortex sent axons to the most lateral sagulum, spann ing the dorsoventral extent, whereas insular cortex axons ended diffus ely in the dorsolateral sagulum. Corticofugal axons ranged from fine b outons en passant to larger globular terminals. The sagulum may repres ent the earliest significant opportunity in the ascending auditory pat hway for corticofugal modulation. The most extensive input arises from the polymodal association areas. The sagulum then projects divergentl y to the dorsal cortex of the inferior colliculus and the dorsal divis ion of the medial geniculate body. The projection from the dorsal divi sion of the auditory thalamus to nonprimary auditory cortex completes this circuit between the forebrain and the midbrain and represents a n exus in the ascending and descending auditory systems. Such circuits c ould play a critical role in auditory-motor adjustments to sound. J. C omp. Neurol. 401:329-351, 1998. (C) 1998 Wiley-Liss, Inc.