Connections of Cat Auditory Cortex: II.Commissural SystemCHARLES C. LEE*AND JEFFERY A. WINERDivision of Neurobiology, Department of Molecular and Cell Biology, University ofCalifornia at Berkeley, Berkeley, California 94720-3200ABSTRACTThe commissural projections between 13 areas of cat auditory cortex (AC) were studiedusing retrograde tracers. Areal and laminar origins were characterized as part of a largerstudy of thalamic input and cortical origins of projections to each area. Cholera toxin betasubunit (CT␤) and cholera toxin beta subunit gold-conjugate (CT␤G) were injected separatelywithin an area or in different areas in an experiment. The areas were identified indepen-dently with SMI-32, which revealed differences in neurofilament immunoreactivity in layersIII, V, and VI. Each area received convergent AC input from 3 to 6 (mean, 5) contralateralareas. Most of the projections (⬎75%) were homotopic and from topographically organized lociin the corresponding area. Heterotopic projections (⬎1 mm beyond the main homotopicprojection) constituted ⬇25% of the input. Layers III and V contained ⬎95% of the commis-sural neurons. Commissural projection neurons were clustered in all areas. Commissuraldivergence, assessed by double labeling, was less than 3% in each area. This sparse axonalbranching is consistent with the essentially homotopic connectivity of the commissuralsystem. The many heterotopic origins represent unexpected commissural influences converg-ing on an area. Areas more dorsal on the cortical convexity have commissural projectionsoriginating in layers III and V; more ventral areas favor layer III at the expense of layer V,to its near-total exclusion in some instances. Some areas have almost entirely layer III origins(temporal cortex and area AII), whereas others have a predominantly layer V input (anteriorauditory field) or dual contributions from layers III and V (the dorsal auditory zone). Atopographic distribution of commissural cells of origin is consistent with the order observedin thalamocortical and corticocortical projections, and which characterizes all extrinsic pro-jection systems (commissural, corticocortical, and thalamocortical) in all AC areas. Thus,laminar as well as areal differences in projection origin distinguish the auditory corticalcommissural system. J. Comp. Neurol. 507:1901–1919, 2008.© 2008 Wiley-Liss, Inc.Indexing terms: interhemispheric; divergence; convergence; SMI-32; laminar origins; areal originsThe commissural connections in sensory neocortex un-derlie the construction of unitary representations of spaceor the body from the independent peripheral contributionsto each hemisphere (Gazzaniga, 2000). These connectionsfollow unique rules of organization specific to each modal-ity. Thus, in the primary visual cortex only regions repre-senting the vertical meridian project commissurally(Hubel and Wiesel, 1967; Segraves and Rosenquist, 1982;Miller and Vogt, 1984; Abel et al., 2000), and in primarysomatic sensory cortex the corpus callosum links proximalbody representations preferentially, with distal extremi-ties receiving lesser projections (Jones and Powell, 1968;Wise and Jones, 1976; Rouiller et al., 1994). By contrast,in the primary (AI) auditory cortex (AC) commissuralconnections link tonotopically and binaurally matchedsubregions across the representational axis of character-istic frequency (Imig and Brugge, 1978; Ru¨ ttgers et al.,1990; Rouiller et al., 1991; Morel et al., 1993) in a clus-tered arrangement (Code and Winer, 1985) that appearsto support a modular organization, at least in primaryauditory cortex (AI) (Middlebrooks et al., 1980). Despitethe functional diversity of the 13 areas of auditory cortexGrant sponsor: National Institutes of Health; Grant number: R01DC2319-29.*Correspondence to: Charles C. Lee, Department of Neurobiology, Uni-versity of Chicago, 947 E. 58th St., MC0926, Chicago, IL 60637.E-mail: [email protected] 1 November 2006; Revised 13 November 2007; Accepted 19November 2007DOI 10.1002/cne.21614Published online in Wiley InterScience (www.interscience.wiley.com).THE JOURNAL OF COMPARATIVE NEUROLOGY 507:1901–1919 (2008)© 2008 WILEY-LISS, INC.(Winer, 1992), the patterns organizing the interhemi-spheric connections are largely unknown outside of theprimary fields. Thus, the commissural connections of non-tonotopic auditory areas, and their areal and laminarrelations relative to those in AI, are the principal subjectsof this study. A second goal was to apply measures oftopography to these projections to assess their degree oforder. Perhaps the commissural projections of nonprimarycortex are less ordered than those in the primary areas,given the virtual absence of a regular arrangement ofcharacteristic frequency in the nonprimary regions(Schreiner and Cynader, 1984; Ehret, 1997). A third ques-tion is the pattern of interareal divergence (degree ofbranching) among commissural neurons and, if such cellsexist, which fields are their targets.In the primary auditory areas commissural connectionsare topographic, clustered, and link homotopic regionspredominantly (Code and Winer, 1986; Rouiller et al.,1991). Other, sparser input arises from similar character-istic frequency locations in tonotopic fields other than AI(Imig and Brugge, 1978; Rouiller et al., 1991; Winer,1992), consistent with a highly parallel architecture andmodest convergence from heterotopic (tonotopically or to-pographically mismatched) sources (Lee et al., 2004b).Also at issue is whether the various nontonotopic fieldsare preferentially and reciprocally interconnected com-missurally, as they are in the monkey (Hackett et al.,1999).The laminar origins of these projections in each area arerelevant for commissural function. In AI these arise al-most entirely from layers III and V (Code and Winer,1985; Rouiller et al., 1991), but their sources elsewhereare unknown in the cat, even in the other primary areas.Since each layer is a potential source for information seg-regation, their laminar profiles can offer clues about arealsequences of processing.To clarify area-specific differences in AC commissuralconnections we investigated the contralateral cells of ori-gin projecting to 13 auditory areas in 25 experiments inwhich two sensitive retrograde tracers were injected ei-ther within an area or in different areas (Fig. 1B: inset).The first type of experiment assessed within-area variabil-ity, the second directly compared