Many response properties in primary auditory cortex (AI) are segregated spatially and organized topographically as those in primary visual cortex. Intensive study has not revealed an intrinsic, anatomical organizing principle related to an AI functional topography. We used retrograde anatomic tracing and topographic physiologic mapping of acoustic response properties to reveal long-range (>1.5 mm) convergent intrinsic horizontal connections between AI subregions with similar bandwidth and characteristic frequency selectivity. This suggests a modular organization for processing spectral bandwidth in AI.corticocortical projections ͉ cortical modules L ittle is known about how intrinsic connections in cat primary auditory cortex (AI) are organized and what their functions are. These connections form a discontinuous and elongated spatial pattern (1-3) different from that in the primary visual cortex (4 -6). After artificial rerouting of visual thalamic afferents into AI, intrinsic connectivity takes on spatial and functional characteristics of primary visual cortex (VI) (7). However, the relationship between acoustic response properties and intracortical connectivity in AI is unknown. Studies of the spatial organization of frequency (2,3,8) and binaurality (8 -11) show that these features cannot account for the topography of AI horizontal connections. Injections of AI with anterograde tracers that involve supragranular cortical layers find many intrinsic terminal patches in the dorsoventral (isofrequency) axis and few that cross the caudorostral (cochleotopic) dimension of AI. The frequency preference of the patches was believed to match the injection site because most labeling followed the dorsoventral axis (2, 3). However, it has been suggested that the dorsal patches of labeling are tuned preferentially to higher frequencies (1); hence, the frequency alignment of intrinsic connections in AI is still unresolved. AI neurons also segregate according to binaural properties, and the spatial pattern of excitatory and inhibitory binaural bands covaries with bands of commissural terminal labeling (8 -11). However, the spatial arrangement of AI horizontal intrinsic connections differs from that of binaural bands (1).Features other than characteristic frequency (CF) and binaurality that are represented in AI include intensity dependence, excitatory bandwidth, preferred speed of frequency sweeps, and response latency (12)(13)(14)(15)(16)(17)(18)(19). Each feature has a slightly different topographic organization. However, most align dorsoventrally to a central cluster of neurons with narrow bandwidth (NB) tuning (19). This region crosses all frequencies and is flanked above and below (along the isofrequency axis) by clusters of broadbandwidth (BB) neurons (12,14,19). We mapped bandwidth topography (14) to target deposits of a tracer in the central NB region. We found multiple patches of retrograde labeling at consistent and predictable locations within AI. One, and perhaps all, of these patches contain neurons with spectra...