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To determine the dendritic fields, mitral, displaced mitral, middle tufted, and granule cells in the rabbit olfactory bulb were stained by intracellular injection of HRP. The secondary dendrites of mitral cells were distributed mostly in the inner half of the external plexiform layer (EPL). Those of displaced mitral cells extended mainly into the middle and superficial sublayers in the EPL. The secondary dendrites of middle tufted cells were distributed mostly in the superficial portion of the EPL. Mitral cells extended their secondary dendrites in virtually all directions within a plane tangential to the mitral cell layer (MCL) and thus had a disklike projection field with a radius of about 850 microns. Displaced mitral cells had similar dendritic projection fields in the tangential plane but with somewhat distorted shapes. The secondary dendrites of middle tufted cells had a tendency to extend in particular directions. From the projection pattern of the gemmules on the peripheral processes, granule cells were classified into three types. Type I granule cells had gemmules both in the superficial and in the deep sublayers of the EPL. The peripheral processes of Type II granule cells were confined to the deep half of the EPL. The gemmules of Type III granule cells ere distributed in the superficial half of the EPL. The differing dendritic ramification among mitral, displaced mitral, and middle tufted cells suggests the separation of the dendrodendritic synaptic interactions with granule cells in different sublayers in the EPL. It also suggests a functional separation of the sublayers of the EPL.

The present study examined auditory cortical neurons, the responses of which depended on the duration of noise bursts. We recorded from 150 neurons with response latencies exceeding 30 msec and from 28 neurons with OFF responses to auditory stimuli in the dorsal zone of cat auditory cortex. Of 150 long-latency neurons, 132 displayed some form of duration selectivity. Seventy-eight were classified as selective for long durations. Among the long-duration-selective neurons, 30 responded only to noise burst stimuli with durations longer than a minimal threshold and were classified further as duration threshold neurons. Of 132 duration-selective neurons, 41 responded selectively to noise bursts of short duration; 13 showed maximal responses to noise bursts of a particular duration and could be regarded as duration-tuned neurons. OFF-response neurons included ones that were long-duration-selective, duration-tuned, and nonduration-selective. Duration tuning has been described previously only at the midbrain level in amphibians and bats. The present finding of sensitivity to sound duration in at least one region of cat auditory cortex indicates that this form of neural tuning may be important for hearing in all vertebrates, and for processing of sound at multiple levels in the auditory pathway. The duration tuning in the cat auditory cortex was much broader, and the best duration was distributed over a wider range than in the bat inferior colliculus. We suggest that the duration selectivity of the long-latency neurons results from integration along the time domain of a stimulus during the latent period.

Two types of terminations were observed on corticothalamic fibers arising from cells in different layers of cat auditory cortex. Injections of the anterograde tracers Phaseolus vulgaris leucoagglutinin (PHA-L) or biocytin were made into single cortical loci that included both superficial layer 5 (5a) and layer 6 in the primary auditory cortex (AI). These resulted in labeling of terminal fibers with small (approximately 1 micron) and large (approximately 2 microns) boutons in the medial geniculate complex (MG) and the lateral nucleus of the posterior complex. Large boutons were found in the deep and superficial dorsal nuclei, in the ventrolateral nucleus, and, less frequently, in the medial nucleus of the MG. They usually ended in grape-like clusters of boutons. By contrast, small boutons were found densely in the pars lateralis and pars ovoidea of the ventral nucleus, and to a lesser extent in the medial nucleus of MG. In the anterior third of the ventral nucleus, where the highest density of labeled fibers was observed, the small bouton terminations formed a plate-like plexus. In the inferior colliculus (IC), most terminal boutons on labeled corticotectal fibers were of large size. To reveal the cells of origin of the axons ending in the two different types of corticothalamic terminations, biocytin injections were localized in either layer 5 or layer 6 of AI or PHA-L injections were made into middle layers, including layer 5a but excluding layer 6. Virtually all labeled terminals found in the MG after layer 5 injections were of large size, while those found after layer 6 injections were of small size. The distribution of terminals of single-labeled axons was extensive and variable. For example, an axon recovered after a layer 5 injection of biocytin ended in at least seven patches of clusters of large boutons along much of the anteroposterior axis of MG. Our previous findings showed two neuronal populations situated in superficial layer 5 and in layer 6 of AI and projecting to the thalamus. The axons of these cells had different patterns of collateral distributions in the cortex. The present study shows that the extrinsically projecting axons of these two populations also have different terminal morphologies and distribution patterns in the MG. The findings suggest that the corticothalamic pathway in the cat auditory system consists of at least two feedback projections originating from different cortical layers that exert different influences on distinct thalamic neuronal populations.

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