The non-lemniscal auditory cortex in ferrets: convergence of corticotectal inputs in the superior colliculus

Abstract

Descending cortical inputs to the superior colliculus (SC) contribute to the unisensory response properties of the neurons found there and are critical for multisensory integration. However, little is known about the relative contribution of different auditory cortical areas to this projection or the distribution of their terminals in the SC. We characterized this projection in the ferret by injecting tracers in the SC and auditory cortex. Large pyramidal neurons were labeled in layer V of different parts of the ectosylvian gyrus after tracer injections in the SC. Those cells were most numerous in the anterior ectosylvian gyrus (AEG), and particularly in the anterior ventral field, which receives both auditory and visual inputs. Labeling was also found in the posterior ectosylvian gyrus (PEG), predominantly in the tonotopically organized posterior suprasylvian field. Profuse anterograde labeling was present in the SC following tracer injections at the site of acoustically responsive neurons in the AEG or PEG, with terminal fields being both more prominent and clustered for inputs originating from the AEG. Terminals from both cortical areas were located throughout the intermediate and deep layers, but were most concentrated in the posterior half of the SC, where peripheral stimulus locations are represented. No inputs were identified from primary auditory cortical areas, although some labeling was found in the surrounding sulci. Our findings suggest that higher level auditory cortical areas, including those involved in multisensory processing, may modulate SC function via their projections into its deeper layers.

Keywords: auditory cortical fields; corticofugal input; multisensory integration; neural tracers; orientation behavior; sound localization.

Figure 1

The experimental design. Neural tracer injections in the superior colliculus (SC) and in the auditory cortex were made to label corticotectal cells retrogradely and their terminals in the SC anterogradely. (A) Dorsal view of the SC after the cortical hemispheres have been removed, showing the location of the tracer injections. Dotted lines and small C and D letters indicate the anteroposterior level of the coronal sections shown in panels (C) and (D), respectively, in which the laminar organization of the SC can be observed. (B) Lateral view at the level of the ectosylvian gyrus taken from a flattened, tangential section stained with SMI₃₂ monoclonal antibody. Tracer injections were placed in the different auditory fields (see Table 1), as indicated. The limits between the different layers in the SC were identified primarily on the basis of acetycholinesterase staining (C) and SMI₃₂ immunocytochemistry (D). Calibration bars are 2 mm in (A) and (B) and 0.5 mm in (C) and (D).

Figure 2

Distribution of retrogradely labeled cells in the auditory cortex after rhodamine (FR) injection in the superior colliculus. (A) Coronal section at the level of the left SC showing the injection site in the deep SC (SGP). (B) 3D reconstruction of auditory cortex showing the location of retrogradely labeled neurons. (C) Photomicrograph of a flattened, tangential section at the level of the ipsilateral auditory cortex. The retrogradely labeled cells are numerous, especially in the anterior part of the ectosylvian gyrus (AEG). Calibration bars are 1 mm in (A) and 2 mm in (B) and (C).

Figure 3

Morphology of retrogradely labeled corticotectal neurons in layer V after rhodamine (FR) injection in the superior colliculus. (A,C) Coronal sections at the level of the anterior and middle part of the ectosylvian gyrus, respectively, showing labeled neurons that were mainly located in the AEG (A), the deep part of the suprasylvian sulcus and the dorsal part of the PEG (arrows) (C). (B) Higher-magnification photomicrograph of a Nissl-counter-stained section at the level of the AEG; sets of two to three labeled neurons can be observed in layer V (asterisks). (D) Higher-magnification photomicrograph showing the pyramidal morphology of a labeled neuron in layer V, whose thick apical dendrite runs orthogonal to the cortical layers. This photomicrograph was taken at the location shown by the frame in (C). Calibration bars are 1 mm in (A) and (C), 0.5 mm in (B) and 0.1 mm in (D).

Figure 4

Examples of two cases with BDA tracer injections in the auditory cortex. (A,B) Photomicrographs showing the location of the injection sites in AVF (A, flattened tangential section) and PEG (B, coronal section). (C,D) Drawings of coronal sections at the level of the SC with each dot representing a terminal. Gray and white layers were combined for the intermediate (stratum intermediale, SI) and deep (stratum profundum, SP) SC. The number of the section in each drawing indicates its anteroposterior position, with zero indicating the most posterior corner of the SC. Calibration bars are 2 mm for (A) and 1 mm for (B–D).

Figure 5

Quantification of the corticotectal inputs to the SC. Six cases with injection sites in the AEG (n = 3) and PEG (n = 3) were used for quantification. (A) The normalized density of terminals (see Materials and Methods for details) is plotted in different layers of the left and right SC. The projection is predominantly uncrossed with a very minor contralateral component. No differences between the intermediate and deep layers of the SC (SGI and SGP) were found. The density of terminals was higher when input came from the AEG than from the PEG, but this difference was not significant (ANOVA, F_(7,23) = 1.01, p = 0.46). For each individual case, the clustering index is plotted against dispersion (measured as the ratio between the area of labeling in the SC and the area of the injection site in the cortex). These measures show that terminals from AEG (B) tend to be more clustered and less dispersed than terminals from PEG (C). Dotted lines in (B) and (C) represent the mean and the squares represent three times the standard deviation of the mean; terminals in SGI are shown in blue and terminals in SGP in green.

Figure 6

Distribution of cortical terminals in the SC. The proportion of terminals is plotted after dividing the SC into quadrants. The highest proportion was found in the posterior half of the SC, especially in the medial quadrant (ANOVA, F_(3,3) = 9.865, p = 0.001), and no differences in this distribution were found with injection site location in the cortex (ANOVA, F_(1,3) = 0.008, p = 0.93).

Figure 7

Morphology of terminals in the superior colliculus. Camera lucida drawings (A,D) and photomicrographs (B,E) of terminal fields in the SC after tracer injections in the AEG (A and B, case F0505) and in the PEG (D and E, case 0536). Locations of the terminal fields in the SC are shown in the lower magnification drawings. Some end-terminals are labeled with asterisks and en passant terminals with arrows. Calibration bars are 20 μm. (C) Histogram showing the percentage of the different types of terminals in the SC. En passant terminals were significantly more numerous (**p < 0.001) than the end-terminals, irrespective of the location of the injection sites in the cortex.