Topography of visual and somatosensory projections to mouse superior colliculus.

In adult mice of the C57BL/6J strain the projection of the visual field was systematically mapped under direct vision. As in other vertebrate species the nasal (anterior) field projected anterolaterally, and the inferior field posterolaterally. Values of magnification-1 (m-1, or degrees of visual field per millimeter tectal surface) were calculated over most of the tectum, for measurements in the coronal and sagittal planes. Whereas m-1 was fairly constant for measurement pairs in sagittal planes, for coronal planes there was a rather large, elongated, horizontally oriented area in the upper field of vision within which m-1 was smaller than elsewhere. In this area m-1 was anisotropic, with a ratio of almost 2:1 between sagittal and coronal planes. In a previously study we had observed that many cells recorded in deeper tectal layers responded to somatosensory stimulation, with whiskers especially conspicuous. In a given penetration perpendicular to the tectal surface, somatosensory receptive fields recorded in the deeper tectum were always concerned with that group of whiskers or with those parts of the body that crossed the regions of visual field represented in the superficial layers directly above. Given this information on the visual coordinates associated with certain somatosensory fields, the detailed mapping of the visual field onto the tectum made it possible to prepare a map of the somatosensory projection on the tectum. The resulting representation differed markedly from maps described for the classic somatosensory pathway. In the tectum the somatosensory map was dictated by the visual-field projection rather than by the peripheral tactile innervation density. Whiskers were thus featured much more prominently in the tectum, and structures close to the eye, such as the pinna and cheek, receive more representation than the tail or hindpaws.