Reciprocity of receptive field images and point images in the superior colliculus of the cat.

The largest receptive fields mapped during microelectrode penetrations through the upper strata of the cat's superior colliculus vary substantially in size and shape in different collicular regions. Previous work has shown that when these large fields are plotted in the retinotopic map of the colliculus, their profiles become elliptical and vary little in size and orientation over the central regions of the map. Such profiles are called the receptive field images (RFIs) in the visual coordinate system of the colliculus. Of particular interest here is the relationship of these RFIs to the region of the colliculus occupied by cell whose receptive fields include a common visual point. We call this region the point image in the colliculus. A straightforward geometric argument indicates that the point image in the upper collicular strata should have the same size, shape, and orientation as the RFIs of the large-field cells recorded here, if, indeed, these RFIs are translationally invariant as suggested by the earlier studies. The experiments reported here directly assessed the translational invariance of large-field RFIs in the superior colliculi of individual cats. The results support the previous inference that the profiles are oval with their long axes oriented mediolaterally. The mediolateral and anteroposterior dimensions of the profiles were about 2.7 and 2.0 mm, respectively, wherever they were measured in the central regions of the colliculus. Since the boundaries of the point images in the superficial strata must have similar shape and dimensions, these data indicate that a visual point is "seen" by collicular cells spread through a substantial fraction of the tissue receiving the projection of the contralateral visual field.