A direct projection from the retina to the intermediate gray layer of the superior colliculus demonstrated by anterograde transport of horseradish peroxidase in monkey, cat and rat.

The anterograde transport of horseradish peroxidase (HRP) was used to re-examine the retinal projection to the superior colliculus in the monkey, cat and rat. By a somewhat novel application of the HRP in which the enzyme is deposited intravitreally in two or three sequential installments at 24 h intervals and by modifications that increase the sensitivity of the tetramethylbenzidine reaction procedure, we have successfully mapped the distribution of a significant number of retinal ganglion cell axons below the stratum opticum in the intermediate gray layer of the superior colliculus. Although the deep retinotectal axons project to the contralateral colliculus in all animals used, such axons can be followed as well, but in lesser numbers, to the ipsilateral intermediate gray layer in the cat and even more so in the monkey. The deep retinotectal axons here demonstrated may mediate the short latency responses of deep tectal neurons observed in earlier physiological studies and can no longer be considered as inconsequential to the visuo-oculomotor functions of the deep collicular layers.