Identification of the trochlear motoneurons by retrograde transport of horseradish peroxidase

A common technique for demonstrating projections in the brain is to electrically stimulate one part of the brain and record mass or field potentials from another part. We showed in the visual system of the cat, where connections between retina, lateral geniculate nucleus, and superior colliculus are very well known, that the recording of field potentials is not at all sufficient to demonstrate connections. The most prominent potential after electrical stimulation of the optic tract is the field potential created by the Y-ganglion cell fibers of the optic nerve. We recorded this potential in the optic nerve head of the eye, in the lateral geniculate nucleus, and in the superior colliculus. To our surprise, we also could record this potential 7 mm in front of the retina, with the electrode in the vitreous, and 5 mm above the lateral geniculate nucleus and the superior colliculus, where there are no direct inputs from the optic tract. These results show quite clearly that field potentials can “stray” much farther than the underlying anatomical structure projects.