Identifying the injection site

M. Hofmann presents a photomicrograph apart from the one in Fig. 1A of my article (Schmidt, Reference Schmidt2020), and alleges that the true injection site is located in the tegmentum, but he does not present a series of brain sections with heavily labeled structures in the nearby brain region including the brain section in my article (Hofmann, Reference Hofmann2021: see Section “Alleged ‘nucleus corticalis injection’”). I did that now with a series of 10 semi-sections (Fig. 1; see also Supplementary material). They are arranged from rostral (Fig. 1A) to caudal (Fig. 1J). M. Hofmann used the track of the glass micropipette as evidence to justify his allegation. It is indicated in Sections 4–8 (Fig. 1D–1H; arrowheads). Section 4 (Fig. 1D) includes Fig. 1A of Schmidt (Reference Schmidt2020) and Section 8 (Fig. 1H) includes Fig. 1A of Hofmann (Reference Hofmann2021). The injection site in Fig. 1A of Hofmann (Reference Hofmann2021) may look like a single entity, but in fact, it consists of two parts, a dorsal and a ventral part of labeled fiber bundles, and therefore, it cannot be the injection site (Fig. 1K and 1L). These two fiber bundles can be followed rostrally and clearly seen in Section 7 (Fig. 1G). The true injection site is located more rostral. It can be found in the Sections 4 and 5 (Fig. 1D and 1E; arrows). Perhaps it would have been better to show the injection site in Section 5 (Fig. 1E) instead of Section 4 (Fig. 1D) in my article, but I think that this is not a mistake. I wrote in the second paragraph of the Results section that the injection site lies in the “entire caudal portion” of the nucleus corticalis (Schmidt, Reference Schmidt2020). This would mean Sections 4 and 5 (Fig. 1D and 1E).

Consequently, the labeled processes in the tectum opticum in Fig. 1B of Schmidt (Reference Schmidt2020), observed after tracer injection into the nucleus corticalis, are the dendrites of the neurons of the nucleus corticalis. Their distribution in the tectum opticum serves as reference for the labeled structures shown in Fig. 1F and 1J (Schmidt, Reference Schmidt2020).

Other labeled structures

M. Hofmann presents a set of photomicrographs of other brain structures with labeled cells in the whole brain for the selected injection cases in my article. I did not check them. All these afferents to the nucleus corticalis (Fig. 1C–1P; Hofmann, Reference Hofmann2021) or tectum opticum (Figs. 2B–2J and 3B–3L; Hofmann, Reference Hofmann2021) would not contribute to the pattern in the nucleus glomerulosus in Fig. 2 of my article (Schmidt, Reference Schmidt2020). It depends on the neurons of the nucleus corticalis and summarizes the findings of Fig. 1. The connection is “tectum opticum (dendrites of nucleus corticalis neurons)—nucleus corticalis (cell bodies of nucleus corticalis neurons)—nucleus glomerulosus (terminating axons (glomeruli) of nucleus corticalis neurons)”. As it can be inferred from the title of my article, the major aim was to describe these “two different areas of the nucleus glomerulosus” (Schmidt, Reference Schmidt2020).

However, important labeled structures regarding my article are neglected. This applies, for example, to the labeled cells in the dorsal telencephalon whose projections to the tectal layer stratum griseum centrale I cited as “unpublished observations” in the first paragraph of the Discussion section (Schmidt, Reference Schmidt2020). I would have expected them among the photomicrographs in Fig. 3 of Hofmann (Reference Hofmann2021). They are not shown.

Topographical organization

M. Hofmann concluded that the projection pattern in the nucleus glomerulosus may be caused by a topography, because one injection into the tectum opticum is very dorsal (the stratum fibrosum et griseum superficiale injection: Fig. 1E; Schmidt, Reference Schmidt2020) and the second one extreme lateral (the stratum griseum centrale injection: Fig. 1I; Schmidt, Reference Schmidt2020) (Hofmann, Reference Hofmann2021: see Section “Tectal injections”). I disagree with his conclusion. The connection “tectum opticum—nucleus corticalis—nucleus glomerulosus” can be considered as one system that consists of two subsystems due to the different layers in the tectum opticum (Fig. 2; Schmidt, Reference Schmidt2020). These two subsystems are the connections “stratum fibrosum et griseum superficiale—nucleus corticalis—nucleus glomerulosus” and “stratum griseum centrale—nucleus corticalis—nucleus glomerulosus”. In my opinion, a topographical organization may occur within each of the two subsystems, but not across them. Investigations regarding a topographical organization, however, would be subject of further studies, as I mentioned in the third paragraph of the Discussion section (Schmidt, Reference Schmidt2020).