Different sets of afferents are demonstrated by the fluorescent tracers fast blue and rhodamine

The fluorescent retrograde tracers Rhodamine B Isothiocyanate (RITC) and Fast Blue (FB) were injected either into the thalamic nucleus rotundus or into the complexus neck muscle in pigeons. Both in the rotundal and the peripheral motor preparations RITC yielded a 3–5 times larger number of afferent ipsilateral neurones than FB. While RITC additionally labelled a large number of contralateral afferents, virtually no labelled cells were detected contralateral to the injection site using FB. This selective omission of the contralateral input with FB was identical for both neural systems despite differences in injection volume, survival time, and transport length. In the present case, this structure-specific sensitivity of FB would lead to the erroneous conclusion that contralateral afferents are virtually non-existent in the visual tectofugal and the peripheral motor system. Thus, these results make it likely that the choice for a certain tracer may lead to different connectional interpretations of a neural system. Although the mechanism for these structure-specific sensitivity differences are unknown, it is suggested that tracers with lower effectivity like FB may be unable to label afferent structures with fewer axon terminals.