Proposal by Georg Striedter and David Perkel on
how to Change the Nomenclature of the Avian Forebrain
May 13, 2002
- Change names of avian forebrain structures that are obviously misleading in terms of their implied homologies to mammalian structures.
- Keep changes to a minimum.
- Don't change names that are not actively misleading.
- Minimize changes to names for which all alternatives are also (potentially) misleading.
- Retain as much of the existing nomenclature (including abbreviations) as possible.
- Avoid changes to names that could become problematic down the road as additional information becomes available.
- Base new terms on homologies if (and only if) the homologies are well established.
- Explain the rationale behind the proposed changes, including a treatment of alternative possibilities.
- Borrow freely from other proposals.
- The Proposal:
- Diencephalon: No names should be changed in this region at this time, since new regions are still being described regularly (e.g. VIA and ALa) and the homologies here are still very much in flux. For example, some authors think nucleus rotundus is homologous to LP/pulvinar, while others make a reasonable argument for its homology to part of the mammalian posterior complex. Similarly, the dorsomedial thalamic zone of birds might be largely homologous to the mammalian intralaminar nuclei, but more ventral/posterior nuclei in the avian dorsal thalamus might also be homologous to at least part of the mammalian intralaminar complex.
- Paleostriatum primitivum (PP): Change name to “pallidum” Topological, molecular developmental, connectional, histochemical, and physiological criteria all suggest that PP is homologous to the mammalian pallidum. Since in birds there appears to be no clear division into internal and external segments, it seems prudent to use “pallidum” rather than “globus pallidus”.
- Paleostriatum augmentatum (PA): Change name to “lateral striatum”. Topological, molecular developmental, connectional, histochemical, and physiological criteria all suggest that PA is homologous to at least part of the mammalian striatum. Since LPO (see below) may also be homologous to part of the mammalian striatum, and since PA lies lateral to LPO in some transverse sections, it is best to call PA “lateral striatum”. This term has been used previously in the literature. The term “dorsal striatum” is less desirable.
Parolfactory Lobe (LPO): Change name to “medial striatum”. Electrophysiological data suggest that, in songbirds and chickens, LPO contains both “striatal” and “pallidal” cell types that are intermingled with one another. Thus, LPO may not be strictly homologous with mammalian dorsal striatum.. However, most LPO cells seem to be striatal in nature and, therefore, the term medial striatum seems appropriate if one is willing to cede that the avian “medial striatum” contains some neurons that are probably homologous to mammalian “pallidal” neurons. This could be due to either developmental migration or to changes in developmental gene expression within the avian LPO.
- Hippocampus (H) and Parahippocampal Area (APH): There is no immediate need to change these names, as most authors agree that they are homologous to at least part of the greater hippocampal formation in mammals. Moreover, since more detailed homologies between avian and mammalian hippocampal regions are still controversial, it would be premature to change the names.
- Wulst (W), including all parts of the hyperstriatum except HV: The name “Wulst” is awkward but not actively misleading; therefore it need not be changed. Since W is widely agreed to be homologous to the mammalian dorsal pallium (and, yes, this term has become more widely used in the mammalian literature), “dorsal pallium” might be a good synonym for the “Wulst” region. Another potentially useful synonym for "Wulst" could be "Region W" (see Table 1) because the latter term would enhance consistency with some of the other names we propose (e.g. Region HV and Region N). The term “hyperstriatum” is misleading since none of the avian hyperstriatal areas are homologous to any part of mammalian striatum. Replacing it with a term such as “hyperpallium” is not necessary and might be misleading if it is interpreted as being somehow “above pallium”. Instead, the concept of a hyperstriatum should be retired. One could simply refer to “areas HIA, HIS, HA, and HD” within the Wulst (much as one can refer to neocortical areas by abbreviations; eg. Area MT). Functional regions within the Wulst (e.g. the separate visual, somatosensory, and motor regions) could be referred to as such; they might be abbreviated as V1, S1 and M1 if there is sufficient agreement regarding their homologies to the mammalian counterparts.
- Ventral hyperstriatum (HV): This region is not part of the Wulst or dorsal pallium. Therefore, its name should be distinct from that of the other “hyperstriatal” areas. “Region HV” might do. If one wants to specify what telencephalic division HV belongs to, one could say “Region HV of the lateral pallium”. The developmental data strongly support this hypothesis and no data strongly challenge this hypothesis.
- Neostriatum: This is the most difficult structure in the whole “renaming problem” because the current name is the most misleading, yet most of the obvious alternatives have serious drawbacks.
- The Problem: The term “neostriatum” (N) implies a homology to the mammalian striatum that is almost certainly false. Instead, the molecular and developmental data indicate that the avian N is homologous to part of the mammalian claustro-amygdaloid complex.
- Some Possible Solutions: In terms of connections, functions, and degree of differentiation, N is so different from the mammalian claustro-amygdaloid region that it makes little sense to rename N as a part of the claustrum or amygdala. One could refer to it as “ventral pallium” if one accepts the terminology proposed by Puelles and co-workers for the early embryonic telencephalic zones (this terminology is likely to persist for some time regardless of what we do with the adult avian brain nomenclature). However, it would then be awkward to rename all of the many subdivisions in N. For, example, the “magnocellular nucleus of the anterior neostriatum” (MAN) would become the “magnocellular nucleus of the anterior ventral pallium (MAVP). Such changes are unlikely to be adopted widely, and, if they were, would cause massive discontinuities in the literature. Another possibility is that one could change “neostriatum” to “nidipallium”. This would preserve the abbreviation, but carries no meaning in a comparative context since the term would be bird-specific.
- Our Suggested Solution: One could just refer to it as “region N of the ventral pallium”. (In practice, people would probably use this term rarely, since the developmental biologists would probably use “ventral pallium” and the students of adult avian brains would probably refer to specific subdivisions of region N. Nonetheless, if authors make an effort to mention this full name at least once in their papers, some continuity in the literature could be maintained.)
- How to Name "Neostriatal" Subdivisions: The various subdivisions of “region N” could be named according to their current abbreviations, as illustrated below:
- lMAN: nucleus lMAN (within region, or within the ventral pallium, as appropriate in context)
- HVC: nucleus HVC
- NIF: nucleus NIF
- Field L: nucleus L (L1,2,3)
- Frontal neostriatum (NF): area NF
- And so on. We favor a distinction between nuclei and areas (depending on level of cytoarchitectural distinctness), but would avoid using the term “field” for anything other than field L (to avoid confusion with Rose's various “fields”)
- Piriform Cortex and Dorsolateral Corticoid Area: These do not need to be renamed.
- Archistriatum: This is another trouble spot. Much of the avian archistriatum is almost certainly homologous to much of the avian amygdala, but there is little agreement about more than that. Moreover, some would argue that much of the avian "neostriatum" is also homologous to part of the mammalian amygdala, as is "nucleus taeniae" (which is considered by some to be part of the avian archistriatum). Therefore, it would be unwise to rename "archistriatum" simply as "amygdala", even though that would nicely conserve the abbreviations. Referring to it as "archipallium" is also less than ideal because there are disagreements about whether or not all of the archistriatum is pallial (some of it may be subpallial, but no one seems quite sure about what adult regions come from subpallium). In other words, we need more data on the archistriatum before we can find a good new name for it. In the meantime, we favor simply calling it "region A" (including "nucleus RA", "area Ai", etc.).
Table illustrating the proposed renaming scheme
Old NamesNew NamesMajor RegionNucleus or SubdivisionMajor RegionNucleus or SubdivisionPaleostriatum primitivum (PP)PallidumPaleostriatum augmentatum (PA)Lateral striatumParolfactory Lobe (LPO)Medial striatumArea X (songbirds)Nucleus XLPOm (parrots)Nucleus LPOmWulstRegion W (synonyms: Wulst, Dorsal Pallium)Accessory hyperstriatum (HA)Area HAIntermediate hyperstriatum (HI)Area HIDorsal hyperstriatum (HD)Area HDVentral Hyperstriatum (HV)Region HVNeostriatum (N)Region NEctostriatum (E)Nucleus EFrontal neostriatum (NF)Area NFlMANNucleus lMANHVcNucleus HVCNucleus basalis (NB)Nucleus BField LNucleus LNucleus interface (NIF)Nucleus NIFArchistriatum (A)Region ANucleus robustus archistriatalis (RA)Nucleus RA