It is commonly accepted that neocortical learning in adult mammals is mediated by changes at the synaptic level (Hebb 1949; Kandel 2006) rather than by the addition of new neurons, as seems to be the case for adult songbirds (Goldman and Nottebohm 1983). In mammals, even though reports have been made about adult neurogenesis at the hippocampal formation these claims have been tainted by investigators conflating adolescence with adulthood (Charvet and Finlay 2018, see footnote 1; for those interested in the thousands of studies on hippocampal neurogenesis in mammals enter ‘neurogenesis in mammals’ on Google). For humans, it is well established that there is no neocortical/hippocampal neurogenesis in adults (Sanai et al. 2011; Sorrells et al. 2018), which concurs with the original suspicion (Dugue, Rakic et al. 2022).
So, what about songbirds? Adult neurogenesis occurs throughout a songbird’s telencephalon, which includes the hippocampal formation (Barnea and Pravosudov 2011). Neurogenesis varies seasonally in many telencephalic structures [the high-vocal center, HVC; the arcopallium, RA; and Area X; see Fig. 1], and it has been related to the learning of new songs. All new neurons originate from the ventricular zone, and they migrate outward to the structures. The addition of new neurons has been related to the seasonal learning of songs, and the life of each song learned lasts one season. The magnitude of neurogenesis is related to the number of songs learned (Brenowitz and Larson 2015). As well, the neurogenesis has been correlated with environmental enrichment, stress, exercise, and the spatial memory of food items and migration as it pertains to the hippocampus, which is located at the lateral ventricle in the telencephalon above the HVC (see Fig. 1). There is no evidence for neurogenesis in the cerebellum.
Thus, the “neocortex” of the songbird much like the neocortex of the mammal is involved in the storage of information such as sound elements, visual objects, and so on, but the storage of songs in the adult songbird is potentiated seasonally by the addition of new neurons, a potentiation that does not occur in the neocortex of any adult mammal. Even if the hippocampal formation of adult mammals were potentiated by neurogenesis [for which there is little evidence when examined carefully: Chart and Finlay 2018; Dugue, Rakic et al. 2022; Sanai et al. 2011; Sorrells et al. 2018], this neurogenesis would not be involved in the storage of information because the hippocampus is a transmitter of information (Corkin 2002; Knecht 2004; Morrison and Hof 1997; Munoz-Lopez et al. 2010; Roux et al. 2021; Scoville and Milner 1957; Squire et al. 2001; Squire and Zola-Morgan 1991; Xu et al. 2016). Finally, the absence of neurogenesis in the cerebellum of adult songbirds indicates that it functions much like the cerebellum of the mammal using synaptogenesis to concatenate movement routines to be executed automatically using neocortical-cerebellar loops (Tehovnik, Hasanbegović, Chen 2024).
Footnote 1: Barbara Finlay of Cornell University was trained at MIT under Peter Schiller and Gerry Schneider. Barbara is an expert on mammalian development, the objective aging of mammals, and on quantitative neuroanatomical methods.
Figure 1: The connectivity of the songbird telencephalon indicating the RA, the HVC, and Area X of the telencephalon that interconnects with nucleus XII that innervates the trachea and syrinx muscles for singing. For further information see text. This is a variation of figure 1 from Kubikova et al. (2014).
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